On wiring the split load CU, will it be OK to put both the cooker and
hob onto the RCD'd busbar? or should they go onto the non RCD side?
I was thinking of putting both ring mains on the RCD and the
cooker/hob, and putting both lighting ccts on the non RCD side so I'm
not plunged into darkness if there is a fault elsewhere !!

CU is 100A, 80A rcd

cheers

On 2 Oct 2006 08:54:12 -0700, "Staffbull" wrote:

On wiring the split load CU, will it be OK to put both the cooker and
hob onto the RCD'd busbar? or should they go onto the non RCD side?
I was thinking of putting both ring mains on the RCD and the
cooker/hob, and putting both lighting ccts on the non RCD side so I'm
not plunged into darkness if there is a fault elsewhere !!

CU is 100A, 80A rcd


Perform some risk benefit analysis.

When you get electrocuted by a faulty hob/oven do you want the lights
to remain on? Or, when the RCD trips because of an earth leakage on
the hob/oven would you prefer salad for dinner?

Maybe you don't like salad


--

Matt wrote:
On 2 Oct 2006 08:54:12 -0700, "Staffbull" wrote:

On wiring the split load CU, will it be OK to put both the cooker and
hob onto the RCD'd busbar? or should they go onto the non RCD side?
I was thinking of putting both ring mains on the RCD and the
cooker/hob, and putting both lighting ccts on the non RCD side so I'm
not plunged into darkness if there is a fault elsewhere !!

CU is 100A, 80A rcd


Perform some risk benefit analysis.

When you get electrocuted by a faulty hob/oven do you want the lights
to remain on? Or, when the RCD trips because of an earth leakage on
the hob/oven would you prefer salad for dinner?

Maybe you don't like salad


??????


--

Don't worry - there is lots of info regarding split CUs in the d-i-y
archives. After a lengthy study, I have come to the conclusion that
the cooker stays off the RCD (as you have suggested) as most cookers
'leak' anyway.

"Staffbull" wrote in message
ups.com...
On wiring the split load CU, will it be OK to put both the cooker and
hob onto the RCD'd busbar? or should they go onto the non RCD side?
I was thinking of putting both ring mains on the RCD and the
cooker/hob, and putting both lighting ccts on the non RCD side so I'm
not plunged into darkness if there is a fault elsewhere !!

CU is 100A, 80A rcd

cheers

I would normally put the cooker on the non-rcd side. However, if the cooker
switch is of the sort that also has a 13amp socket on it and it is likely
that the socket may be used to power outdoor powertools then it must go on
the RCD side of the CU.

Adam

ARWadsworth wrote:
"Staffbull" wrote in message
ups.com...
On wiring the split load CU, will it be OK to put both the cooker and
hob onto the RCD'd busbar? or should they go onto the non RCD side?
I was thinking of putting both ring mains on the RCD and the
cooker/hob, and putting both lighting ccts on the non RCD side so I'm
not plunged into darkness if there is a fault elsewhere !!

CU is 100A, 80A rcd

cheers

I would normally put the cooker on the non-rcd side. However, if the cooker
switch is of the sort that also has a 13amp socket on it and it is likely
that the socket may be used to power outdoor powertools then it must go on
the RCD side of the CU.

Adam

Cheers, what about the seperate Hob? RCD it?

"Staffbull" wrote in message
ups.com...

ARWadsworth wrote:
"Staffbull" wrote in message
ups.com...
On wiring the split load CU, will it be OK to put both the cooker and
hob onto the RCD'd busbar? or should they go onto the non RCD side?
I was thinking of putting both ring mains on the RCD and the
cooker/hob, and putting both lighting ccts on the non RCD side so I'm
not plunged into darkness if there is a fault elsewhere !!

CU is 100A, 80A rcd

cheers

I would normally put the cooker on the non-rcd side. However, if the
cooker
switch is of the sort that also has a 13amp socket on it and it is likely
that the socket may be used to power outdoor powertools then it must go
on
the RCD side of the CU.

Adam

Cheers, what about the seperate Hob? RCD it?


Same applies, for "cooker" read "hob".

Adam

ARWadsworth wrote:
"Staffbull" wrote in message
ups.com...

ARWadsworth wrote:
"Staffbull" wrote in message
ups.com...
On wiring the split load CU, will it be OK to put both the cooker and
hob onto the RCD'd busbar? or should they go onto the non RCD side?
I was thinking of putting both ring mains on the RCD and the
cooker/hob, and putting both lighting ccts on the non RCD side so I'm
not plunged into darkness if there is a fault elsewhere !!

CU is 100A, 80A rcd

cheers

I would normally put the cooker on the non-rcd side. However, if the
cooker
switch is of the sort that also has a 13amp socket on it and it is likely
that the socket may be used to power outdoor powertools then it must go
on
the RCD side of the CU.

Adam

Cheers, what about the seperate Hob? RCD it?


Same applies, for "cooker" read "hob".

Adam

Thanks

On 2 Oct 2006 10:19:09 -0700, "Ren"
wrote:

Don't worry - there is lots of info regarding split CUs in the d-i-y
archives. After a lengthy study, I have come to the conclusion that
the cooker stays off the RCD (as you have suggested) as most cookers
'leak' anyway.

Which is fine right until the point the oven element fails and the
break in the earth conductor in the consumer unit that has gone
un-noticed for months/years/forever means the fuse doesn't blow and
the oven casing rises to mains potential.

A fatal shock is about the 6 inches of reach between the oven and the
nicely bonded "split level" gas hob sat right above it.

Apart from the last bit which was narrowly avoided by the owner
switching off the cooker at the wall switch the above is actually what
happened on an installation a few weeks ago. The earth had failed
right in the middle of a sleeved run in the consumer unit. A simple
visual examination wouldn't have picked it up.

Put the cooker on the RCD!


--

Matt wrote:
On 2 Oct 2006 10:19:09 -0700, "Ren"
wrote:

Don't worry - there is lots of info regarding split CUs in the d-i-y
archives. After a lengthy study, I have come to the conclusion that
the cooker stays off the RCD (as you have suggested) as most cookers
'leak' anyway.

Which is fine right until the point the oven element fails and the
break in the earth conductor in the consumer unit that has gone
un-noticed for months/years/forever means the fuse doesn't blow and
the oven casing rises to mains potential.

A fatal shock is about the 6 inches of reach between the oven and the
nicely bonded "split level" gas hob sat right above it.

Apart from the last bit which was narrowly avoided by the owner
switching off the cooker at the wall switch the above is actually what
happened on an installation a few weeks ago. The earth had failed
right in the middle of a sleeved run in the consumer unit. A simple
visual examination wouldn't have picked it up.

Put the cooker on the RCD!


--

I'm now as confused as a baby raccoon !!!

My first thought was to RCD everything apart from the lights, is this
sound? the cooker and hob both have seperate radials as they are more
than 2m apart (as per regs).
What size MCB should serve the cooker & hob (seperateley) the CU came
with five 32's one 40 three 6's and one 16.

"Matt" wrote in message
...
On 2 Oct 2006 10:19:09 -0700, "Ren"
wrote:

Don't worry - there is lots of info regarding split CUs in the d-i-y
archives. After a lengthy study, I have come to the conclusion that
the cooker stays off the RCD (as you have suggested) as most cookers
'leak' anyway.

Which is fine right until the point the oven element fails and the
break in the earth conductor in the consumer unit that has gone
un-noticed for months/years/forever means the fuse doesn't blow and
the oven casing rises to mains potential.

A fatal shock is about the 6 inches of reach between the oven and the
nicely bonded "split level" gas hob sat right above it.

Apart from the last bit which was narrowly avoided by the owner
switching off the cooker at the wall switch the above is actually what
happened on an installation a few weeks ago. The earth had failed
right in the middle of a sleeved run in the consumer unit. A simple
visual examination wouldn't have picked it up.

Put the cooker on the RCD!

A simple installation test or PIR would have picked that up.

Adam

On 2 Oct 2006 11:39:51 -0700, "Staffbull" wrote:

My first thought was to RCD everything apart from the lights, is this
sound?

Yes

the cooker and hob both have seperate radials as they are more
than 2m apart (as per regs).
What size MCB should serve the cooker & hob (seperateley) the CU came
with five 32's one 40 three 6's and one 16.

The MCB needs sizing in accordance with whatever size cable you are
protecting. Without these "unknowns" it's just a guessing exercise.

The guess could be 32A, 20A or 16A

--

Matt wrote:
On 2 Oct 2006 11:39:51 -0700, "Staffbull" wrote:

My first thought was to RCD everything apart from the lights, is this
sound?

Yes

the cooker and hob both have seperate radials as they are more
than 2m apart (as per regs).
What size MCB should serve the cooker & hob (seperateley) the CU came
with five 32's one 40 three 6's and one 16.

The MCB needs sizing in accordance with whatever size cable you are
protecting. Without these "unknowns" it's just a guessing exercise.

The guess could be 32A, 20A or 16A

--

6mm cable (4mm is like hens teeth around here for some reason) and
cooker 8m hob 11m long

Staffbull wrote:
On wiring the split load CU, will it be OK to put both the cooker and
hob onto the RCD'd busbar? or should they go onto the non RCD side?
I was thinking of putting both ring mains on the RCD and the
cooker/hob, and putting both lighting ccts on the non RCD side so I'm
not plunged into darkness if there is a fault elsewhere !!

CU is 100A, 80A rcd

cheers


I think the best compromise is a 100mA RCD for everything, with 30mA RCD
or RCBO for where there is a real danger.

In practice, although I haven't gotten to the 30mA RCBO for outside
sockets, I now have almost zero nuisance tripping on a BIG house, and
yet whenever there has been a genuine fault, the RCD has tripped.

(extension cable in the rain outside. 2Kohm short between washing
machine motor coil and earth)

You have to make your mind up whether its better to be left in the dark
everytime there is a power surge or a big motor cuts in, or whether you
are prepared to risk being fried by 100mA .

Even 30mA will kill you if you are unlucky.

But two sweaty hands measures at about 3k, which is like 80mA on a 250v
circuit.

Ren wrote:
Don't worry - there is lots of info regarding split CUs in the d-i-y
archives. After a lengthy study, I have come to the conclusion that
the cooker stays off the RCD (as you have suggested) as most cookers
'leak' anyway.


Which is why they are EARTHED.

Matt wrote:
On 2 Oct 2006 10:19:09 -0700, "Ren"
wrote:

Don't worry - there is lots of info regarding split CUs in the d-i-y
archives. After a lengthy study, I have come to the conclusion that
the cooker stays off the RCD (as you have suggested) as most cookers
'leak' anyway.

Which is fine right until the point the oven element fails and the
break in the earth conductor in the consumer unit that has gone
un-noticed for months/years/forever means the fuse doesn't blow and
the oven casing rises to mains potential.

A fatal shock is about the 6 inches of reach between the oven and the
nicely bonded "split level" gas hob sat right above it.

Apart from the last bit which was narrowly avoided by the owner
switching off the cooker at the wall switch the above is actually what
happened on an installation a few weeks ago. The earth had failed
right in the middle of a sleeved run in the consumer unit. A simple
visual examination wouldn't have picked it up.

Put the cooker on the RCD!


Put it on a 100mA full house trip. Put socketry on 30mA as per regs.

The stove will normally leak enough to get a tingle off if its not earthed.

Staffbull wrote:
Matt wrote:
On 2 Oct 2006 10:19:09 -0700, "Ren"
wrote:

Don't worry - there is lots of info regarding split CUs in the d-i-y
archives. After a lengthy study, I have come to the conclusion that
the cooker stays off the RCD (as you have suggested) as most cookers
'leak' anyway.
Which is fine right until the point the oven element fails and the
break in the earth conductor in the consumer unit that has gone
un-noticed for months/years/forever means the fuse doesn't blow and
the oven casing rises to mains potential.

A fatal shock is about the 6 inches of reach between the oven and the
nicely bonded "split level" gas hob sat right above it.

Apart from the last bit which was narrowly avoided by the owner
switching off the cooker at the wall switch the above is actually what
happened on an installation a few weeks ago. The earth had failed
right in the middle of a sleeved run in the consumer unit. A simple
visual examination wouldn't have picked it up.

Put the cooker on the RCD!


--

I'm now as confused as a baby raccoon !!!

Welcome to the 'safety mens falling down the stairs in the dark, because
you have a 30mA trip and the electronics draws 31mA' brigade.

My first thought was to RCD everything apart from the lights, is this
sound? the cooker and hob both have seperate radials as they are more
than 2m apart (as per regs).

Look. Houses all 'leak' a bit. Apart from stuff like cookers and kettles
that have to balance insulation integrity with high temperatures and
cheap cost, steamy rooms and a sweaty thumb print can cause a few mA
leakage, every RFI filter in every bit of electronics adds one or two
more, the wiring capacitance of all your wires adds a bit more..

I don't regard any wiring as potentially untouchable, so I like an
overall RCD including lights.

BUT experience shows that a full house 30mA trip is always tripping for
no bloody reason at all. Especially if you have as many TV's, computers,
mast head amps, routers, printers and the like as I have.

HOWEVER the regs state that outside sockets have to be on a 30mA RCD.

In practice this means split load or RCBO.

I personally like an RCD protected HOUSE, but set at a level that
doesn't do nuisance trips., Here its 100mA. In a small house or flat,
30mA is probably OK on everything.



What size MCB should serve the cooker & hob (seperateley) the CU came
with five 32's one 40 three 6's and one 16.

Depends on the cooker wiring. And the cooker rating. The MCB is there to
protect the wiring to the cooker though.

In article .com,
"Staffbull" writes:
(4mm is like hens teeth around here for some reason)

It turns out not to be a particularly useful size.
Wholesalers sometimes don't even stock it.

--
Andrew Gabriel

On 2 Oct 2006 12:52:34 -0700, "Staffbull" wrote:


Matt wrote:
On 2 Oct 2006 11:39:51 -0700, "Staffbull" wrote:

My first thought was to RCD everything apart from the lights, is this
sound?

Yes

the cooker and hob both have seperate radials as they are more
than 2m apart (as per regs).
What size MCB should serve the cooker & hob (seperateley) the CU came
with five 32's one 40 three 6's and one 16.

The MCB needs sizing in accordance with whatever size cable you are
protecting. Without these "unknowns" it's just a guessing exercise.

The guess could be 32A, 20A or 16A

--

6mm cable (4mm is like hens teeth around here for some reason) and
cooker 8m hob 11m long

Use a 32A to protect the cable, although if you feel the need to fuse
more closely a typical oven at around 3kW would be "happier" with 16A.


--

On Mon, 02 Oct 2006 19:38:06 GMT, "ARWadsworth"
wrote:


"Matt" wrote in message
.. .

Which is fine right until the point the oven element fails and the
break in the earth conductor in the consumer unit that has gone
un-noticed for months/years/forever means the fuse doesn't blow and
the oven casing rises to mains potential.

A fatal shock is about the 6 inches of reach between the oven and the
nicely bonded "split level" gas hob sat right above it.

Apart from the last bit which was narrowly avoided by the owner
switching off the cooker at the wall switch the above is actually what
happened on an installation a few weeks ago. The earth had failed
right in the middle of a sleeved run in the consumer unit. A simple
visual examination wouldn't have picked it up.

Put the cooker on the RCD!

A simple installation test or PIR would have picked that up.

errm what do you think I was doing to even pick up the fault in the
first place! I'm not in the habit of walking round kitchens, turning
on an obviously faulty oven, finding a bit of well earthed metal and
then simultaneously dabbing a hand on each to check for any wiring
defects

How often do people really have their installations checked? I'd bet
some have never been touched in 20 years, maybe more.


--

Matt wrote:

Which is fine right until the point the oven element fails and the
break in the earth conductor in the consumer unit that has gone
un-noticed for months/years/forever means the fuse doesn't blow and
the oven casing rises to mains potential.

That displays a fundamental misunderstanding about what the 30 mA RCD in
a split load CU (in a TN earthed installation) is for. It's not there
as a form of backup protection in case the earthing is dodgy. Its
primary purpose is to provide supplementary protection against direct
contact with live conductors, particularly outdoors - the cut hedge
trimmer flex scenario, if you like.

The On-Site Guide is quite unequivocal about which circuits should be on
the RCD side of the board:

"30 mA RCDs installed to provide protection to socket outlets likely to
feed portable equipment outdoors should protect only those sockets, see
Fig 3b."

The guide goes on to state that RCDs installed for indirect contact
protection (where the earth fault loop impedance is too high to allow an
OPD to perform this role) should have a rated tripping current of 100 mA
or more.

Put the cooker on the RCD!

.... is the wrong answer!

--
Andy

Staffbull wrote:

On wiring the split load CU, will it be OK to put both the cooker and
hob onto the RCD'd busbar? or should they go onto the non RCD side?

Better on the non RCD side. (with the caveat that the cooker point does
not have an integral socket)

I was thinking of putting both ring mains on the RCD and the
cooker/hob, and putting both lighting ccts on the non RCD side so I'm
not plunged into darkness if there is a fault elsewhere !!

CU is 100A, 80A rcd

30mA trip one presumes?


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
Staffbull wrote:

On wiring the split load CU, will it be OK to put both the cooker and
hob onto the RCD'd busbar? or should they go onto the non RCD side?

Better on the non RCD side. (with the caveat that the cooker point does
not have an integral socket)

I was thinking of putting both ring mains on the RCD and the
cooker/hob, and putting both lighting ccts on the non RCD side so I'm
not plunged into darkness if there is a fault elsewhere !!

CU is 100A, 80A rcd

30mA trip one presumes?


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/


Hi John, yes 30mA for the RCD

On Tue, 03 Oct 2006 00:21:37 +0100, Andy Wade
wrote:

Matt wrote:

Which is fine right until the point the oven element fails and the
break in the earth conductor in the consumer unit that has gone
un-noticed for months/years/forever means the fuse doesn't blow and
the oven casing rises to mains potential.

That displays a fundamental misunderstanding about what the 30 mA RCD in
a split load CU (in a TN earthed installation) is for. It's not there
as a form of backup protection in case the earthing is dodgy. Its
primary purpose is to provide supplementary protection against direct
contact with live conductors, particularly outdoors - the cut hedge
trimmer flex scenario, if you like.

The On-Site Guide is quite unequivocal about which circuits should be on
the RCD side of the board:

"30 mA RCDs installed to provide protection to socket outlets likely to
feed portable equipment outdoors should protect only those sockets, see
Fig 3b."


That quote doesnt back up your assertion. It simply means dont put the
sockets in the garage on the main 30 mA RCD but put them on the non
protected side and let them have their own RCDs.

The guide goes on to state that RCDs installed for indirect contact
protection (where the earth fault loop impedance is too high to allow an
OPD to perform this role) should have a rated tripping current of 100 mA
or more.

Put the cooker on the RCD!

... is the wrong answer!

On Tue, 03 Oct 2006 00:21:37 +0100, Andy Wade
wrote:

Matt wrote:

Which is fine right until the point the oven element fails and the
break in the earth conductor in the consumer unit that has gone
un-noticed for months/years/forever means the fuse doesn't blow and
the oven casing rises to mains potential.

That displays a fundamental misunderstanding about what the 30 mA RCD in
a split load CU (in a TN earthed installation) is for. It's not there
as a form of backup protection in case the earthing is dodgy. Its
primary purpose is to provide supplementary protection against direct
contact with live conductors, particularly outdoors - the cut hedge
trimmer flex scenario, if you like.

The On-Site Guide is quite unequivocal about which circuits should be on
the RCD side of the board:

"30 mA RCDs installed to provide protection to socket outlets likely to
feed portable equipment outdoors should protect only those sockets, see
Fig 3b."

The guide goes on to state that RCDs installed for indirect contact
protection (where the earth fault loop impedance is too high to allow an
OPD to perform this role) should have a rated tripping current of 100 mA
or more.

Put the cooker on the RCD!

... is the wrong answer!

There is no fundamental misunderstanding, and I also know full well
what has been written on the subject. The facts are that in the
circumstances mentioned, placing the cooker on the RCD protected side
could have prevented a situation that may have lead to death. The
cooker still had a functioning grill for which there was opportunity
to use on a few occasions prior to the oven element being replaced.
As it was, the simple action of switching off at the cooker point
inadvertently prevented a fatal shock. If I hadn't had a socket
tester to hand I wouldn't have immediately picked up on the missing
earth until the whole installation was scheduled to be tested a few
weeks later, in more normal circumstances the missing earth could have
been undetected for years.

While I'm not in favour of Part P, is it right to follow the head in
the sand "it will never happen" approach and wait until it kills
someone "important" like MP's daughter or other "celebrity" or do we
get the engineering right in the first place so that faults that may
go undetected do not later lead to a dangerous situation?

The downside of placing almost everything on a 30mA RCD when the
installation and equipment are in good condition is absolutely
minimal, nor does fitting a 30mA RCD doesn't immediately make the
installation less safe. Not that I think that instantly plunging the
house into total darkness is a good idea, given the choice I would put
everything on the 30mA RCD including the lights and provide maintained
lighting at strategic points.

In the case of a central pendant lamp failing where RCD protection is
not provided for the lighting, the user might use table lamps in that
room for the evening so that replacement can take place next day.
Sometimes the user cannot always be sure the light switch is off (ok
they shouldn't be used for isolation purposes but they often are) In
these circumstances accidental contact with live objects such as the
remains of the lamp base, or the contacts in the lamp holder are
possible, and in the case of the elderly or someone not immediately
familiar with lamp replacement quite likely. You wouldn't permit
unshrouded socket outlets which leave exposed live parts, yet go up a
step ladder and reach up to the ceiling and they are deemed
"acceptable"

But it really doesn't matter where you come into contact with mains
electricity, live conductors in the garden or live unearthed metalwork
in the kitchen, either can kill. That a 30mA RCD is deemed completely
suitable in one set of circumstances in the garden, yet in the
kitchen, where damp hands and portable electrical equipment such as
blenders are operated round devices like hobs that may burn through
power cords, RCD protection is frowned upon. Utterly ridiculous. You
might not operate a hedge trimmer or lawnmower in the kitchen but the
potential for injury is probably just as great especially given the
frequency of use. Lawnmower usage of once a week for 6 months of the
year vs cooking once/twice/three times a day for 50 weeks a year.

Full or partial failure of earths can happen due to many
circumstances, of course on a well installed and regularly inspected
system the chances of an earth conductor failing AND a live to earth
fault are probably remote, but the backup can be provided at zero
cost, there is no need to duplicate RCD's, one rated at 30mA trip can
operate in a safe and predicable manner and not be prone to false
tripping. The idea promulgated that 100mA RCD's are suitable as backup
protection for inadvertent contact is laughable in the extreme. Yes
you might prevent someone else getting zapped but one body on the
kitchen floor is one too many. I might end up being the only one in
here holding that view but there are many who totally disagree with
what the IEE place into print on both this subject and it must be said
many others.


--

Matt wrote:

I intended to reply to this a while ago as it raises some interesting
points... only just remembered to do it!

On Tue, 03 Oct 2006 00:21:37 +0100, Andy Wade

That displays a fundamental misunderstanding about what the 30 mA RCD in
a split load CU (in a TN earthed installation) is for. It's not there
as a form of backup protection in case the earthing is dodgy. Its
primary purpose is to provide supplementary protection against direct
contact with live conductors, particularly outdoors - the cut hedge
trimmer flex scenario, if you like.

There is no fundamental misunderstanding, and I also know full well
what has been written on the subject. The facts are that in the
circumstances mentioned, placing the cooker on the RCD protected side
could have prevented a situation that may have lead to death. The

Phrases like "Could have", "may have", and "in the circumstances
mentioned", highlight a significant point: that the incidence of deaths
caused by fixed wiring in any circumstance are vanishingly small[1], and
one must presume that those caused from this particular scenario even
smaller.

[1] I appreciate if you are a relative of one of the handful of people
killed in this way, you will derive little comfort from this - but we
are talking bigger picture here!

cooker still had a functioning grill for which there was opportunity
to use on a few occasions prior to the oven element being replaced.
As it was, the simple action of switching off at the cooker point
inadvertently prevented a fatal shock. If I hadn't had a socket
tester to hand I wouldn't have immediately picked up on the missing
earth until the whole installation was scheduled to be tested a few
weeks later, in more normal circumstances the missing earth could have
been undetected for years.

What did you plug the socket tester into though? A socket on the cooker
point? If so then it would be correct for the oven to be on a RCD
protected supply anyway, simply because of this socket being there.

While I'm not in favour of Part P, is it right to follow the head in
the sand "it will never happen" approach and wait until it kills
someone "important" like MP's daughter or other "celebrity" or do we
get the engineering right in the first place so that faults that may
go undetected do not later lead to a dangerous situation?

Good engineering not only requires that you analyse risks, but also look
at the likelihood of those risks manifesting and causing harm.

It is very easy to take a specific example as say "a RCD protected
supply would be better because it would mitigate the risks in this
circumstance", however that in itself does not suggest it would be good
engineering. Take your example of the oven: the RCD would reduce the
electrocution risk in the circumstance you described, however how
probable is that particular combination of faults? Now assess how
probable a nuisance trip might be as a result of placing the oven on
the RCD. If the likelihood of causing a trip is significantly greater
(and I suspect it would be), then the likelihood of causing injury due
to a trip or fall is also significantly raised. Thousands die each year
from the latter remember.

The downside of placing almost everything on a 30mA RCD when the
installation and equipment are in good condition is absolutely
minimal,

Much depends on what "everything" is, and how much of it there is. One
could argue that the best possible protection would be offered by having
a dedicated RCBO on every circuit. However this is not something you see
often, because cost must also come into the equation.

nor does fitting a 30mA RCD doesn't immediately make the
installation less safe. Not that I think that instantly plunging the
house into total darkness is a good idea, given the choice I would put
everything on the 30mA RCD including the lights and provide maintained
lighting at strategic points.

That is a viable solution - and one permitted by the wiring regs.
However again there is a risk to be assessed there. It is only viable so
long as the emergency lighting is subject to routine maintenance and
testing, otherwise it becomes a liability.

In the case of a central pendant lamp failing where RCD protection is
not provided for the lighting, the user might use table lamps in that
room for the evening so that replacement can take place next day.
Sometimes the user cannot always be sure the light switch is off (ok
they shouldn't be used for isolation purposes but they often are) In
these circumstances accidental contact with live objects such as the
remains of the lamp base, or the contacts in the lamp holder are
possible, and in the case of the elderly or someone not immediately
familiar with lamp replacement quite likely. You wouldn't permit
unshrouded socket outlets which leave exposed live parts, yet go up a
step ladder and reach up to the ceiling and they are deemed
"acceptable"

Placing "out of reach" is acceptable in many circumstances. The effort
required to reach the lamp holder is much greater than a socket
(especially for the most likely candidates to go sticking metal things
in sockets - i.e. kids) and hence the probability of accidental contact
much less. There is also very little danger of the lamp holder being
used to power a device outside via an extension lead. Serious shock
injury in most living rooms due to direct contact with live parts is in
reality going to be very small, since you will typically be well
insulated from any earth reference by carpets and shoes etc.

But it really doesn't matter where you come into contact with mains
electricity, live conductors in the garden or live unearthed metalwork
in the kitchen, either can kill.

Either can, however the former is far more likely to.

That a 30mA RCD is deemed completely
suitable in one set of circumstances in the garden, yet in the
kitchen, where damp hands and portable electrical equipment such as
blenders are operated round devices like hobs that may burn through
power cords, RCD protection is frowned upon.

I am not sure how you have arrived at this interpretation.

RCD protection for kitchen *sockets* is certainly *not* frowned upon,
and in fact, is required in all but a very few cases (where it is
nevertheless still strongly recommenced). As you say, it is the blender
in your hand with its flex melted on the hob that is far more likely to
cause you harm.

Utterly ridiculous. You
might not operate a hedge trimmer or lawnmower in the kitchen but the
potential for injury is probably just as great especially given the
frequency of use. Lawnmower usage of once a week for 6 months of the
year vs cooking once/twice/three times a day for 50 weeks a year.

The probability of the event itself occurring is in many ways less
important than its severity. You may survive 20 minor shocks in a
kitchen environment while relatively well insulated by flooring and
shoes etc, however survivability of one shock in a damp garden is much
reduced. Also note again that RCD protection would be required in both
circumstances anyway, with the exception of for the fixed kitchen
appliance (cooker, boiler etc).

Full or partial failure of earths can happen due to many
circumstances, of course on a well installed and regularly inspected
system the chances of an earth conductor failing AND a live to earth
fault are probably remote, but the backup can be provided at zero
cost, there is no need to duplicate RCD's, one rated at 30mA trip can
operate in a safe and predicable manner and not be prone to false
tripping.

Assuming the combined leakage of the devices it supplies is low enough.
The more traditionally "leaky" devices you knowingly connect to it, the
more likely you to have problems I would have thought.

The idea promulgated that 100mA RCD's are suitable as backup
protection for inadvertent contact is laughable in the extreme. Yes
you might prevent someone else getting zapped but one body on the
kitchen floor is one too many.

Not sure I follow this. The only time a 100mA trip RCD would be mandated
for protection from direct or indirect contact is when EEBADS alone
cannot be relied on to do this (e.g. non power circuits on a TT install
with a high impedance earth connection). In these circumstances a 100mA
RCD will provide very effective protection from indirect contact caused
by a phase to earth fault, and massively improved chances of survival in
the case of direct contact, while not accentuating the risk of injury as
a consequence of a nuisance trip.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
Matt wrote:

I intended to reply to this a while ago as it raises some interesting
points... only just remembered to do it!

On Tue, 03 Oct 2006 00:21:37 +0100, Andy Wade

That displays a fundamental misunderstanding about what the 30 mA RCD in
a split load CU (in a TN earthed installation) is for. It's not there
as a form of backup protection in case the earthing is dodgy. Its
primary purpose is to provide supplementary protection against direct
contact with live conductors, particularly outdoors - the cut hedge
trimmer flex scenario, if you like.

There is no fundamental misunderstanding, and I also know full well
what has been written on the subject. The facts are that in the
circumstances mentioned, placing the cooker on the RCD protected side
could have prevented a situation that may have lead to death. The

Phrases like "Could have", "may have", and "in the circumstances
mentioned", highlight a significant point: that the incidence of deaths
caused by fixed wiring in any circumstance are vanishingly small[1], and
one must presume that those caused from this particular scenario even
smaller.

[1] I appreciate if you are a relative of one of the handful of people
killed in this way, you will derive little comfort from this - but we
are talking bigger picture here!

cooker still had a functioning grill for which there was opportunity
to use on a few occasions prior to the oven element being replaced.
As it was, the simple action of switching off at the cooker point
inadvertently prevented a fatal shock. If I hadn't had a socket
tester to hand I wouldn't have immediately picked up on the missing
earth until the whole installation was scheduled to be tested a few
weeks later, in more normal circumstances the missing earth could have
been undetected for years.

What did you plug the socket tester into though? A socket on the cooker
point? If so then it would be correct for the oven to be on a RCD
protected supply anyway, simply because of this socket being there.

While I'm not in favour of Part P, is it right to follow the head in
the sand "it will never happen" approach and wait until it kills
someone "important" like MP's daughter or other "celebrity" or do we
get the engineering right in the first place so that faults that may
go undetected do not later lead to a dangerous situation?

Good engineering not only requires that you analyse risks, but also look
at the likelihood of those risks manifesting and causing harm.

It is very easy to take a specific example as say "a RCD protected
supply would be better because it would mitigate the risks in this
circumstance", however that in itself does not suggest it would be good
engineering. Take your example of the oven: the RCD would reduce the
electrocution risk in the circumstance you described, however how
probable is that particular combination of faults? Now assess how
probable a nuisance trip might be as a result of placing the oven on
the RCD. If the likelihood of causing a trip is significantly greater
(and I suspect it would be), then the likelihood of causing injury due
to a trip or fall is also significantly raised. Thousands die each year
from the latter remember.

The downside of placing almost everything on a 30mA RCD when the
installation and equipment are in good condition is absolutely
minimal,

Much depends on what "everything" is, and how much of it there is. One
could argue that the best possible protection would be offered by having
a dedicated RCBO on every circuit. However this is not something you see
often, because cost must also come into the equation.

nor does fitting a 30mA RCD doesn't immediately make the
installation less safe. Not that I think that instantly plunging the
house into total darkness is a good idea, given the choice I would put
everything on the 30mA RCD including the lights and provide maintained
lighting at strategic points.

That is a viable solution - and one permitted by the wiring regs.
However again there is a risk to be assessed there. It is only viable so
long as the emergency lighting is subject to routine maintenance and
testing, otherwise it becomes a liability.

In the case of a central pendant lamp failing where RCD protection is
not provided for the lighting, the user might use table lamps in that
room for the evening so that replacement can take place next day.
Sometimes the user cannot always be sure the light switch is off (ok
they shouldn't be used for isolation purposes but they often are) In
these circumstances accidental contact with live objects such as the
remains of the lamp base, or the contacts in the lamp holder are
possible, and in the case of the elderly or someone not immediately
familiar with lamp replacement quite likely. You wouldn't permit
unshrouded socket outlets which leave exposed live parts, yet go up a
step ladder and reach up to the ceiling and they are deemed
"acceptable"

Placing "out of reach" is acceptable in many circumstances. The effort
required to reach the lamp holder is much greater than a socket
(especially for the most likely candidates to go sticking metal things
in sockets - i.e. kids) and hence the probability of accidental contact
much less. There is also very little danger of the lamp holder being
used to power a device outside via an extension lead. Serious shock
injury in most living rooms due to direct contact with live parts is in
reality going to be very small, since you will typically be well
insulated from any earth reference by carpets and shoes etc.

But it really doesn't matter where you come into contact with mains
electricity, live conductors in the garden or live unearthed metalwork
in the kitchen, either can kill.

Either can, however the former is far more likely to.

That a 30mA RCD is deemed completely
suitable in one set of circumstances in the garden, yet in the
kitchen, where damp hands and portable electrical equipment such as
blenders are operated round devices like hobs that may burn through
power cords, RCD protection is frowned upon.

I am not sure how you have arrived at this interpretation.

RCD protection for kitchen *sockets* is certainly *not* frowned upon,
and in fact, is required in all but a very few cases (where it is
nevertheless still strongly recommenced). As you say, it is the blender
in your hand with its flex melted on the hob that is far more likely to
cause you harm.

Utterly ridiculous. You
might not operate a hedge trimmer or lawnmower in the kitchen but the
potential for injury is probably just as great especially given the
frequency of use. Lawnmower usage of once a week for 6 months of the
year vs cooking once/twice/three times a day for 50 weeks a year.

The probability of the event itself occurring is in many ways less
important than its severity. You may survive 20 minor shocks in a
kitchen environment while relatively well insulated by flooring and
shoes etc, however survivability of one shock in a damp garden is much
reduced. Also note again that RCD protection would be required in both
circumstances anyway, with the exception of for the fixed kitchen
appliance (cooker, boiler etc).

Full or partial failure of earths can happen due to many
circumstances, of course on a well installed and regularly inspected
system the chances of an earth conductor failing AND a live to earth
fault are probably remote, but the backup can be provided at zero
cost, there is no need to duplicate RCD's, one rated at 30mA trip can
operate in a safe and predicable manner and not be prone to false
tripping.

Assuming the combined leakage of the devices it supplies is low enough.
The more traditionally "leaky" devices you knowingly connect to it, the
more likely you to have problems I would have thought.

The idea promulgated that 100mA RCD's are suitable as backup
protection for inadvertent contact is laughable in the extreme. Yes
you might prevent someone else getting zapped but one body on the
kitchen floor is one too many.

Not sure I follow this. The only time a 100mA trip RCD would be mandated
for protection from direct or indirect contact is when EEBADS alone
cannot be relied on to do this (e.g. non power circuits on a TT install
with a high impedance earth connection). In these circumstances a 100mA
RCD will provide very effective protection from indirect contact caused
by a phase to earth fault, and massively improved chances of survival in
the case of direct contact, while not accentuating the risk of injury as
a consequence of a nuisance trip.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/


Thanks, cooker and hob are both on their own MCB, non RCD'd and have
their own 45A DP without socket. :-)

John Rumm wrote:

The only time a 100mA trip RCD would be mandated
for protection from direct or indirect contact is when EEBADS alone
cannot be relied on to do this (e.g. non power circuits on a TT install
with a high impedance earth connection). In these circumstances a 100mA
RCD will provide very effective protection from indirect contact caused
by a phase to earth fault, and massively improved chances of survival in
the case of direct contact, while not accentuating the risk of injury as
a consequence of a nuisance trip.



That is my point exactly. I don't want any non RCD protected stuff in my
house

30mA overall is too sensitive.

100mA seems just right.

Anything more is a separate RCBO on such circuits where the leakage
under non fault conditions is less, and the danger overall of being
electrocuted is higher.

John Rumm wrote:

What did you plug the socket tester into though? A socket on the cooker
point? If so then it would be correct for the oven to be on a RCD
protected supply anyway, simply because of this socket being there.

Except in the case where there's little or no chance of the socket being
used to supply portable equipment outdoors. In that situation no RCD is
required - unless of course it's a TT-earthed installation (in which
case RCD protection is required, but need not necessarily be 30 mA).

Take your example of the oven: the RCD would reduce the electrocution
risk in the circumstance you described, however how probable is that
particular combination of faults?

The probability of an open circuit CPC should be very low indeed, if the
installation has been done in accordance with BS 7671 [see all of
543-03-xx]. With T&E cable the chance of the CPC being severed in some
way, but not the phase & neutral conductors seems extremely unlikely -
so where was the open circuit? A bad (loose) connection, or one never
made, perhaps. Why was this not picked up in the initial testing of the
installation? Continuity of protective conductors is the very first test...

The downside of placing almost everything on a 30mA RCD when the
installation and equipment are in good condition is absolutely
minimal,

Much depends on what "everything" is, and how much of it there is. One
could argue that the best possible protection would be offered by having
a dedicated RCBO on every circuit. However this is not something you see
often, because cost must also come into the equation.

It's quite true that if everything is (and remains) in good condition
you can go for years and years with a single whole-house 30 mA RCD and
have no trips. The arrangement still doesn't comply with 314-01-01
though because "inconvenience in the event of a fault" hasn't been
minimised. Nobody would put all their circuits on to a single fuse or
MCB (using suitably rated cable throughout) and then use the argument
that it's OK because the risk of such a fuse blowing "when the
installation and equipment are in good condition is absolutely
minimal!"

[Lampholders]
Placing "out of reach" is acceptable in many circumstances.

That isn't the measure (as defined in BS 7671) being used here though,
to avoid shock by direct contact. The measure is still "protection by
barriers or enclosures" [412-03-xx] with an assumption that the
barrier/enclosure is completed by the lamp itself. Reg. 412-03-04 in
general requires that enclosures can't be opened without the use of a
tool, or some interlocking mechanism to disconnect the power first, but
this reg. goes on to make specific (historical) exemptions for ceiling
roses, pull cord switches and lampholders.

As well as the points you made, another is that the area of skin contact
with the live terminal in a lampholder is too small to give rise to a
shock current likely to be high enough to prove fatal.

The idea promulgated that 100mA RCD's are suitable as backup
protection for inadvertent contact is laughable in the extreme. Yes
you might prevent someone else getting zapped but one body on the
kitchen floor is one too many.

Not sure I follow this.

Again it seems that he is not understanding the difference between the
use of an RCD to provide supplementary protection against shock by
direct contact (30 mA RCD, as required for outdoor equipment sockets by
471-16-01, 412-06-02(ii) and in certain other situations, e.g.
bathrooms, in Part 6) and the use of an RCD to provide protection
against shock by indirect contact. This is a very fundamental
misunderstanding, despite the denial.

The only time a 100mA trip RCD would be mandated for protection from
direct or indirect contact is when EEBADS alone cannot be relied on
to do this

Careful... This is still EEBADS, it's just that the RCD (of whatever
trip current) is providing the Automatic Disconnection of Supply, rather
than an OPD [see all of 413-02-xx]. Also it's not actually mandated in
BS 7671 that such an RCD be 100 mA, merely that one may be used. The
advice that an RCD provided /only/ for this purpose should be at least
100 mA appears in the OSG [sect. 3.6.2]

--
Andy

Andy Wade wrote:

[Lampholders]

Placing "out of reach" is acceptable in many circumstances.


That isn't the measure (as defined in BS 7671) being used here though,
to avoid shock by direct contact. The measure is still "protection by

Yup, appreciate that, but was just highlighting that "out of reach" is
still an effective policy for many circumstances.

As well as the points you made, another is that the area of skin contact
with the live terminal in a lampholder is too small to give rise to a
shock current likely to be high enough to prove fatal.

Yes, that is a good point I had not considered. Is there actually any
published details on the relationship of contact area with shock current
(or lethality)?

The only time a 100mA trip RCD would be mandated for protection from
direct or indirect contact is when EEBADS alone cannot be relied on
to do this


Careful... This is still EEBADS, it's just that the RCD (of whatever

Yup, sorry my bad - being sloppy with terminology.

trip current) is providing the Automatic Disconnection of Supply, rather
than an OPD [see all of 413-02-xx]. Also it's not actually mandated in
BS 7671 that such an RCD be 100 mA, merely that one may be used. The
advice that an RCD provided /only/ for this purpose should be at least
100 mA appears in the OSG [sect. 3.6.2]



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In article ,
Andy Wade writes:
John Rumm wrote:

What did you plug the socket tester into though? A socket on the cooker
point? If so then it would be correct for the oven to be on a RCD
protected supply anyway, simply because of this socket being there.

Except in the case where there's little or no chance of the socket being
used to supply portable equipment outdoors. In that situation no RCD is
required - unless of course it's a TT-earthed installation (in which
case RCD protection is required, but need not necessarily be 30 mA).

I know that's what the regs say, but I always make sure the
socket where the kettle is going to be plugged in is RCD
protected. With kettles being very frequently moved, containing
water, handled with wet hands, never checked/tested in domestic
premises, used until they actually die, and nowadays built as
consumables with limited lives, they seem to me to merit an RCD.
I would never fit a cooker switch with combined socket*, but if
there was one, then I would put that circuit on an RCD (or
better, an RCBO), or swap the cooker switch for one without a
socket.

*At least, I would never fit a cooker switch with combined
socket for that purpose. When I was putting in circuit for the
outdoor sockets on a 10mA RCBO, switched from inside the garage,
one of these combined switch/socket units was ideal. It gives me
a single switch to switch off all the outdoor sockets, combined
with a separately switched socket in the garage where outdoor
appliances might also be plugged in, with the same 10mA protection.

--
Andrew Gabriel

In article ,
Andy Wade writes:
John Rumm wrote:
[Lampholders]
Placing "out of reach" is acceptable in many circumstances.

That isn't the measure (as defined in BS 7671) being used here though,
to avoid shock by direct contact. The measure is still "protection by
barriers or enclosures" [412-03-xx] with an assumption that the
barrier/enclosure is completed by the lamp itself. Reg. 412-03-04 in
general requires that enclosures can't be opened without the use of a
tool, or some interlocking mechanism to disconnect the power first, but
this reg. goes on to make specific (historical) exemptions for ceiling
roses, pull cord switches and lampholders.

As well as the points you made, another is that the area of skin contact
with the live terminal in a lampholder is too small to give rise to a
shock current likely to be high enough to prove fatal.

I searched for any records of electrocutions from an open
lampholder, but could find none for this country. Edison
Screw holders do seem to have caused some in other countries,
almost certainly when the supply wasn't polarised and the
outer screw was the live, including the bulb cap. More
modern ES lampholders don't make contact with the outer
screw contact until the bulb is screwed right in, but in
countries where they've been in use for years, there are
older ones where the screw thread is also the contact, so
they're live all the time and the bulb cap becomes live
the moment it touches the lampholder.

--
Andrew Gabriel

On Sat, 07 Oct 2006 14:59:07 +0100, Andy Wade
wrote:

John Rumm wrote:

What did you plug the socket tester into though? A socket on the cooker
point? If so then it would be correct for the oven to be on a RCD
protected supply anyway, simply because of this socket being there.

Except in the case where there's little or no chance of the socket being
used to supply portable equipment outdoors. In that situation no RCD is
required - unless of course it's a TT-earthed installation (in which
case RCD protection is required, but need not necessarily be 30 mA).

So you, along with the numbnuts at the IET view the likelihood of a
flexible mains lead being damaged by a hot hob as being lower or non
existent compared to severing it with a lawnmower in the garden?

Take your example of the oven: the RCD would reduce the electrocution
risk in the circumstance you described, however how probable is that
particular combination of faults?

The probability of an open circuit CPC should be very low indeed, if the
installation has been done in accordance with BS 7671 [see all of
543-03-xx]. With T&E cable the chance of the CPC being severed in some
way, but not the phase & neutral conductors seems extremely unlikely -
so where was the open circuit? A bad (loose) connection, or one never
made, perhaps. Why was this not picked up in the initial testing of the
installation? Continuity of protective conductors is the very first test...

When the lack of earth was noticed I expected:

a) a nicked conductor where the outer sheath started, either in the
cooker switch or in the consumer unit
b) a loose earth in the consumer unit
c) a loose earth in the cooker switch
d) rodent/mechanical damage on the cable run

The failure was within the consumer unit, the outer sheath had been
removed correctly without nicking the insulation on the live and
neutral conductors, the sleeving was correctly fitted but around
halfway down the sleeved length of around 6 inches the earth conductor
had completely failed. In the vicinity of the failure I couldn't
see any external factors such as the routing of other cables or the
positioning of the front cover etc could contribute to the failure.
The earth sleeving was totally intact, only by probing into the end of
the cable sheath and making direct contact with the "good" side cable
was the fault isolated to the incredibly short section in the consumer
unit.

The conductor didn't immediately appear notched, nor fatigued and no
necking had occurred. If anything I would have said it was a cut from
a pair of very sharp side cutters. I wish now I'd kept the piece for
further examination.

I've no idea why this wasn't picked up on initial installation, the
place was probably rewired in the late 70's/early 80's. Further work
to add wiring for a garage was added in about the last 10 years. There
are no test certificates or other evidence of who did the work
although the way some "tradesmen" work nowadays it's difficult to
distinguish their work from that of outright bodgers. Cable runs at
the back of the consumer unit (hidden in a cupboard) weren't clipped
as they disappeared into a riser duct but other than that it looked a
neat and tidy installation.

The downside of placing almost everything on a 30mA RCD when the
installation and equipment are in good condition is absolutely
minimal,

Much depends on what "everything" is, and how much of it there is. One
could argue that the best possible protection would be offered by having
a dedicated RCBO on every circuit. However this is not something you see
often, because cost must also come into the equation.

It's quite true that if everything is (and remains) in good condition
you can go for years and years with a single whole-house 30 mA RCD and
have no trips. The arrangement still doesn't comply with 314-01-01
though because "inconvenience in the event of a fault" hasn't been
minimised.

Inconvenience on the death of the occupant really ought to be a
greater consideration If the installation is up to scratch then
even a 10mA RCD shouldn't cause any problems due to "nuisance"
tripping.

Nobody would put all their circuits on to a single fuse or
MCB (using suitably rated cable throughout) and then use the argument
that it's OK because the risk of such a fuse blowing "when the
installation and equipment are in good condition is absolutely
minimal!"

No one ever suggested that.

[Lampholders]
Placing "out of reach" is acceptable in many circumstances.

That isn't the measure (as defined in BS 7671) being used here though,
to avoid shock by direct contact. The measure is still "protection by
barriers or enclosures" [412-03-xx] with an assumption that the
barrier/enclosure is completed by the lamp itself. Reg. 412-03-04 in
general requires that enclosures can't be opened without the use of a
tool, or some interlocking mechanism to disconnect the power first, but
this reg. goes on to make specific (historical) exemptions for ceiling
roses, pull cord switches and lampholders.

As well as the points you made, another is that the area of skin contact
with the live terminal in a lampholder is too small to give rise to a
shock current likely to be high enough to prove fatal.

That I find an argument without any basis in fact. The pins on a lamp
holder are around 4mm diameter, giving a csa of around 12.5mm^2. The
normal extension cable/lawnmover cable is around 0.5 or 1mm^2.

Cut the cable in the garden and it is deemed sufficiently "bad" that
RCD protection is mandatory. Burn through a blender cable in the
kitchen and as long as the socket it's connected to is nowhere near
the garden then it is presumably "OK" to handle the exposed
conductors. Change a lamp, while perched up a stepladder and fail to
isolate the circuit and you may contact terminals 25 times the area
that you would in the garden, possibly simultaneously with your other
hand contacting the body of the lampholder with a low impedance earth.
But having no RCD protection and being electrocuted/falling and
breaking your neck in that set of circumstances is deemed "OK"

You can clearly see why some hold the view that BS7671 is seriously
flawed.

The idea promulgated that 100mA RCD's are suitable as backup
protection for inadvertent contact is laughable in the extreme. Yes
you might prevent someone else getting zapped but one body on the
kitchen floor is one too many.

Not sure I follow this.

Again it seems that he is not understanding the difference between the
use of an RCD to provide supplementary protection against shock by
direct contact (30 mA RCD, as required for outdoor equipment sockets by
471-16-01, 412-06-02(ii) and in certain other situations, e.g.
bathrooms, in Part 6) and the use of an RCD to provide protection
against shock by indirect contact. This is a very fundamental
misunderstanding, despite the denial.

No, all I'm saying is that in the circumstances where, despite
testing, either at the time of installation or at a future date, an
earth conductor has failed for whatever reason , AND sometime later a
live to earth fault occurs on equipment that would normally be
earthed, then far better the RCD operates on human contact than the
alternative of leaving an appliance with live metalwork immediately
adjacent to other sound and properly earthed metalwork. A tripped RCD
and having to reset the timer on the video is I would expect
preferable in most peoples eyes (with the obvious exception of the
IET) to an identification visit to the mortuary.

The only time a 100mA trip RCD would be mandated for protection from
direct or indirect contact is when EEBADS alone cannot be relied on
to do this

Careful... This is still EEBADS, it's just that the RCD (of whatever
trip current) is providing the Automatic Disconnection of Supply, rather
than an OPD [see all of 413-02-xx]. Also it's not actually mandated in
BS 7671 that such an RCD be 100 mA, merely that one may be used. The
advice that an RCD provided /only/ for this purpose should be at least
100 mA appears in the OSG [sect. 3.6.2]

The only drawback of the RCD tripping rather than the MCB is that
discrimination by the way of direct indication of the circuit
concerned is not provided. The RCD is perfectly capable of handling
this type of duty. That the occupant of the premises noticed the radio
in the kitchen go off when they accidentally made contact between the
hob and the oven ought to be a reasonable indication to anyone
investigating further.

I still maintain, as do many others that on domestic premises with
PME, RCD protection at around 30mA for the whole installation,
together with maintained lighting is the safest for the user. If
maintained lighting is not/cannot be provided then lighting only on
the non RCD protected circuits is marginally acceptable. Everything
else, especially internal sockets located nowhere near the garden
(such as in the kitchen, children's bedrooms) AND supplies to
appliances with exposed metalwork should go on the RCD protected side
of a split load consumer unit.


--

In article ,
Matt writes:
So you, along with the numbnuts at the IET view the likelihood of a
flexible mains lead being damaged by a hot hob as being lower or non
existent compared to severing it with a lawnmower in the garden?

Over the last 25 years, I've cut part way though a lawn mower cable
and through two hedge trimmer cables, and I've probably repaired 10
or more such incidents done by neighbours/colleagues, but I've never
damaged a flexible mains lead on a hot hob, and I can't actually
recall any incidents where I've come across anyone else doing so.
So I would say that assertion is completely justified IME.

Inconvenience on the death of the occupant really ought to be a
greater consideration If the installation is up to scratch then
even a 10mA RCD shouldn't cause any problems due to "nuisance"
tripping.

A 10mA RCD would be liable to trip with 4 computer systems
connected to it. Each Class I IT appliance is permitted to
leak 0.75mA through its RFI filters, and a PC normally has
two Class I IT components, the base unit and the monitor.
Four of these add up to 6mA, which is within the permitted
tripping range for a 10mA RCD ( 5mA, = 10mA).

An appliance with heating elements is permitted an unspecified
earth leakage until the elements have had a chance to warm up,
because many of the high temperature electrical insulating
materials used in such appliances are hygroscopic. It's not
unknown for a cooker which hasn't been used for a long time
and has no fault to trip a 30mA RCD until it's been on for a
few seconds, which may be impossible if it's on an RCD.
(PAT testing guidelines explicitly state such appliances should
be allowed to heat up if their earth leakage is too high when
cold.)

[Lampholders]
As well as the points you made, another is that the area of skin contact
with the live terminal in a lampholder is too small to give rise to a
shock current likely to be high enough to prove fatal.

That I find an argument without any basis in fact. The pins on a lamp
holder are around 4mm diameter, giving a csa of around 12.5mm^2. The
normal extension cable/lawnmover cable is around 0.5 or 1mm^2.

What's the relevance?

Cut the cable in the garden and it is deemed sufficiently "bad" that
RCD protection is mandatory.

Ah, you misunderstand the potential fatal fault scenario.

Cutting right through the cable is most unlikely to be a problem.
You may blow the fuse anyway, but even if the blade contact time
is too short and it doesn't, touching the live end would be unlikely
to pass enough current to you to do harm, and unlikely to invoke a
grasp reaction keeping you in contact with it.

The much worse scenario is that you don't cut right through the
cable, but end up with the cable conductors caught in the blade.
Now you have, say, a lawn mower, on which you have a good grip,
probably with damp/sweaty hands, which has become live, and you
can't let go. Even ignoring the possibility of catching the cable
with the cutting blade, you still have an appliance which gets
much rougher treatment than a kitchen blender, and may get its
cable snagged on some other part of it or something else sharp
in the garden, exposing live conductors which are in danger of
coming in contact with the mower's metal bodywork, with the same
result.

Burn through a blender cable in the
kitchen and as long as the socket it's connected to is nowhere near
the garden then it is presumably "OK" to handle the exposed
conductors. Change a lamp, while perched up a stepladder and fail to
isolate the circuit and you may contact terminals 25 times the area
that you would in the garden, possibly simultaneously with your other
hand contacting the body of the lampholder with a low impedance earth.

As I said, you are worrying about the wrong fault scenario.
In any case, the grasp reaction is not going to keep you in
contact with the live pin, actually quite the opposite.

But having no RCD protection and being electrocuted/falling and
breaking your neck in that set of circumstances is deemed "OK"

You can clearly see why some hold the view that BS7671 is seriously
flawed.

It's not uncommon when people don't understand it.
Largely, it's been driven by actual events, rather than imagined
ones, so on the whole it does represent protection from real risks.
Please find a single reported case, ever, of someone being
electrocuted by the pins in a BC lampholder, as I couldn't.
(Yes, I know there are cases outside the UK for Edison Screw
lampholders, where the outer screw contact is the live, but that
doesn't apply in the UK.)

However, I am less hopeful about the Regs future. IEE's involvement in
the Part P fiasco was a very marked departure from their previously
excellant grasp of risk assessments with appropriate solutions, and
I fear the Wiring Regs might go off down the same non-scientific
political route, which will discredit it.

--
Andrew Gabriel

Matt wrote:

It's quite true that if everything is (and remains) in good condition
you can go for years and years with a single whole-house 30 mA RCD and
have no trips. The arrangement still doesn't comply with 314-01-01
though because "inconvenience in the event of a fault" hasn't been
minimised.


Inconvenience on the death of the occupant really ought to be a
greater consideration

It is. Falling down stairs and breaking your neck is considered to be a
far more likely inconvenience than melting your kettle cable.

If the installation is up to scratch then
even a 10mA RCD shouldn't cause any problems due to "nuisance"
tripping.

My computer setup on its own would probably trip that...

adjacent to other sound and properly earthed metalwork. A tripped RCD
and having to reset the timer on the video is I would expect
preferable in most peoples eyes (with the obvious exception of the
IET) to an identification visit to the mortuary.

To an extent this is a moot point. In most installations done today, it
is a fairly safe bet that most if not all the socket outlets will be RCD
protected. Rumour has it that the 17th edition of the regs will make
this standard practice. Remember also that the original question was not
about a socket outlet, but a fixed and hard wired appliance.

The only drawback of the RCD tripping rather than the MCB is that
discrimination by the way of direct indication of the circuit
concerned is not provided.

and/or you fall and break your neck...

The RCD is perfectly capable of handling
this type of duty. That the occupant of the premises noticed the radio
in the kitchen go off when they accidentally made contact between the
hob and the oven ought to be a reasonable indication to anyone
investigating further.

I still maintain, as do many others that on domestic premises with
PME, RCD protection at around 30mA for the whole installation,

If you mean the *same* 30mA (i.e. one RCD shared between all circuits),
then that is neither practical in most cases, or desirable in any. If
instead you mean that each circuit should have RCD protection at a trip
threshold no higher than 30mA (e.g. RCBOs or multiple CUs), then it is a
more plausible suggestion. (although not warranted by the accident
statistics it would seem)

together with maintained lighting is the safest for the user. If
maintained lighting is not/cannot be provided then lighting only on
the non RCD protected circuits is marginally acceptable. Everything
else,

Including fire alarms?

especially internal sockets located nowhere near the garden
(such as in the kitchen, children's bedrooms) AND supplies to
appliances with exposed metalwork should go on the RCD protected side
of a split load consumer unit.



--
Cheers,

John.

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