I've just discovered that the kitchen sink in the house I've just moved into
has no earth bonding (plastic hot & cold supplies).

I'm going to fit a waste-disposal unit, so that would provide an earth
through it's metalwork to the sink.

Is this enough, or should I run earth cable from the sink back to the
consumer unit (& if so, how thick would it need to be - about 25' from the
sink to the CU)

thanks,

Jim

JJJ wrote:

I've just discovered that the kitchen sink in the house I've just moved into
has no earth bonding (plastic hot & cold supplies).

Kitchen sinks do not require 'earth bonding'. Supplementary bonding of
such extraneous-conductive-parts is only necessary in bath and shower rooms.

I'm going to fit a waste-disposal unit, so that would provide an earth
through it's metalwork to the sink.

Is this enough,

It's OK.

or should I run earth cable from the sink back to the consumer unit
(& if so, how thick would it need to be - about 25' from the sink to
the CU)

That's not necessary, but make sure that the main bonding from the main
earth terminal to the incoming gas and water service pipes (& oil, if
applicable) is in place, close to the point of entry of each service.
The minimum size for this is 10 mm^2.

--
Andy

Tim (Remove NOSPAM. wrote:

What if incoming water pipe is plastic? (no oil or gas) The Earth rod is
then sufficient??

No - if all or most of the house pipework is in copper (or other metal,
naturally), then that should be bonded to your main earth terminal close
to the transition from plastic-incomer to copper-internal - usually
this'll be at your internal stopcock. The idea is that the bulk of the
metal pipework in your house should provide a low-resistance path for
any fault current: that way, if some fault (e.g. in a hot water boiler,
immersion heater, or less likely misplaced nail
touching-but-not-severing L-in-cable and going on to a water pipe - I
said less likely, OK!?) brings your supply Live into contact with that
pipework, (a) a big enough current will flow to make any fuse/MCB pop
nice and quickly, (b) even while the fuse/MCB is deciding how quickly to
pop, the pipework doesn't rise to a voltage far above earth (cos the low
resistance of its connection to earth means most of the 240V is dropped
on the L side of the fault).

But if your pipework's all or nearly-all plastic (e.g. just visible
copper tails going to radiators), there's no need to bond the short
segments of copper pipework.

HTH - Stefek

Tim (Remove NOSPAM. wrote:

What if incoming water pipe is plastic? (no oil or gas)

If the house is plumbed in copper the need for main bonding is
unaffected. If the house is plumbed mostly in plastic then no main
bonding to the water service is required.

The Earth rod is then sufficient??

Err, what earth rod? Are you talking about a TT installation?

--
Andy

Stefek Zaba wrote in message ...
Tim (Remove NOSPAM. wrote:

What if incoming water pipe is plastic? (no oil or gas) The Earth rod is
then sufficient??

No - if all or most of the house pipework is in copper (or other metal,
naturally),

snip

I have difficulty with the standard answers to the question of bonding
in a kitchen

(a) a big enough current will flow to make any fuse/MCB pop
nice and quickly,

hopefully yes, but a fuse/mcb won't trip on the current/voltage that
gives a fatal shock - you need an RCD - even then that takes 0.4s - or
40 half cycles each reaching a peak 340volts.

(b) even while the fuse/MCB is deciding how quickly to
pop, the pipework doesn't rise to a voltage far above earth (cos the low
resistance of its connection to earth means most of the 240V is dropped
on the L side of the fault).

Something like that was the subject of the recent inquest. The metal
shelf went to 240v & the earthed casing of the (?)dishwasher stayed
resolutely at zero volts.


I wonder whether over-earthing/bonding in a kitchen can compromise
safety: perhaps more regard should be paid to the merits of relying on
natural protection from clothes, shoes etc & minimising exposed metal
surfaces in a kitchen.

jim_in_sussex wrote:

I have difficulty with the standard answers to the question of bonding
in a kitchen

(a) a big enough current will flow to make any fuse/MCB pop

nice and quickly,


hopefully yes, but a fuse/mcb won't trip on the current/voltage that
gives a fatal shock - you need an RCD - even then that takes 0.4s - or
40 half cycles each reaching a peak 340volts.

One might argue that the MCBs purpose is to protect the installation
from overload, not to prevent electrocution. That is what the RCD (if
fitted) is for.

Something like that was the subject of the recent inquest. The metal
shelf went to 240v & the earthed casing of the (?)dishwasher stayed
resolutely at zero volts.

Had adequate RCD protection been in place then this would probably not
have not ended in a fatality though.

I wonder whether over-earthing/bonding in a kitchen can compromise
safety: perhaps more regard should be paid to the merits of relying on
natural protection from clothes, shoes etc & minimising exposed metal
surfaces in a kitchen.

Can work both ways I guess. Typically there is no requirement for
supplementary bonding on a kitchen anyway, and there are pretty strong
arguments for earthing appliances in these circumstances. A faulty earth
on an appliance could also result in a severe shock hazard when combined
with other faults.

--
Cheers,

John.

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

John Rumm wrote:


One might argue that the MCBs purpose is to protect the installation
from overload, not to prevent electrocution. That is what the RCD (if
fitted) is for.

One might, but one would be in error. The little slogan is "protection
by EEBADS" - that's protection of people, against electric shock, by
Earthed Equipotential Bonding and Automatic Disconnection of Supply. The
*point* of the low-resistance earthing we commonly practice (not just
the supplementary bonding in bathrooms, but earthed cases on metal
appliances) is to produce a big enough flow so that the MCB or fuse will
operate fast enough. RCD's are a useful top-up, esp. where good earthing
can't be assured (portable appliances outdoors being the clearest case),
but good old EEBADS is the design and implementation which has kept
death-by-electrocution down to very low levels in the UK for, yay, 40
yearses or more.

Stefek

hopefully yes, but a fuse/mcb won't trip on the current/voltage that
gives a fatal shock - you need an RCD - even then that takes 0.4s - or
40 half cycles each reaching a peak 340volts.

No it doesn't. That may be the spec, but the RCD will probably trip at the
next zero crossing point (or the next one in a particular direction,
anyway).

The spec for a 30mA RCD was designed to ensure adequete shock protection
without leading to unnecessarily complex design or excessive nuisance
tripping.

& minimising exposed metal surfaces in a kitchen.

Indeed. The IEE recommend even in bathrooms that any metal surfaces should
be isolated if possible, rather than earthed. It is only if they will be
earthed anyway (perhaps at a different potential to other parts) that
supplementary bonding is required.

Christian.

Stefek Zaba wrote:

John Rumm wrote:


One might argue that the MCBs purpose is to protect the installation
from overload, not to prevent electrocution. That is what the RCD (if
fitted) is for.

One might, but one would be in error. The little slogan is "protection
by EEBADS" - that's protection of people, against electric shock, by
Earthed Equipotential Bonding and Automatic Disconnection of Supply. The
*point* of the low-resistance earthing we commonly practice (not just
the supplementary bonding in bathrooms, but earthed cases on metal
appliances) is to produce a big enough flow so that the MCB or fuse will
operate fast enough. RCD's are a useful top-up, esp. where good earthing
can't be assured (portable appliances outdoors being the clearest case),
but good old EEBADS is the design and implementation which has kept
death-by-electrocution down to very low levels in the UK for, yay, 40
yearses or more.

Yup agreed - perhaps my statement was too hard on EEBADS! it is indeed a
major life saver. However there are still catagories of faults for where
no amount of earthing is going to give sufficent protection though, like
the incident refered to above, or the classic mower/hedge trimmer
through the cable scenario.


--
Cheers,

John.

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

In message ,
(jim_in_sussex) wrote:


hopefully yes, but a fuse/mcb won't trip on the current/voltage that
gives a fatal shock - you need an RCD - even then that takes 0.4s - or
40 half cycles each reaching a peak 340volts.


Leaving aside the merits or otherwise of RCD protecting and/or bonding
everything in sight, I thought the specification for the "life saving"
30mA trip RCD was 40ms, not 400ms (i.e. 0.04s, not 0.4s) though
admittedly this 40ms is at 5x30mA. Having said that, even 150mA isn't a
lot of current - even a really manky earth should allow that amount
through (V/I=1600R).

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... If you sold hats, babies would be born without heads.

"Martin Angove" wrote
| (jim_in_sussex) wrote:
| you need an RCD - even then that takes 0.4s -
| Leaving aside the merits or otherwise of RCD protecting
| and/or bonding everything in sight, I thought the specification
| for the "life saving" 30mA trip RCD was 40ms, not 400ms

The requirement is for a 0.4s disconnection time (through EEBADS) on socket
circuits, whether that is achieved by fuse, MCB or RCD, and is in the wiring
regs. The BS for the RCD is a different matter.

Owain

In message ,
"Owain" wrote:

"Martin Angove" wrote
| (jim_in_sussex) wrote:
| you need an RCD - even then that takes 0.4s -
| Leaving aside the merits or otherwise of RCD protecting
| and/or bonding everything in sight, I thought the specification
| for the "life saving" 30mA trip RCD was 40ms, not 400ms

The requirement is for a 0.4s disconnection time (through EEBADS) on socket
circuits, whether that is achieved by fuse, MCB or RCD, and is in the wiring
regs. The BS for the RCD is a different matter.


Yes, I realise that the requirement for the circuit is 0.4s (or 5s
depending on the circuit), but Jim wrote something which implied to me
that he thought that an RCD took 0.4s too; the bit which went:

a fuse/mcb won't trip on the current/voltage that gives a fatal shock -
you need an RCD - even then that takes 0.4s

In testing RCDs I have to admit that even at 1x current I've only once
seen a (30mA, non-delay) RCD take longer than 40ms to trip, and that was
because it was mechanically knackered. At 5x current most seem to manage
around 10ms. Again though, I realise that this is a feature of the RCD,
not a specific requirement of the BS disconnection times.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Exam is a four-letter word for torture...

At 5x current most seem to manage around 10ms. Again though, I realise
that this is a feature of the RCD, not a specific requirement of the BS
disconnection times.

It's probably quite difficult to design an RCD that doesn't instantaneously
trip at the next crossing point (or the next crossing point of a particular
polarity). It would need some sort of additional time delay mechanism, like
those intended for TT system incomers.

Christian.

In message ,
"Christian McArdle" wrote:

At 5x current most seem to manage around 10ms. Again though, I realise
that this is a feature of the RCD, not a specific requirement of the BS
disconnection times.

It's probably quite difficult to design an RCD that doesn't instantaneously
trip at the next crossing point (or the next crossing point of a particular
polarity). It would need some sort of additional time delay mechanism, like
those intended for TT system incomers.


I see what you mean - that if there is sufficient current flowing (about
1x the operating current) the thing should trip. Does it trip at the
crossing point (when the current is effectively zero), or at the peak
voltage, hence peak current? The tests I carry out (and which usually
involve a lot of running up and down ladders) are quite interesting:

1: at half rated current the device should not trip, tested at 0
degrees, 180 degrees and then 0 degrees again.

2: at rated current it should trip within a specified time, test and
discard reading at 0 degrees and then take reading at 180 degrees and 0
degrees again.

3: at 5x rated current it should trip within the specified time (e.g.
40mS), three tests as in 2:

That's usually nine tests in all, and the nearest convenient socket at
which to plug the tester in is rarely within arm's reach of the RCD :-)

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Unable to locate Coffee -- Operator Halted!

Does it trip at the crossing point (when the current is effectively zero)

It trips when the current is zero. Anything else would torch the contacts.
Even a manual switch will "let go" at a crossing point if there are any.
This is why a normal switch is rated higher for currents at AC than DC.

Note that the voltage being zero will only coincide with current being zero
if the circuit is power factor corrected. But switching voltage is easy.
It's current that hurts.

That's usually nine tests in all, and the nearest convenient socket at
which to plug the tester in is rarely within arm's reach of the RCD :-)


I fitted a double socket next to the consumer unit for one ring circuit. The
other ring circuit is for the adjacent room anyway...

Christian.

In message ,
"Christian McArdle" wrote:

Does it trip at the crossing point (when the current is effectively zero)

It trips when the current is zero. Anything else would torch the contacts.
Even a manual switch will "let go" at a crossing point if there are any.
This is why a normal switch is rated higher for currents at AC than DC.


There's a difference between "letting go" and "actuating". MCBs have arc
extinguishing chambers precisely because they are highly likely to trip
just about anywhere *except* at a zero point - they are largely
mechanical devices after all with absolutely no electronics. Are RCDs
any different? My gut feeling was that the coil would be actuated at a
certain current - which is likely to be other than at the zero voltage
point - and that at that point the thing is mechanically "tripped".
Quite how long it takes for the arc to extinguish is another matter. I
have never opened up an RCD but would be surprised if there is any
electronics in there to "hold off" until the zero point, merely in the
interests of extending contact life.

Note that the voltage being zero will only coincide with current being zero
if the circuit is power factor corrected.

Obviously, but for domestic use it's usually good enough to say that the
pf is reasonably good unless there's a lot of fluorescent lighting
about.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Since you're going to die anyway, can we use you as a shield?

Are RCDs any different?

Well, the mechanism notwithstanding, the current will flow until the next
zero crossing point. I didn't mean to imply that there was any magic or
electronics in this, just that it is what AC switches do.

However, RCDs are often much more complicated than MCBs and do often have
internal analogue electronics, rather than being simple mechanical/inductive
devices. This means that they could be designed to have the ability to sense
an impending zero crossing point before attempting to move the contacts. I
don't know if they actually do that, though, but I'd guess not.

The effect on the system as a "black box" though is that the current will
cease at a zero crossing point whether the RCD attempts to nurse its
contacts or not.

Christian.