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.

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