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.


--