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.


--