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