Date : Thu, 03 Sep 2009 23:02:16 +0100
From : jgh@... (Jonathan Graham Harston)
Subject: Leccy @ Acorn World '09
Mick Champion wrote:
> > and I have to go round the house turning stuff off, to get it back on
again,
> > I suspect it's inrush current exceeding 32A!
If it's 32A, it's probably a MCB or RCD. If it's a RCD, then that
operates by monitoring the current on the phase, neutral and earth
conductors. If phase<>neutral or earth>0, then it trips. You can
get erroneous trips because of the power-on or power-off
characteristics or certain appliances. The earth/neutral monitoring
can be a right bugger if you're working on a final circuit with the
circuit break switched off, if you happen to touch the earth to the
neutral, an RCD earlier on in the circuit will trip - even though
the final circuit is isolated.
> If I remember correctly (for I can't find a definitive answer on t'web),
http://mdfs.net/Docs/Electrical
> 2.5 mm twin and earth cable (commonly used in ring main circuits) is
> rated at between 19 and 23 amps. A 32 amp breaker is only permissible
It depends on what you're using it for and how it's installed. OSG
Table 6D1 gives you all the information you need.
2.5mm copper T+E with PVC insulation clipped direct is rated at
27A. The rating drops progressively as you group it with other
cables, increase the length and enclose it within, eg, walls and
floors. Within thermally insulated material the rating drops to
19.5A, and if supplying cooking appliances drops by another 30%
regardless of the actual appliances.
You have to remember that most electricians are electrical fitters,
not electrical engineers, and are only competant within a
standardised installation with standardised and unexpected final
circuits.
> because each socket has two cables feeding it from either side of the
> ring effectively doubling up the rating. (Say 38 amps to be on the safe
Nooooo. times SQR(2), not times 2. Check your parallel resistance
calculations.
--
J.G.Harston - jgh@... - mdfs.net/User/JGH
BBC BASIC for Windows and Internationalisation
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