On Tue, 4 Oct 2005 19:14:39 +0100 someone who may be "Jim Ingram"
wrote this:-

Could someone please expain to me what the requirements are for
ensuring a balanced road on a ring main.

In most cases, test that the ring actually is a ring.

As is often the case dishwasher and
washing machine are next to each other and would be relatively close
to the start of the ring. I would estimate about 4 m in from the cu
with a total length of circuit of around 17m. If the two appliances
were on together this might be around 6.5 kW giving a current of 27A
of which I estimate 76% would travel down the shorter leg.

Not a problem. These things were thought through some time ago.

I have a vague recollection of a figure of 20% going the "long way"
being the worst case. That is for a standard circuit, which has a
maximum floor area for a number of reasons one of which is to do
with loads.

This would
be getting fairly close to the cables capacity. An additional load
near this point could take the cable to above its rating (~24A for
2.5mm2) and not trip the 32A mcb. I would have thought this situation
to be just about acceptable, as at least some of the loads will be
fairly short lived (minutes rather than hours).

Cable ratings depend on the installation. They are also time
dependent. 2.5mm2 cable will safely carry many hundreds of amps for
a short period, which is what happens in a short circuit. In an
earth fault, in flat twin and earth wiring, the smaller protective
conductor carries the same current as the live. A 32A MCB will take
around 30s to operate at 100A, which the circuit wiring will take if
it is designed, installed and tested correctly. A lot of thought has
gone into the Wiring Regulations over a long time and co-ordination
of protective devices with cable capacity. 100% safety is not
possible, don't use electricity if you want that, but it is a good
compromise with the competing factors.



--
David Hansen, Edinburgh | PGP email preferred-key number F566DA0E
I will always explain revoked keys, unless the UK government
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