In message ,
"dennis@home" wrote:


"Martin Angove" wrote in message
...
In message ,
"dennis@home" wrote:

[diversity/ring circuits]
I understand them..
I just don't agree with them.



Without diversity modern living would be impossible. In the
one-radial-per-room scenario we would have at least eight radials in
this small house. Even if they were all 16A that's a non-diversified
8*16A you have to cater for which is 128A which is well over the
capacity of even the largest normally available service fuse (100A).
Without diversity you'd have to assess a cooker at its maximum rating in
the same way as a shower. One 8kW cooker and one 8kW shower makes 66A
while many houses are still on 60A service fuses.

Modern radials are multiple connection affairs not the old ones with round
fuses in the plugs. 8-)

But for the purposes of (non) diversity you have to calculate maximum
demand at the rated capacity of the circuit. If you allow diversity your
maximum demand (sockets circuits) is largest+40% of the rest (IEE
guidelines in the OSG). There can be a big difference between (say) 8
radial socket circuits serving the house and two or three rings.

[...]
Where can you buy 20A fuse wire these days? Actually, on that subject,
that's an area where a 3036-protected ring may actually be safer than a
radial: fuse wire of 20A is not commonly available any more, so if a 20A
fuse blows it may be replaced with 30A wire. Wire greater than 30A is
likewise not commonly available so a blown 30A ring fuse is only likely
to be replaced with 30A wire. (Let's ignore the cases where fusewire
isn't used at all and blown fuses are replaced with bits of copper
stripped from T&E)

I think you will find you have to use a MCB in a modern radial circuit
(probably for the reason you state).


BS3036 rewireable fuses are still allowed in domestic situations for all
standard final circuits. You have to derate the cable to account for the
greater let-through of energy, but MCBs are by no means mandated.

To be at serious risk of damaging a correctly functioning ring you have
to have a series of improbable coincidences. Either that or the central
heating has broken down, it's the middle of the winter, and you've
managed to find the one homeowner in the town who has more than three or
four portable electric heaters.

Why is it so difficult to explain that designing a system which can
(apparantly) function normally with a hidden fault present is bad
engineering.
It is worse if that fault drops the margins of that circuit below what is
safe.
This is the case for a ring main.
It cannot be considered to be good practice these days when there are
alternatives (like fitting a 20A MCB to the ring).


I don't think anyone is disputing the fact that it is possible for a
ring to "break" without any noticeable effects until such time as a
sustained overload occurs. What we are arguing is that the circumstances
you describe are rare; other faults happen first and are coped with
better by a ring. Likewise, even in the case of a break, the chances of
the actual load (rather than the potential load) on any one isolated
part of the ring being sufficient to cause a problem are low: you'd have
to have the break happen near the CU with most appliances connected on
the "long" side of the ex-ring.

And it is only small overloads we are really considering. The cable is
perfectly safe protected by a 30/32A device for short circuits where
hundreds of Amps flow. If it weren't, it wouldn't be allowed for spurs.

You are quibbling the difference between a 27A cable protected at 20A
and a 27A cable protected at 30/32A where the fault causes a sustained
current of up to 45A to be drawn in one leg of the ring. Above that and
the MCB will eventually trip (50A will trip a 32A MCB in around 1,000
seconds). The debate then becomes whether 45A continuously will damage
2.5mm2 cable in such a way as to be the cause of (say) a fire, or
whether it is more likely that this low-level overheating will cause
insulation to flow, eventually causing a live conductor to short on the
CPC causing a dead short, blowing the MCB. I suspect the latter.

It isn't perfect, but it isn't (necessarily) unsafe.

If you're worried, why not wire your rings in 4mm2 cable? At least this
is a more practical solution than downgrading the MCB to 20A. Spurring
is a bit more difficult as getting 3x4mm2 cables into a terminal is
nigh-on impossible...

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
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