How are single socket spurs adequately protected on a 32A ring?

messagenews: snipped-for-privacy@t35g2000yqj.googlegroups.com...

Well that would be the problem.

Reply to
dennis
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Sounds like a load twaddle to me. Care to cite the edition of BS7671?

Reply to
John Rumm

But dennis you disagree with most things as a matter of principle - that alone add little weight to an argument.

That's not true, since there is no requirement for any fuse in the spur

- the spur could quite legitimately have an unfused flex outlet on the end of it, and everything would remain well protected. The overload protection of the spur being enforced by controlling the size of the load - not by fusing, and the fault protection being maintained at the origin of the circuit by the 32A MCB.

Is it equally easy to uprate your vacuum cleaner to 5kW for this non sequiter to have any relevance?

Yes your could probably bodge a 4 way trailing lead and stick two 3kW fan heaters on it, but as has been observed in testing the lead will fail long before the fixed wiring is at risk.

You seem to think that plug fuses are designed to protect against overload, this is not the case. They are there for the fault protection of the flex connected to them. The common exception to this is with multiway extension leads, but this is not of relevance to the fixed wiring.

If we extend your argument to its logical conclusion, every table lamp ought to have a 4mm^2 flex so that it matches the trip threshold of the circuit breaker.

Which is exactly what the current circuit designs that permit 2.5mm^2 cable on 32A breakers do. The breaker provides fault protection at all times. Overload protection some of the time.

It highlights why rings tend to perform better than radials with regard to earth faults. It also helps explain why when high integrity earthing is use on a radial circuit, it is done by turning the protective conductor into a *ring*

A 32A radial in 4mm^2 T&E is a standard circuit. Just as is a 20A one in

2.5mm^2 cable. The 32A version is however far more useful as a general purpose socket circuit.
Reply to
John Rumm

messagenews: snipped-for-privacy@t35g2000yqj.googlegroups.com...

The same way he tests a match to make sure its not a dud... ;-)

Reply to
John Rumm

messagenews: snipped-for-privacy@t35g2000yqj.googlegroups.com...

Its pretty straight forward if you know anything about manufacturing processes. You pull out a sample of production on a regular basis and subject that to destructive testing against an agreed BS test procedure. If they fail to meet spec, you bin or rework that batch of production output.

Reply to
John Rumm

vbg

To be fair you are pretty screwed if the fuses are illegal copies from China with all the correct BS numbers stamped on them. However the 2.5 T&E unfused spur is not the bit that will suffer if they turn out to be nails and not fuses, it will be the melting appliance or the appliance flex.

-- Adam

Reply to
Adam Wadsworth

messagenews: snipped-for-privacy@t35g2000yqj.googlegroups.com...

Hence the asta certificate, there was a time when batch testing wasn't mandatory. Of course there may even be cases where the fuses are faked, sorry make that we know there are cases where they have been faked.

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you really want to continue stating that making the plug fuse essential for safety is a good idea?

Reply to
dennis

messagenews: snipped-for-privacy@t35g2000yqj.googlegroups.com...

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But MCBs have been faked too

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Reply to
Andy Burns

So you think its permitted to wire in a four way without any plug or FCU?

You have no way to know this. It is not difficult to find 2.5 mm flex and then there is no reason why it should suffer any more than the fixed wiring. There are even heat resistant flexes about that will suffer no damage at all while the main cable melts. The idea that something else will fail first is not reasonable unless that something else is designed to fail first.

the whole argument is about the plug fuse protecting the spur from overload and you know that. If the 32A breaker provided protection to stop the spur being overloaded there wouldn't be any argument as it would be designed as I want it to be.

But you just said the plug fuse doesn't protect the spur from overload so now you think its OK to have part time protection. Don't design anything I have to use if that is OK to you.

That's fine I haven't said you can't have a ring, just that the cables shouldn't rely on it being a ring or to have spurs that need plug fuses to prevent overload.

It is if you don't use 2.5 mm spurs anywhere. If you do then you may as well use a ring as its no safer. The 20A circuit doesn't rely on the plug fuse to prevent overloading the cables like the others do.

Reply to
dennis

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True but there should be more chance of one being spotted by an electrician than there is of the average person spotting a fake fuse. and they are testable.

Reply to
dennis

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>>> True but there should be more chance of one being spotted by an

There is an article in switchedON (issue 18) that may well have a bearing on this debate.. "The national standard for extension leads, BS 1363-2: 1995, specifies a maximum 'continuous use' rating of 13 A for the fitted plug and socket-outlet. The test current is set marginally above this at 14 A for type-testing, to determine compliance with the product standard. Standard BS 1362: 1973, covering the fuse link, requires the fuse to operate at 1.9 times rated current (that is 24.7 A) within 30 minutes, and also that the fuse shall have a non-fusing current of 1.6 times rated current (that is 20.8 A). It is therefore possible for an extension lead to carry a total current of

20 A for an indefinite period. While this may not be a common situation, the product testing undertaken was to determine whether it is possible to overload extension leads such that they might pose a fire hazard."

Assuming they have the figures correct.. it is quite easy to get 40A out of a double spur for an indefinite period using two fully working extension sockets and some loads. This would probably trip an MCB before any serious heating of the cable. However it means that you could draw 32A indefinitely, this exceeds the rating if the 2.5 mm cable by a significant amount and it requires no circuit modification at all, not even a faulty fuse. The only question is which melts first, the flex or the cable and that depends on what they are made of and how they are cooled. So I guess I was wrong, you don't need a fault to overload the spur just an ignorant user (most of the population at a rough guess).

Reply to
dennis

No, an unfused spur can have a maximum of one single or one double socket. I assume however you are referring to a 4 way trailing lead, which is not designed to be "wired in" at all.

Actually I do.

Seen many extension leads with 4.0mm^2 flex?

Link?

2.5mm^2 flex has a lower current carrying capacity than the fixed wiring, and also is less likely to benefit from contact with masonry or other building materials that will aid dissipation.

Commercial 4 way extension leads are almost exclusively wired with

1.5mm^2 flex however.

Clutching at straws dennis. If you are clued up enough to assemble an extension lead with heavy gauge high temperature flex, you are also unlikely to go replacing a fuse with a nail and plugging multiple fan heaters into it. Assuming however for the sake of argument you do manage to stick a sustained 40A load into a single socket, then its a pretty safe bet the plug and or socket will give out before its supply cable.

2.5mm^2 T&E with maximum current carrying capacity of 27A Vs cheap thermoplastic 4 way socket (with dennis' patented nail in place of the fuse) and 1.5mm^2 flex with a 15A max rating...

your call.

That seems to be *your* argument, however its based on an incorrect assumption.

The plug fuse is not there to protect the spur from anything; overload, or fault current. Its there to provide fault protection to the appliance flex, that is all.

I expect that most people want is a system designed to work and perform safely in the real world. What we have evidently does so very well as evidenced by our extremely low rate of accident and injury result from fixed wiring.

If you want to do it differently, by all means do so, you don't need our permission. However don't seek to justify this by making false claims about proven engineered designs.

That's correct. It doesn't.

No, the design requires full time protection.

Well since I am only advocating the use of standard designs as implemented up and down the country, with a long proven track record of safety, its not going to be a choice you get to make other than perhaps in your own home should you choose to rewire it.

However look on the bright side, many of the things I have designed are intended to kill you, so you should be grateful for the remote possibility that one might fail.

Which misses the fundamental point of the design in the first place. i.e. that you can supply significant amount of power over a wide area, using a cable that is easy to work with.

Which would make no difference to performance since the load on each is limited by the specification of only one single or double socket per spur....

A 4mm^2 radial for general purpose sockets will in many circumstances perform less well than a ring since it tends to have a higher earth loop impedance. Its also more difficult to wire and in many cases saves no copper. Hence why its fairly rare to see in practice.

None of the standard circuits rely on plug fuses to prevent overloading.

Reply to
John Rumm

Earlier you stated "there is no requirement for any fuse in the spur - the spur could quite legitimately have an unfused flex outlet on the end of it" if so I can put a trailing socket on it.

Actually you don't as you can't define the flex type or how it is cooled.

I don't need 4 mm the spur is 2.5 mm so the flex would only need to operate better than that.

To what?

Clutching at straws John? I can easily put 40A into a double spur without needing a faulty fuse as ordinary 13A fuses will run at 20A continuous.

But according to you it fails to do that. A 32A breaker can't protect 2.5mm T&E from overloads so what do you think does?

Just remember that the regs are the bare minimum that the IEE thinks are safe enough at the cost they have decided upon. The OSG provides guidence for those that can't or won't do the maths to prove what they do is OK. You obviously don't understand the design compromises or what protects what in the circuits. I recommend you don't continue with this debate until you get a clue..

The fact that there is two sockets on a spur does not limit the current below the cable rating. It is quite obvious that it doesn't, you can even see it quoted in trade magazines like switchedON issue 18 if you bother to loo;. It is easy to get 40A continuously without blowing the plug fuses from a double socket.

Yes they do, you are wrong and refuse to accept that you are wrong. You cling to straws and keep changing the argument just to avoid admitting it. You are also a hypocrite as you accuse other of doing the same, even when they don't. You only have to look at this post to prove that this is true of you.

If the circuits were protected by breakers upstream we wouldn't have this debate and if you really have designed stuff you know that its illogical to protect the cable at the wrong end using something the householder controls.

Reply to
dennis

Oh gee...

#1 BS1362 : 1973 fuse standard:

- Fuse shall disconnect at 1.9x rated current in

Reply to
js.b1

The irony here is that, AFAIR, the counterfeit fuses just lack sand filling, so that, in all probability they'd operate OK in practice to clear moderate overloads. Their real danger is that they explode and arc if called on to break the full rated 6 kA fault. I suggest that their danger has probably been somewhat overstated since (a) in 99+% of cases the prospective fault current on a final circuit is far less than that and (b) the upstream device will provide backup protection.

The counterfeit MCBs, OTOH, are *really* scary.

Reply to
Andy Wade

Thanks for that description. It actually agrees with what I said. The protection isn't there and it relies on statistics and supposition.. the user won't do this, the user wont do that, etc., if he does do it something else will probably fail (and kill him?) first.

The 10A fuse is probably a good idea, but not for heaters, fridges, etc. but for the fourway strips. The 16A that it would fail at is more likely to protect the flex than the 20A the 13A fuse will run at. You would have to drop it to 8A to bring overload protection to the actual spur cabling.

Its pointless debating it with people that just quote the regs are the answer when its the regs that are in dispute. The evidence is that there is no overload protection for the spur and that its easy overload it without introducing fault conditions and its only assumed use that protects it.

I think the matter is closed.

Reply to
dennis

It does not.

It is a theoretical risk which is extremely unlikely to be realised. More of a problem is substandard 4-way multi-adapters who struggle with 10A continuous never mind 13A.

It could be solved with a minor amendment to a standard changing 13A fuse spec to 1.25x (I suspect the 1.6x came about due to production tolerances).

Reply to
js.b1

As bad as 2.5 for a fuse?

I went to a job this year where all the MCBs were 32A and the Zs was

18ohm (a neighbour had installed the CU). They has a fault on the bathroom light (nice big burn marks on the ceiling) but somehow the fault cleared.

-- Adam

Reply to
Adam Wadsworth

Yes, or worse:

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I went to a job this year where all the MCBs were 32A and the Zs was

so much for Part P stopping such things. No RCDs in sight, presumably?

Reply to
Andy Wade

:

It was a "nice" metalclad CU with the tails passing through different knockouts:-)

So again, it shows that you do not instantly die just because of a bad setup even when there is a fault.

-- Adam

Reply to
Adam Wadsworth

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