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

In a bout of man flu, I lay in bed wondering: If the MCB is supposed to protect the cable to the fitting, and the fuse in the plug is supposed to protect the cable to the appliance, how is the single bit of 2.5mm T+E run to a single socket spur adequately protected by anything?
You've basically got 2 bits of 2.5mm in parallel (usually unequal lengths), feeding a single bit of 2.5mm. 32A MCB.
Whereas if it was a standard radial circuit, you'd have just the single bit of 2.5mm, and you'd only be allowed a 20A MCB on it. (IIRC they even dropped the 2.5mm rating down to 18A, which is "correct" in accordance with the calculations, but then increased it back to 20A because that's what everyone does anyway).
I'm not worried. I just don't see the logic.
I have 10cm of 2.5mm T+E protected by a 40A MCB on the cooker circuit (feeding a socket that can only supply the gas hob ignition) and that seems very wrong too - but wiring a socket in 6mm is just too painful. However, the cooker circuit may yet drop back to 32A (because I suspect that's all it needs) which makes it no worse than the socket spur discussed above.
?
Cheers, David.
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On 15/12/10 19:54, David Robinson wrote:

Fuse in the plug at max of 13A.
Odd as it may seem, overload protection *can* exist downstream as well as upstream.
In the case or a short circuit in the back of the socket, the 32A breaker will still protect the cable (ie interrupt the current before the cable overheats).
--
Tim Watts

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Thanks Tim.
So on a 20A radial, the 20A is to protect against overloads (lots of "up to 13A" loads, potentially causing damage over time without ever tripping a 32A) whereas a short circuit would generate enough current to trip a 32A before the cable suffered harm?
Where can I find that calculation? I saw those MCB and fuse trip-time vs current graphs the posted the other week, but can't remember reading a shot circuit disconnect time requirement.
Do I need to change my 40A to a 32A, or leave it as it is?
Cheers, David.
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NO. A 20 radial is a circuit with a 20A MCB. A spur from a 32A ring is a different thing. ( my other post did not make that one clear)

http://wiki.diyfaq.org.uk/index.php?title=Fuse
for the graphs.

I would change it. Far easier than doing calculations. However I would not use the 2.5 T&E for use with an electric cooker without making some very accurate calculations.
--
Adam



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Its self protecting exactly as a spur on a 32A ring is (it only has a single 13 socket). I would change it too, most electricians would reject it as they don't know how to do the calculations and its just easier to use the regs than to explain why its OK.
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Yes it is at the moment, but it will not be if the OP installs an electric cooker, and he has hinted that he may do so.

(the regs tell you what is OK and they give you the calculations, the regs and what is OK are inclusive not exclusive)
I have to take the bigger view. I might be able to show that a certain cooker is safe when installed with the 2.5 T&E, however the customer may change the cooker for a higher powered one or sell the house and a new owner may fit a higher powered cooker. I would rather see a cooker circuit correctly fused to the actual cable rating and not to some diversity calculations that mean nothing when a cooker is swapped.
--
Adam



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You can do anything you like if you are competent to do the calcs, I believe that's in the regs somewhere. The majority of electricians mean the onsite guide when they refer to the regs.

That I agree with.. diversity is a PITA. It doesn't even work when someone swaps a few single plugs for doubles in a ring and then plugs in a few fan heaters at one end. That sort of things makes rings run out of spec. I suppose its regarded as unlikely that some will use 9kW of heating in a room, even one they are trying to dry out after the recent floods. ;-)
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The same rules and regs apply to both the full BS7671 and the OSG.

That makes no difference in most cases. Most people do not have a massive store of fan heaters ready to be used.

For a short time that will not matter.

But you know damn well the correct tool is a dehumidifer not a heater for this job.
--
Adam



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OK, 9 kW of heating + 1kW of dehumidifier.
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But that never happens. I doubt that three 3kW fan heaters will stay on for long if they are in the same room. They have thermostats.
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The problem being that there is no way to know, it might be a big room with the doors and windows open with a gale blowing through, they would stay on then. The reason why I brought it up is because I have seen it happen BTW. Not everyone would think about running an extension lead from somewhere else to make sure there wasn't a problem. They would leave it running until the room was dry or it all failed.
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You are missing one important point.
Ring mains are designed for normal domestic use not for clearing floods.
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Shame that very few know that it can be dangerous, something that could be designed out.
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The IET must be stupid. Why did they not let you design the new regs for them?
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So you don't think they got the compromise between cost and safety on ring mains wrong then? That would be why they added radials with breakers that actually match the cables because they didn't need to. Who knows, sometime in the future when all the old electricians have retired rings will finally die.
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On 16/12/10 08:58, dennis@home wrote:

Rings have notable safety benefits over radials, namely that the CPC is doubled up over two paths.
Also, show me a practical 32A radial with multiple socket drops noting the BS standard restrictions on terminal capacities of socket and FCU accessories.
--
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They also have several safety problems. They can have latent faults that the householder will only find out about when its too late. Things like broken earths (which negates your above argument) can just be there undetected for years unless you have regular inspections and real continuity tests. This isn't a solution though as the act of doing the test will make the faults more likely to occur.

Why do you need a 32A radial? I don't think you will find them in the OSG. 4mm will do the job with many accessories and you could always crimp in a joint if the accessory is to small. I wouldn't bother with a 32A radial to replace a ring.
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On 16/12/10 09:38, dennis@home wrote:

Because 20A is fuck all use to me.
I do not want to be thinking - are these sockets on the same 20A circuit - damn I cannot plug this 3kW appliance in because there are already two high load appliances on that circuit.
I already also have enough RCBOs in my CU - I don't have space for even more, let alone the expense.

Bugger the OSG, it is a guide only. I think you WILL find them in Appendix 15 of the 17th.

4mm doesn't cut it for Reference Method B (a common installation method in my house).

--
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But you need to do that if you want to plug them into one end of a 2.5 mm ring, even more so with three appliances. It causes imbalance currents in the ring that can take them out of spec. Its why fixed heating shouldn't go on a ring, they don't like high load stuff at one end.
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20A radials often use more cable when installing than a ring, offer very little in the way of diversity and require just as much (if not more) work to install than a ring.
There are times when they are useful but not very often.
--
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