Garage socket & pattress question and 17th Edition wiring regs

[meow2222]

32A is what it'll pass indefinitely. Real life trip current is far higher

fuses don't limit current at all. A 13A fuse can pass around 20A for a long time

PSCC is anywhere upto 6,000A. Whatever PSCC actually is it'll normally trip the

32A mcb first. If its north of the mcb's breaking capacity it'll take out the incomer fuse instead.

It would carry anything upto 6kA. Its not a problem for the very short time involved.

its within the 27A cable rating. But what real life loads would draw 26A continuous?

more like thousands of amps

no need. 20A's enough for a double socket.

no need

:)

NT

[Stephen H]

Incidentally. There has been a thought about ring mains that has bothered me for some time.

in a perfect world, you have a continuous live loop and a continuous neutral loop on a 32A MCB ring main, from teh CU to all the sockets and then back to the CU.

What if you had a break in the cable between the CU and the last socket? This converts the ring main to a radial circuit.

Lets assume the ring main is then loaded to its full capacity of 32A.

This would mean that *ALL* the current would be flowing along one route between the CU and the first socket (assuming multiple appliances plugged in a various points around the radial.

Can a single 2.5mm2 T&E really sustain a 32A continuous load?

Also as I understand it, you should only have a 20A mcb on a radial circuit wired in 2.5mm T&E. if you complete the radial into a ring, then the MCB can be changed to a 32A MCB.

[Alexander Lamaison]

snip

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I'm pinning this reply in my newsreader as I've never seen it explained so clearly before. Perhaps this Q&A is also worth sticking on the Wiki somewhere?

Alex

[John Rumm]

Indeed it does...

In reality you could have a break anywhere - and would then get a pair of radials.

However each circuit topology has strengths and weaknesses. There are several types of faults (high resistance connections being the most common, followed by disconnected conductors, and finally disconnected cables). Each will have different effects.

The options are covered in some detail here:

Depends on the circumstances and how quick it can shed heat. It also very much depends on how long you can keep that load on for. Rings typically serve general purpose use sockets, which form a diverse load.

Yup

Obviously.

[John Rumm]

Well to be fair this has been discussed a few times in the past, so I have had practice ;-))

I think most of it ought to be in the wiki... although if you find a bit missing feel free to either highlight what, or for that matter add it yourself (drop me an email if you want a wiki account) ;-)

Fault and overload currents are discussed here (and in other places):

The effects on different circuit types along with that of other common fault conditions is discussed here:

The responses of fuses and MCBs here: