Radials, Rings and Spurs

[phil_ballard]

Sorry, having a bit of a dense day today

One may take a spur off a ring main, not a spur off a spur, or off an already-spurred socket. I understand the rule, but can't work out why it's so? Is it to do with the ring protection being 32A rather than 16A as for a radial?

I have a feeling the answer's going to be really obvious and therefore somewhat embarassing ....

 

[plugwash]

rings are a special case cuircuit in which the protection at the start is rated higher than the cable

as such design rules must be adopted that minimise the risk of overloading the cable without sending an overloading the breaker

note that the cable in a ring must have at least two thirds the rating of the protective device

 

[phil_ballard]

But doesn't this mean that (if the spur cable must have the same rating as the ring cable) then even spurring as permitted allows the spur cable to be overloaded before the cct protection breaks? Also, if the (permitted) spur goes to a double socket (which I think is allowed) how is this different to a (disallowed) spur to a single then spurred from that to another single?

NB I'm certainly not arguing the toss with you plug, just trying to understand

 

[ban-all-sheds]

The theory is that since you're only allowed 1 socket on it, you can't overload it.

What you've missed in your analysis is the little-known fact that a double socket is still only rated at 13A - you cannot take 26A from it, and if you do so for any length of time it will overheat dangerously. Maybe even catch fire.

Even so, I still don't like even single unfused spurs - it's a personal position, but I would always advise against them.

 

[phil_ballard]

Good point, there was an interesting thread recently about the current rating of double sockets .... hmm. Still think it's a little odd, then, that they allow what they do with an underrated cable (or at least, rated < the protection device)

Thanks plugwash, I can see why you have that opinion re fusing of spurs

 

[ban-all-sheds]

And if you are installing an FCU, I would advise wiring it into the ring just like you would do with a socket.

 

[phil_ballard]

otherwise it, I suppose, becomes an unfused spur (as far as the fuse, anyway)?

 

[ban-all-sheds]

Yup.

OK - in reality, you can't overload that bit of cable, and assuming it's short-ish, and concealed/protected like the rest of the ring it's hard to conceive of any fault condition that could arise that would be a hazard to the cable because it has "only" 30/32A protection.

So maybe it is a bit anal to wire FCUs directly onto the ring whenever possible, but hey - if you aren't in the vicinity of a socket, it saves the cost of a JB

 

[phil_ballard]

OK - in reality, you can't overload that bit of cable

... unless the fault is at the supply side of the FCU, e.g. someone sticking a nail through the cable? But then, I suppose, you'd get a short (and blow the fuse/MCB) rather than a sustained too-high current of, say, 40A .....

Interesting, thanks guys

 

[plugwash]

But doesn't this mean that (if the spur cable must have the same rating as the ring cable) then even spurring as permitted allows the spur cable to be overloaded before the cct protection breaks?

plug fuses should stop and major overload of the spur cable

Also, if the (permitted) spur goes to a double socket (which I think is allowed) how is this different to a (disallowed) spur to a single then spurred from that to another single?

two singles on a spur used to be permitted

the problem was they got replaced with doubles in a lot of cases