Spur off spur

[danny199]

Hi,

Just need to check something. Sparky installed two new plug sockets. To do this, cut into the main ring, put a junction box in, one cable out to socket. Then from there connected one more plug. I understand to spur off a spur you should use a fuse between them?

 

[StephenRowley]

Why are you still using fuses?

Because they we've got a 32amp ring main. And appliances with wiring that are designed to handle 13amps max(often less) so we stick a fuse in the plug to protect the appliance, saves on copper.

 

[fairlight]

You said not technically correct but acceptable. A two gang socket is type tested with a load of 20A (14A and 6A) across the outlets and whilst in practice this may not be the case it is fact. Based on this a two gang socket will draw no more than 20A, argue this with British Standards. Two one gang sockets have the potential to draw 26A. To apply a departure you could not guarantee a lesser standard of safety is provided by using two one gang sockets.

'Based on this a two gang socket will draw no more than 20a'!!….. Does it have a current limiter then?? A twin socket can, and will, have two 3kw 13a appliances plugged into it just the same as two singles because the plug fits both. The type testing is based on average loadings likely to be applied in average circumstances, the same average circumstances apply whether it is a twin or two singles. British Standards can say what they want, Joe Public will never ever be heard to say. " Ah, two 3kw appliances I need to use, I cant plug those into a twin because it'll only take 20a, I'd better find two separate singles to plug them into".

 

[westward10]

As I said in practice this may not be the case.

 

[sinewove]

As I said in practice this may not be the case.

I believe MK claim their twin sockets are good for 2x13A loads

 

[Kidsolo]

used for fusing down when you are connecting lower load

Sure, you are down sizing the fuse so that socket alone can't exceed 13 amps, while the rest of the circuit can. But one needs to ask "why"? Why not just use the protection set aside for the ring circuit? It is still part of the main circuit as an overall load, so why does a stray socket suddenly need it's own fuse?
and why a ring circuit? sure, it splits the load, but why not just make 2 separate circuits? you have already run the cable back to the switchboard. That way you have two separate circuits, both sharing the load of all the sockets... but you have the added advantage of only tripping one circuit at a time, instead of losing power on everything.

 

[Pete999]

Sure, you are down sizing the fuse so that socket alone can't exceed 13 amps, while the rest of the circuit can. But one needs to ask "why"? Why not just use the protection set aside for the ring circuit? It is still part of the main circuit as an overall load, so why does a stray socket suddenly need it's own fuse?
and why a ring circuit? sure, it splits the load, but why not just make 2 separate circuits? you have already run the cable back to the switchboard. That way you have two separate circuits, both sharing the load of all the sockets... but you have the added advantage of only tripping one circuit at a time, instead of losing power on everything.

Sure, you are down sizing the fuse so that socket alone can't exceed 13 amps, while the rest of the circuit can. But one needs to ask "why"? Why not just use the protection set aside for the ring circuit? It is still part of the main circuit as an overall load, so why does a stray socket suddenly need it's own fuse?
and why a ring circuit? sure, it splits the load, but why not just make 2 separate circuits? you have already run the cable back to the switchboard. That way you have two separate circuits, both sharing the load of all the sockets... but you have the added advantage of only tripping one circuit at a time, instead of losing power on everything.

I guess it's because the UK wiring systems are the envy of the World!!

 

[Andy78]

I believe MK claim their twin sockets are good for 2x13A loads

I replaced quite a few burnt out MK logic sockets that have been used with 2 x 10A+ loads

 

[fairlight]

Sure, you are down sizing the fuse so that socket alone can't exceed 13 amps, while the rest of the circuit can. But one needs to ask "why"? Why not just use the protection set aside for the ring circuit? It is still part of the main circuit as an overall load, so why does a stray socket suddenly need it's own fuse?
and why a ring circuit? sure, it splits the load, but why not just make 2 separate circuits? you have already run the cable back to the switchboard. That way you have two separate circuits, both sharing the load of all the sockets... but you have the added advantage of only tripping one circuit at a time, instead of losing power on everything.

"Why?" only applies when the spur is to feed more than one point. A fused spur enables a single cable (2.5mm) rated at < the 32a source OCPD to feed any number of points because the load is limited to 13a. Two twin sockets on an unfused spur would have a load potential exceeding the current carrying capacity of a single 2.5mm cable which could result in thermal damage to that cable.

 

[Pete999]

Help an Aussie understand.
The fuse connected to the spur, where is that physically situated in the installation? alongside the outlet, back in the switchboard, or somewhere else?
and do you guys still use fuses, or is that your name for a breaker?

A ring circuit has two actives in the bottom of the main breaker and two neutrals in the neutral link?

Help an Aussie understand.
The fuse connected to the spur, where is that physically situated in the installation? alongside the outlet, back in the switchboard, or somewhere else?
and do you guys still use fuses, or is that your name for a breaker?

A ring circuit has two actives in the bottom of the main breaker and two neutrals in the neutral link?

Take a look at the diagram in post 4 all will become clear

 

[Kidsolo]

But if I was doing an EICR two doubles on a spur would be a code 2, two singles would be a code 3

The diagram clearly says "one single or one double". it doesn't say two singles equal one double

 

[Kidsolo]

Two twin sockets on an unfused spur would have a load potential exceeding the current carrying capacity of a single 2.5mm cable which could result in thermal damage to that cable.

This doesn't make any sense at all. Two unfused sockets doesn't equal "unprotected", it is still protected by the 32 amp breaker (just like all the other sockets on the ring circuit). All you are doing is limiting that socket and any connected after it, to 13 amps. it is still part of the 32 amp circuit.
If two fused sockets is going to exceed the load potential of 2.5 mm, then what do you think all those other sockets going to do to the current carrying capacity of the 2.5 mm in that circuit?

 

[Kidsolo]

so we stick a fuse in the plug to protect the appliance, saves on copper.

I probably didn't say it right, might be my accent.... why are you guys still using fuses? they are old news

 

[StephenRowley]

and why a ring circuit? sure, it splits the load, but why not just make 2 separate circuits? y

because we cheap and want to save money on copper and circuit breakers.

 

[DPG]

I probably didn't say it right, might be my accent.... why are you guys still using fuses? they are old news

Strange opinion - fuses have many uses. Think of discrimination/where you have a submain fed from a fuse.

 

[fairlight]

The diagram clearly says "one single or one double". it doesn't say two singles equal one double

It doesn't, and I never said Bs7671 says two singles equals one double. I said in my opinion there is no difference in practice because both allow the connection of any two 13a appliances with a 13a plug.

This doesn't make any sense at all. Two unfused sockets doesn't equal "unprotected", it is still protected by the 32 amp breaker (just like all the other sockets on the ring circuit). All you are doing is limiting that socket and any connected after it, to 13 amps. it is still part of the 32 amp circuit.
If two fused sockets is going to exceed the load potential of 2.5 mm, then what do you think all those other sockets going to do to the current carrying capacity of the 2.5 mm in that circuit?

You are completely missing the point. A ring is protected by a 32a breaker because each socket on the ring is effectively fed by TWO 2.5mm cables, not one. If you reduce the supply cable to ONE 2.5mm with a spur off the ring then the load carrying capacity of that part of the circuit is effectively halved, it's no longer capable of carrying 32a. That's why the spur cable would need to have a reduced overcurrent device to protect only the spur if it supplies more than one twin outlet.(or more than one single to be in compliance with the guidance in 7671.