Fused spur from ring main

[liam5168]

Hi,

I have a detached garage (1x pir led sensor floodlight, 1x double socket and 1x light pendant inside) which has an electrical supply wired from the house left hand side ring main. It's wired using 2.5mm SWA mostly in the ground but the FCU (13A) is at the end of the SWA (in the garage) not the start (in the home).

Had electrician in to add: 2x exterior led lights on a timer and a single exterior socket to the garage. The electrician has said FCU needs to be at the start of the SWA to protect it from fault overload on the ring main side.

Is this needed?

(I'm hoping I've written this clearly enough but please feel free to question)

Please and thanks

 

[flameport]

Not likely to make any difference whether the FCU is in the house or the garage.

protect it from fault overload

Fault protection and overload protection are two different things.

The FCU will prevent overloads regardless of where it is installed.
If it's in the house, it will also provide fault protection for the SWA.
If it's in the garage it won't, but the circuit breaker or fuse that protects the ring almost certainly will.

 

[liam5168]

So, I guess the answer is yes it's needed at the house end as the ring main has a 32A MCB?

Not likely to make any difference whether the FCU is in the house or the garage.


Fault protection and overload protection are two different things.

The FCU will prevent overloads regardless of where it is installed.
If it's in the house, it will also provide fault protection for the SWA.
If it's in the garage it won't, but the circuit breaker or fuse that protects the ring almost certainly will.

 

[JohnW2]

So, I guess the answer is yes it's needed at the house end as the ring main has a 32A MCB?

Not really. I suggest that you read flameport's post again.

Kind Regards, John

 

[ericmark]

The regulations do refer to 3 meters, but really this is when using 300 amp bus bars to feed a 63 amp distribution board. It is not really for a ring final, and if the cable is unlikely to be damaged it make no difference if overload at supply or destination end, it will disconnect the supply with an overload.

More important is how far around the ring final the branch is,

The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable (Regulation 433.1.5 refers). This can generally be achieved by:
(i) locating socket-outlets to provide reasonable sharing of the load around the ring
(ii) not supplying immersion heaters, comprehensive electric space heating or loads of a similar profile frog the ring circuit
(iii) connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit
(iv) taking account of the total floor area being served. (Historically, limit of 100 m² has been adopted.)

The cable of a ring final is rated 20 amp or more, but the overload is rated at 30 or 32 amp, so we hope the distribution of load will not result in over 20 amp being drawn on either leg, so the spur taken in centre of ring is no problem, but a spur near to the consumer unit may be, as it would be easy for a garage supply to be near the maximum for an extended period, and fitting a FCU in the house will not change that.

There would be little point changing the location of the FCU if coming from a ring final it will be RCD protected, the electrician could not have done the work he did unless RCD protected, and a direct short will still trip the 32A MCB or he would not have fitted the extras, and an earth leakage would trip the RCD so in real terms no worries.

He would have issued a minor works certificate and he would have measured the loop impedance to ensure a fault would trip the 32A MCB and he would have also tested the tripping times of the RCD and it will all be written down, if he didn't like it, the cure is simple don't do it, so yes would have been better if the FCU was in house, but he clearly did not really think there was a problem or he would not have done the job and issued a minor works certificate.

 

[JohnW2]

The regulations do refer to 3 meters ...

Only if the circuit does not have other adequate fault protection which, as flameport said, would almost certainly be provided by the ring circuit's OPD.

.... More important is how far around the ring final the branch is ....

That is surely pretty irrelevant in the OP's case of a spur protected (wherever along its length) by a 13A fuse, isn't it? It's no different from having a single 13A socket 'on the ring', and that could be as close as one wanted to an end of a ring without, itself, creating any risk of any part of the circuit being overloaded, couldn't it?

Kind Regards, John