Spur From Ring

[IQ Electrical]

p/262 . Uufused ↔ Spur . is Protected by .. 2 – 13A Fuses ( 1 – Double Socket ) & Yes Eath Loop Impedance / Volt Drop

Page 362

 

[amberleaf]

sorry . thank you

 

[telectrix]

Taking from IET

cable is 3 meters or less you may protect it on the end of the cable rather than the source see 433.2.2. Since a 13 amp socket has to have 13 amp fuses in the plug then a 2.5mm cable feeding a single 13 amp outlet either with one or two sockets in it is considered as being protected by the fuses in the plug. Hence the use of spurs from ring mains. Over 3 meters then it would need protecting as source hence a radial with one socket would need a 25 amp MCB mainly because it is over 3 meters cable run. Is that what you were asking and what is an RF circuit?

so an easy solution would be to reduce the ring final MCB from 32A to 25A? rather than adding an enclosed MCB just for the spur.

 

[IQ Electrical]

I think we're looking at different issues here, your post implied a maximum unfused spur length of 3 metres.

This is what I'm saying:

There is no maximum length specified in BS7671:2008 for an unfused spur taken from a standard ring final circuit as set out in appendix 15, page 362. (obviously Zs and Volt Drop must be considered).

The On-site Guide however has this to say: 'as a rule of thumb, unfused spur lengths should not exceed 1/8 the cable length from the spur to the furthest point of the ring.'

 

[amberleaf]

Am not turning this in to an AGUMENT . Full Stop .

* Unfused Spur . the Two are different .
* Fused Spurs

this is way it ends .

 

[IQ Electrical]

Am not turning this in to an AGUMENT . Full Stop .

* Unfused Spur . the Two are different .
* Fused Spurs

this is way it ends .

It's a discussion!

I'm not arguing but if someone posts that the maximum length for an unfused spur is 3 Metres then I dispute it because it simply is not true!

And it ends when a few more people read this thread and confirm that I'm absolutely correct.

 

[RogertheBodger]

Thanks Guys, that was a great exchange, followed it with interest, not clear on the outcome,
IF a double socket on a spur IS pulling 26 amps, how is the single 2.5 protected?

 

[malcolmsanford]

It's a discussion!

I'm not arguing but if someone posts that the maximum length for an unfused spur is 3 Metres then I dispute it because it simply is not true!

And it ends when a few more people read this thread and confirm that I'm absolutely correct.

I have to agree here with IQ and take for guidance the 1/8 rule of thumb suggested in the OSG.

 

[IQ Electrical]

Thanks Guys, that was a great exchange, followed it with interest, not clear on the outcome,
IF a double socket on a spur IS pulling 26 amps, how is the single 2.5 protected?

In best case scenario, the 2.5 will handle 27 Amps but in reality, the socket outlet will fail long before the cable.
Most twin outlets are rated at 20A with MK being a known exception at 26A.
The installation design in your scenario would need to be altered to suit the excess load.

 

[RogertheBodger]

In best case scenario, the 2.5 will handle 27 Amps but in reality, the socket outlet will fail long before the cable.
Most twin outlets are rated at 20A with MK being a known exception at 26A.
The installation design in your scenario would need to be altered to suit the excess load.

True, but punters don't know this, in a domestic installation where a spur is used in this way, and I can see it happening on cold nights when the C/H is on the blink, what then? if the cable shorts or the double socket melts we can't call that circuit protected can we?

 

[tonys]

I always agree with IQ only because hes an Engineer and I know he does not make a statement unless hes checked his Technical books first.

 

[malcolmsanford]

True, but punters don't know this, in a domestic installation where a spur is used in this way, and I can see it happening on cold nights when the C/H is on the blink, what then? if the cable shorts or the double socket melts we can't call that circuit protected can we?

If the punter decides to plug 2x 3kw heaters into those sockets for a 6 hour period then the cable is quite capable of taking it, what wil most likely not take it are the accessories, the same BS 1363 sockets on the spur as there are on the ring final.

As long as IQ as pointed out you stick to the 1/8 rule, you ensure that the EFLI and VD is still within parameters then that spur should be deemed safe.

 

[IQ Electrical]

You still have fault protection and overcurrent protection but the BS 1363 socket standard rates an outlet whether twin or single at 13A plus an additional short time test at 20A for a twin (14A in one side, 6A in the other).

I suppose the danger is no greater in a socket on the ring final itself, subject to the same BS 1363 standard.

 

[jaresquire]

As we all know, RF circuits are a compromise that have an exception in the Regs whilst they still linger. They were invented to give an extra question or two on testing in the 2391 exam.

 

[RogertheBodger]

You still have fault protection and overcurrent protection but the BS 1363 socket standard rates an outlet whether twin or single at 13A plus an additional short time test at 20A for a twin (14A in one side, 6A in the other).

I suppose the danger is no greater in a socket on the ring final itself, subject to the same BS 1363 standard.

I agree with all that you have pointed out, the OP was asking the question - how is a double socket spur on 2.5 protected from overload by a 32A mcb - I still think it's a good question!