Spur From Ring

[brizospark]

It is considered OK to spur a socket off of a ring main, but my question is this - If the ring is protected by a 32a mcb, this is not sufficient for protecting the 2.5mm cable used for the spur off the ring, maybe I am misunderstanding this can someone please explain?

 

[Deleted member 26818]

You're allowed one double-socket, how much current could be drawn by it, and would that current damage the 2.5mm² cable?

 

[brizospark]

What if it is a double socket?

 

[brizospark]

Double socket - 26amp, bit on the high side for 2.5mm depending on run length etc

 

[amberleaf]

Spur from Ring .

A Spur can be Added to any part of the Circuit providing the rules are Followed .
- if there is not an Existing Socket near enough

You are Limited in Length of Cable you are allowed to Use in the Circuits . 3m

( Plain English ) 17th Edition //////////////////////////// p/74 “ Right “ let’s get the Spur out the Way HERE .

Spur’s ] 433.2.2. is the Regulation that Control’s the Maximum Length of a Spur & Do Not use any Calculation’s

The device protecting a Conductor against Overload may be Installed along the Run of that Conductor if the Part of the run between the Point where a Change “ Occurs “ ( in Cross-Sectional-Area . method of Installation . Type of Cable or Conductor . or in Environmental Conditions ) & the position of the Protective Device has “ Neither Branch Circuit’s nor Outlet’s for Connection of Current-Using Equipment & fulfils at least ONE of the Following Conditions .

(i) it is Protected against Fault Current in Accordance with the Requirements stated in Section 434 .
(ii) its length does NOT Exceed 3m . it is Installed in such a manner as to Reduce the Risk of Fault to a Minimum . & it is Installed in such a Manner as to Reduce to a Minimum the Risk of Fire or Danger to Person(s) ( see also Regulations 434.2.1. )

( 13A – FCU at the Point where you connect to Ring ( Wording !! A Branch from a Ring to Radial onto a Spur )

Definition :- Spur . p/29 .A branch from a Ring or Radial Final Circuit . ( Not Defined as a Circuit of it’s Own Right )

 

[Deleted member 26818]

Is the maximum current likely to be drawn, and if it is, is it likely to be drawn for very long?

 

[amberleaf]

If using a Spur . you are Protecting the Equipment with a Fuse ?? 3a 5a 13A ↔ is smaller 32A you have got Discrimination
The Fuse is there to save the Cable ! ( Iz )

 

[IQ Electrical]

Spur from Ring .

A Spur can be Added to any part of the Circuit providing the rules are Followed .
- if there is not an Existing Socket near enough

You are Limited in Length of Cable you are allowed to Use in the Circuits . 3m

( Plain English ) 17th Edition //////////////////////////// p/74 “ Right “ let’s get the Spur out the Way HERE .

Spur’s ] 433.2.2. is the Regulation that Control’s the Maximum Length of a Spur & Do Not use any Calculation’s

The device protecting a Conductor against Overload may be Installed along the Run of that Conductor if the Part of the run between the Point where a Change “ Occurs “ ( in Cross-Sectional-Area . method of Installation . Type of Cable or Conductor . or in Environmental Conditions ) & the position of the Protective Device has “ Neither Branch Circuit’s nor Outlet’s for Connection of Current-Using Equipment & fulfils at least ONE of the Following Conditions .

(i) it is Protected against Fault Current in Accordance with the Requirements stated in Section 434 .
(ii) its length does NOT Exceed 3m . it is Installed in such a manner as to Reduce the Risk of Fault to a Minimum . & it is Installed in such a Manner as to Reduce to a Minimum the Risk of Fire or Danger to Person(s) ( see also Regulations 434.2.1. )

( 13A – FCU at the Point where you connect to Ring ( Wording !! A Branch from a Ring to Radial onto a Spur )

Definition :- Spur . p/29 .A branch from a Ring or Radial Final Circuit . ( Not Defined as a Circuit of it’s Own Right )

This 3-metre limit does not apply to the pre-designed ring final circuit spur as in appendix 15.

 

[malcolmsanford]

I ddin't think reg 433.2.2 was in conjunction with a ring final circuit to be honest.

 

[IQ Electrical]

I ddin't think reg 433.2.2 was in conjunction with a ring final circuit to be honest.

Neither do I

 

[amberleaf]

IET Inspect/test course was a FCU fed from a socket on a ring to a spur of 1.5mm2 to a fixed appliance, the short length between socket and FCU can be in 1.5mm2 as long as short & unlikely to be damaged.


This 3-metre limit does not apply to the pre-designed ring final circuit spur as in appendix 15.
IQ Electrical
So what you are saying you can run a Fuse Spur 10 / 20 /40 Meters . lets go the full hog here 100m fused Spur ( your View Please ) when does the size cable fall into this

 

[amberleaf]

IQ Electrical So let me get this right . 433.2.2. so its not in Appendix 15 it does not Comply . the Regulations is a Big Book .

 

[IQ Electrical]

IQ Electrical So let me get this right . 433.2.2. so its not in Appendix 15 it does not Comply . the Regulations is a Big Book .

I'm talking about the standard pre-designed ring final circuit.

The limiting factors should as always be earth loop impedance and volt drop.

There is no stipulated maximum unfused spur length but 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.

How's that for a weird obscurity!

 

[amberleaf]

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

 

[amberleaf]

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?

 

[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!

 

[tonys]

how is a double socket spur on 2.5 protected from overload by a 32A mcb - I still think it's a good question!

The overload is governed by the two 13 amp plug tops, in such much 26amp is the maximum that can be drawn and odds of both the sockets being used to the maximum is very slim and if they were and others sockets were also being used the 32amp mcb would more than likely trip.

Diversity works in the real world, so you have to rely on it along with ones experience.

 

[Deleted member 9648]

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.

You are absolutely correct.

 

[Chr!s]

I would say overload has been omitted 433.3 due to the nature of the load, that being the fact that only one single or double may be connected.

Protection against fault current would still be provide via the 32 amp mcb.

Socket outlets now require rcd protection so zs wont be the overriding factor it will be the volt drop.

Chris

 

[IQ Electrical]

I would say overload has been omitted 433.3 due to the nature of the load, that being the fact that only one single or double may be connected.

Protection against fault current would still be provide via the 32 amp mcb.

Socket outlets now require rcd protection so zs wont be the overriding factor it will be the volt drop.

Chris

That's assuming a domestic-type situation.

A socket spurred off in an office under the control of etc. etc. in 20mm galvanised conduit or any of the other surface/earthed scenarios will still be limited by Zs and volt drop.

 

[Chr!s]

That's assuming a domestic-type situation.

A socket spurred off in an office under the control of etc. etc. in 20mm galvanised conduit or any of the other surface/earthed scenarios will still be limited by Zs and volt drop.

Well yes i was assuming a domestic situation, though non domestic will not necessarily be excluded from RCD protection, that would be down to designer type of load and any risk assessments etc

Chris