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Topic Title: T&E ring circuits with high leakage current
Topic Summary: Size of high integrity CPC in OSG
Created On: 04 September 2012 10:55 AM
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 04 September 2012 10:55 AM
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jonno

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Joined: 06 September 2006

Latest OSG page 79 figure 7.5.3 shows a ring final circuit with two high integrity protective conductors of minimum size 1.5mm2 each.

So, does wiring ring final circuits in 2.5mm2 T&E (1.5mm2 earth conductor) comply with regs for use with standard 13A sockets supplying computers, where there may be a high leakage current?

Just finding the regs and OSG a bit confusing today!
 04 September 2012 11:06 AM
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AJJewsbury

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So, does wiring ring final circuits in 2.5mm2 T&E (1.5mm2 earth conductor) comply with regs for use with standard 13A sockets supplying computers, where there may be a high leakage current?

Only if the two c.p.c. have independent connections throughout the circuit - i.e. you'd need dual-earth-terminal accessories and put the two c.p.c.s under different screws on the DB earth bar.
- Andy.
 04 September 2012 11:19 AM
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jonno

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Thanks Andy.

Yes, I was aware of connecting the cpc's independently. Its just for some reason I thought the two cpc's had to be larger, 2.5mm2 or 4mm2 was in my mind.
 04 September 2012 11:26 AM
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AJJewsbury

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You can use larger c.p.c. instead of duplicating - in general your choices are:

10mm² c.p.c., or

4mm² c.p.c. + additional mechanical protection (e.g. flexible conduit), or

duplicated "ordinary" size c.p.c.

(compare with minimums of 2.5mm²+mechanical protection or 4mm² for ordinary protective conductors (c.p.c.s or bonding conductors) outside of wiring systems).

- Andy.
 04 September 2012 01:16 PM
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spinlondon

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Both of the CPCs have to meet the requirements for a CPC separately.
So if you have two radials, then you can link the two CPCs with an apropriate conductor, as both the CPCs meet the requirements for a CPC.
If you were to convert the two radials into a RFC, then an extra CPC would have to be run, and that new CPC would have to meet the requirements for a CPC, 2.5mm² if mechanically protected or 4mm² if not.
 04 September 2012 01:26 PM
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AJJewsbury

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If you were to convert the two radials into a RFC, then an extra CPC would have to be run, and that new CPC would have to meet the requirements for a CPC, 2.5mm² if mechanically protected or 4mm² if not.

Sorry Spin, but I'm not following your logic on that one. If I were to convert two (straight) radials into a ring, I'd run a length of T&E (or equivalent for the wiring system in use) between the two last points - which would contain L, N & c.p.c. just as if it had been installed as a ring in the first place. Why would I need an extra 2.5mm²/4mm² conductor?
- Andy.
 04 September 2012 10:03 PM
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spinlondon

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Because you would then only have the one CPC not two, and the single CPC would not be of a sufficient CSA.
 04 September 2012 10:18 PM
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AJJewsbury

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Because you would then only have the one CPC not two, and the single CPC would not be of a sufficient CSA.

Sorry, still not with you....

Say I had two simple straight radials, in say 2.5/1.5 T&E. I then convert to a ring by linking the two end points with another length of 2.5/1.5 T&E. All accessories are dual earth terminal and the two ends of the c.p.c. terminated separately at the DB (and both sets of live conductors connected to a single OPD). Doesn't this comply with 543.7.2.101(i) - the ring c.p.c. providing two paths from any point to the earth bar?

- Andy.
 04 September 2012 10:24 PM
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mawry

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I'm with Andy, Ring final circuit with a ring protective conductor is acceptable with Dual earth terminals
 04 September 2012 10:42 PM
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spinlondon

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The minimum CSA for a single CPC is 4mm². 543.7.1.103(ii)
Where two CPCs are used, each must meet the requirements of section 543. 543.7.1.103(iii)
The minimum CSA for a CPC when not incorporated as part of a cable is 2.5mm² if mechanically protected, 4mm² if not. 543.1.1.
If you've wired your RFC in T&E, then you have one CPC which does not meet the minimum CSA of 4mm².
You would then require a second CPC which would have to be either 2.5mm² or 4mm².
When using two CPCs, accessories with two earth terminals are required. 543.7.1.104.
 04 September 2012 10:54 PM
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Jobbo

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Not quite getting what you are saying Spin

I inially thought that you were saying that a 4.0mm would be required between the circuits if not mechanically protected, as Andy originally suggested

BS7671 states

(c) where two or more similar radial circuits supply socket-outlets in adjacent areas and are fed from the same distribution board, have identical means of short-circuit and overcurrent protection and circuit protective conductors of the same cross-sectional area, then a second protective conductor may be provided at the final socket-outlet on one circuit by connection to the protective conductor of the adjacent circuit

As 2 conductors are now present regulation 543.7.1.104 is now applicable as Andy, again stated

Regards

Jobbo

Edited due to iPhone error
 04 September 2012 11:01 PM
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spinlondon

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No there's only one CPC, as Andy has converted the two radials into a RFC.
If he had left the two radials alone and just linked the two CPCs, there would be two CPCs, or more correctly, one CPC wired in a ring.
 04 September 2012 11:21 PM
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mawry

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But as Andy has stated above, a ring final circuit 543.7.2.101(i) a ring final circuit with a ring protective conductor is an acceptable arrangement. For socket outlet final circuits.
 04 September 2012 11:55 PM
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spinlondon

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Originally posted by: mawry

But as Andy has stated above, a ring final circuit 543.7.2.101(i) a ring final circuit with a ring protective conductor is an acceptable arrangement. For socket outlet final circuits.

Only if that ring protective conductor provides the high integrity protective conductor connection complying with the requirements of 'Regulation' sic 543.7.1.
I'd like someone to show how any conductor can meet the requirements of Regulation 543.7.1
 05 September 2012 06:06 AM
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Jobbo

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Ok, what are you referring to exactly

Looking at

(iii) Two individual protective conductors, each complying with the requirements of Section 543. The two protective conductors may be of different types e.g. a metallic conduit together with an additional conductor of a cable enclosed in the same conduit.
Where the two individual protective conductors are both incorporated in a multicore cable, the total cross- sectional area of all the conductors including the live conductors shall be not less than 10 mm2. One of the protective conductors may be formed by the metallic sheath, armour or wire braid screen incorporated in the construction of the cable and complying with Regulation 543.2.5

A ring final circuit wired in twin and earth as suggested looks fine

Regards

Jobbo
 05 September 2012 10:35 AM
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spinlondon

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Jobbo, are you thinking that an RFC has two CPCs?
 05 September 2012 11:23 AM
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davezawadi

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I think that Andy has a valid point here. You need to consider what the purpose of the dual connections of the CPC are. It is to prevent any single failure of a CPC connection from removing the means of earthing. This is provided by a suitably installed ring circuit (using multiple connections to each point, and without any spurs) as a single CPC failure is safe. It also has the advantage that the high integrity is tested each time that the ring is tested properly, which is rarely the case with other circuit topologies, unless extensive design information is available to the inspector. It would be wise to check that failure of the CPC loop would not take ZS out of range, but then you have all the numbers from the testing.

-------------------------
David
CEng etc, don't ask, its a result not a question!
 05 September 2012 12:48 PM
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spinlondon

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The whole point of the high integrity earth conductor connection, is to guard against a failure.
One option is to use a single CPC of sufficient CSA so as that failure is unlikely, 10mm² if not mechanically protected, 4mm² if both copper and mechanically protected.
The other option is to use two individual CPCs both of which should be capable of acting as a CPC on their own, no good if one of the CPCs fail and the other isn't up to the job.
Where two CPCs are used, dual connections are required, no point in having two CPCs if one failed connection will take out both CPCs.

The CPC of an RFC wired in T&E is unlikely to satisfy the 10mm² or 4mm² requirement, and use of dual connectors will not help guard against failure.
In fact use of dual connectors will increase the possibllity of failure by introducing more points where failure can occur.
 05 September 2012 03:45 PM
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Parsley

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As long as the requirements of 543.1.3 have been met, I can't see where 2.5mm T&E wired in a ring configuration with a 1.5mm cpc can't be used as long as dual earth sockets and separate terminals in the DB have been provided.

Whether T&E should be used in the first place if it's an commercial building is a another question (434.5.2)

Regards
 05 September 2012 07:16 PM
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AJJewsbury

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Ah, I think I see where Spin's coming from now - I think he's saying that a ring shaped c.p.c. is a single c.p.c. not a duplicated one. I can see how section 543.7 could be read that way - indeed if broken you could argue that resulting Zs might be too high so it wouldn't then functional correctly as a c.p.c..

I don't think that's the traditional interpretation though - I'm pretty sure a simple ring c.p.c. as long been accepted as a 'duplicated' c.p.c. - (going back to the 15th with single sockets and a prohibition on spurs if I recall correctly, although without a duplicate terminal requirement). I think GN 8 explicitly states that ring c.p.c. with separate terminations does satisfy the requirements of 543.7.

- Andy.
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