What’s best practice terminating 2 earths

[Jako13!]

Hi guys what’s best practice when you’ve got a CPC and the fly lead of the swa for the armour and the cores are too big to terminate into the earth bar of a consumer unit?

 

[SparkySy]

@timhoward
I think that is a practical solution and see no real issue with it as long as they are big enough to be sufficient for mechanical protection and they are identified for the circuits they protect.

 

[Lucien Nunes]

Terminating four strands of a 7-strand conductor in one terminal and three in another does not equate to creating parallel conductors in the conventional sense. Two paralleled cores of a cable need to be equal to ensure predictable current sharing where the CCC and adiabatic fault withstand capability are determined by the thermal and electrical characteristics of the cable. Within 10mm of the terminal bar, the thermal characteristics of the bar swamp those of the cable, while the electrical characteristics of the main cable length swamp those of the termination. Therefore provided both terminations are sound and can be relied upon to remain sound, the split termination will be 'better' than all conductors in one hole because both the electrical and thermal contact resistance are likely to be lower.

Notice I am not referencing the regulations here, only the physics.

 

[ipf]

Surprising how the physics and common sense angles often align themselves, ain't it?

 

[telectrix]

Terminating four strands of a 7-strand conductor in one terminal and three in another does not equate to creating parallel conductors in the conventional sense. Two paralleled cores of a cable need to be equal to ensure predictable current sharing where the CCC and adiabatic fault withstand capability are determined by the thermal and electrical characteristics of the cable. Within 10mm of the terminal bar, the thermal characteristics of the bar swamp those of the cable, while the electrical characteristics of the main cable length swamp those of the termination. Therefore provided both terminations are sound and can be relied upon to remain sound, the split termination will be 'better' than all conductors in one hole because both the electrical and thermal contact resistance are likely to be lower.

Notice I am not referencing the regulations here, only the physics.

the voice of reason speaks. my tuppence worth is that you have a copper core cpc and a steel armour also as cpc. their respective resistances can never be the same, so the same size argument can't be applied. tin hat firmly wedged over lug holes.

 

[pc1966]

the voice of reason speaks. my tuppence worth is that you have a copper core cpc and a steel armour also as cpc. their respective resistances can never be the same, so the same size argument can't be applied. tin hat firmly wedged over lug holes.

If the copper cable is able to meet all limits for adiabatic, etc, on its own then you don't have to consider the parallel SWA there.

Of course if you have a SWA-fault then it will have to deal with the fault current but typically it will be OK as most SWA armour does meet the adiabatic limit, and in the cases where it does not so you need the copper CPC, then if both ends of armour on the copper CPC then again probably OK as fault current has sort-of two paths on the armour to return.

 

[brianmoooore]

Surely the regs concerned with unequal parallel earth paths are intended to be applied where one or both of the cpcs is undersized from being able to safely carry the full fault current on its own.
In this case, where the parallel earths are a full size SWA core and the SWA armour/banjo tail, and both should be designed for the full fault current, then the unequal length argument is irrelevant.

Edit: I wrote this post without having seen pc1966's post above, as a result of having severe internet drop outs for the last month. It appears my post is echoing his.

 

[timhoward]

I think this thread has been talking about two different things at the same time, as @westward10 noted:

It may well not cause an issue but it creates parallel conductors of differing csa the length is not relevant. The suggestion of a swa core and sheath is totally different.

The OP was talking about the sheath and core as far as I can tell.
Somewhere en-route we assumed it was a more classic scenario of running out of large enough terminals on the CPC bar and we started talking about the merits (and regulations/physics) regarding splitting the 7 strands between two terminals. But I don't think this was the main point.

 

[Jako13!]

Yeah mainly labelling, say usually if it was circuit 2 it would be terminated to 2 on the earth bar but if I have an extra earth fly lead it’s then oversized for the same terminal. So wondering regs wise the way round this?