Twin and Earth CPC

[Cookie]

IEC 60364-5-54 states that phase conductors 16mm2 and smaller require a CPC of the same size, over 16 to 35mm2 a 16mm2 CPC and over 35mm2 a half size CPC is required.

My questions is why am I seeing catalogs with harmonized twin and earth with reduced size CPCs over 2.5mm2? And why doesn't the half size rule kick in when wire is over 4mm2 instead of 16mm2?

 

[Julie.]

Thanks so the IET is broken down into quite small publications where as BS7671 includes the whole lot in one publication.

Yes, it is explained in BS7671

I think he has just looked at table 54.7 without reading the regulations - as I suggested earlier.

there is NO requirement under the IEC version or the UK version that the cpc is the same size

 

[pc1966]

I was always told by older sparks that Rings and reduced cpcs were due to copper shortages during the war....

Not exactly. They were introduced post-WW2 partly with a view to the shortages of materials, but also a surprisingly forward-looking realisation by the IEE that new homes would need far more outlets.

As a result it would be very wasteful to carry on with the pre-WW2 practice of many radial circuits with different fuse sizes at the DB and single unfused plugs (of which 2A, 5A, 15A and very rarely 30A existed in round-pin) depending on the appliance needs.

So the ring with one 20A or 30A fuse (as it was at the time) and many sockets due to diversity of use, and from which a common design of "13A plug" that could be fused 1,2,3,5,7,10 or 13A meant a far more efficient system.

 

[Risteard]

slight variation on the theme, I had a roll of twin and earth recently and the cpc was insulated in green/yellow, made a change... I checked and turns out this is apparently standard in Ireland. suppliers had received wrong pallet and not realised. i think if it was put on sale over here it would sale. I would buy it as it saves sleeving.

It's about 30% dearer than standard T&E though. (Of course part of that is that there's a very limited amount made due to I.S. 201 cable being for such a small market. The other major factor is the extra copper content.)

 

[pc1966]

It's about 30% dearer than standard T&E though. (Of course part of that is that there's a very limited amount made due to I.S. 201 cable being for such a small market. The other major factor is the extra copper content.)

The copper content often dominates cost these days.

Do you have an idea why the Irish standards body went for equal size / insulated CPC?

 

[Risteard]

The copper content often dominates cost these days.

Do you have an idea why the Irish standards body went for equal size / insulated CPC?

At a guess it was to harmonise with continental Europe, but that's just an assumption on my part. (Although it's different to the cable types used there. That said, NYM-J is also used frequently.)

 

[Cookie]

Yes, it is explained in BS7671

I think he has just looked at table 54.7 without reading the regulations - as I suggested earlier.

there is NO requirement under the IEC version or the UK version that the cpc is the same size

Wait- I thought table 54.3 was mandatory when the adiabatic method isn't being used?

 

[Ian1981]

Wait- I thought table 54.3 was mandatory when the adiabatic method isn't being used?

It’s one or the other, so yes technically if you don’t use table 54.7 then the thermal constraints of the protective conductor should be confirmed.

 

[davesparks]

Waste time was a bit harsh but the money we save will eventually be lost to testing, where as if we used the same size CPC we would only need to test PEFC/Ze at origin like we do PSCC.

Nonsense, the same testing is required whatever the size of the cpc.
An equal size cpc requires testing exactly the same as a reduced size one does.

 

[Mark Wright]

Continuity but it would be a bit pointless proving it's of adequate size when we would already know it was.

 

[Julie.]

Wait- I thought table 54.3 was mandatory when the adiabatic method isn't being used?

Yes, but if you are wiring standard circuits using T&E then you will meet the requirements given by the adiabatic method.

Your question was along the lines of 'why is this cable produced' (with smaller CPC) - well because it is perfectly acceptable to install if you size it correctly.

 

[davesparks]

Continuity but it would be a bit pointless proving it's of adequate size when we would already know it was.

Proving it is of adequate size is done by calculation before you start installing.

Zs proves continuity of the complete return path back to source using a higher current than is normally possible with a dead test.

 

[Mark Wright]

All I'm saying is if the CPC was the same size your concern that ADS could take place would be as much as your concern that a short Line to Neutral would trip the MCB - basically we don't even think about it because we know the Line and Neutral are of the same size so it doesn't even cross our minds we take it for granted.

 

[Ian1981]

All I'm saying is if the CPC was the same size your concern that ADS could take place would be as much as your concern that a short Line to Neutral would trip the MCB - basically we don't even think about it because we know the Line and Neutral are of the same size so it doesn't even cross our minds we take it for granted.

Unlike Line to neutral faults, line to earth faults increase the risk of dangerous mains voltage appearing on exposed conductive parts, which is why we are required to test that disconnection times are met and that earthing and bonding is correctly maintained through out the installation.
The regulations talk about that if a circuit downstream of an rcd is installed, then as long as continuity of the protective conductor is confirmed by adequate testing and that the rcd has been confirmed to provide ADS, then yes measuring earth fault loop impedance is not necessarily required on rcd protected circuits, regulation 643.7.1

 

[Mark Wright]

To me that's just all the more reason to use a CPC at the same size but anyway each to there own etc.

 

[Ian1981]

To me that's just all the more reason to use a CPC at the same size but anyway each to there own etc.

Which is fine if using any other cable than twin and cpc cable.
No ones going to wire domestic installations in conduit and singles tho are they?