UK Selectivity on concealed sub main supply

[John-SJW]

• The CPC (circuit protective conductor) is all the CPC conductors of all circuits that together are joined to the main earthing terminal near the DNO's fuse, joining all exposed conductive parts.
• The earth conductor connects to the DNO's earth or an earth rod from the main earth terminal.
• The bonding conductors connect extrenuous metal parts (gas pipe & water pipe, steel frame of building, etc) to the main earth terminal.

That separates them. into logical divisions. Realistically, the term includes the CPC, earthing conductor and the equipotential bonding conductors.

In this case the 6mm 'earth' wire in the 16mm T&E looks to be an earth, not a CPC as they in this installation are after the CU all joined at te CU's earth bar. Because it is in a T&E cable does not automatically make it a CPC.

 

[davesparks]

• The CPC (circuit protective conductor) is all the CPC conductors of all circuits that together are joined to the main earthing terminal near the DNO's fuse, joining all exposed conductive parts.
• The earth conductor connects to the DNO's earth or an earth rod from the main earth terminal.
• The bonding conductors connect extrenuous metal parts (gas pipe & water pipe, steel frame of building, etc) to the main earth terminal.

That separates them. into logical divisions. Realistically, the term includes the CPC, earthing conductor and the equipotential bonding conductors.

In this case the 6mm 'earth' wire in the 16mm T&E looks to be an earth, not a CPC as they in this installation are after the CU all joined at te CU's earth bar. Because it is in a T&E cable does not automatically make it a CPC.

The protective conductor in the T+E is the circuit protective conductor of the distribution circuit which feeds the CU. This may, or may not also need to perform the function of a main bonding conductor.

 

[Mark.fardon]

The protective conductor in the T+E is the circuit protective conductor of the distribution circuit which feeds the CU. This may, or may not also need to perform the function of a main bonding conductor.

Hi Dave please clarify - under which circumstances would athe CPC need also perform the function of a bonding conductor given this arrangement? ? We are not “exporting” the PME earth to an outbuilding here this is all internal to the same property.

 

[westward10]

Hi Dave please clarify - under which circumstances would athe CPC need also perform the function of a bonding conductor given this arrangement? ? We are not “exporting” the PME earth to an outbuilding here this is all internal to the same property.

You never export a PME earth you merely utilise a cpc which is derived from a PME earthing.

 

[pc1966]

It might be pedantic, but only the DNOs are allowed to run PME / TN-C networks.

 

[Mark.fardon]

You never export a PME earth you merely utilise a cpc which is derived from a PME earthing.

That’s why I put the word in inverted commas. But if, for example’, “utilising a cpc which is derived from PME supply” to a detached garage with say metal foundations needing to be bonded the cpc could also act as a bonding conductor if adequately sized for example by using 3core SWA

 

[John-SJW]

Back to the problem of this topic. The 16mm T&E cable that runs from the meter to CPU is an "earth". All the circuits in the CU (via their CPCs) run back to the DNO's earth terminal at the meter via this T&E. The T&E is extended meter tails, although split with protection, meter tails.

Bonding of the gas pipe near the meter does not use this T&E cable. If the water pipe is bonded at the other side of the building, this does not use it either. The gas & water bonding just gives gas and water pipes the same voltage potential as electrical system metal parts in a fault situation, so a full 240v is not running right across your heart killing you.

So, the size of the earth cable needs to be calculated - see John Wards vid that I posted. In his calculation I was surprised he came up with a size less than 6mm. If that calculation is applied to this installation with a result that the size is less than 6mm, then the cable can be left in place.

All it needs is RCD protection at the meter to protect the embedded T&E. Looks like a Smart RCD (auto resetable) will solve it with mcb's fitted at the CU. The 63A mcb can be ditched as long as the MCBs at the CU are not all far higher than they need to be, so the DNOs fuse does not blow.

 

[pc1966]

The 63A mcb can be ditched as long as the MCBs at the CU are not all far higher than they need to be, so the DNOs fuse does not blow.

No, if you have more than 3m then you must provide protection against a short circuit on the cable run. You cannot rely on the DNO fuse for that as it is not under your control and could be changed by them at any point in the future.

Ditching the MCB for a fuse would result in better selectivity so it is a good plan, but unfortunately there are virtually no fuse options to match typical CUs, and for currents above 32A typically they are 1.5 or 2 "modules" wide.

In that case your real problem becomes space to fit both a fused-switch and RCD, in particular as the only auto-recloser one to meet the 63A or so rating appears to be the Schneider one that is 4 modules wide already.
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That said, if the DNO can arrange to pull the fuse, you could replace the DP isolator with a DP fused-switch, then there is probably enough room for a typical 4-module "garage CU" to house the auto-recloser RCD and tidy up the mess currently present.

 

[John-SJW]

OK, keep the 63A mcb adding a smart RCD, then: overcurrent, L to N fault and L to E fault protection. Have only mcbs at the CU. All providing the 6mm earth is fine. And install 16mm tails. Cold be conflict with CU mcbs and 63A mcb.

As T&E is under plaster, need RCD protection?

I thinking, is a way to keep the T&E which is a major job to replace.

 

[pc1966]

As T&E is under plaster, need RCD protection?

That is the underlying problem!

If it were in SWA then only the fused-switch for hard cable faults would be needed, so no real selectivity issues with the CU on MCB or RCD action.

 

[westward10]

The earth in that cable is a cpc.

 

[Mark.fardon]

Back to the problem of this topic. The 16mm T&E cable that runs from the meter to CPU is an "earth". All the circuits in the CU (via their CPCs) run back to the DNO's earth terminal at the meter via this T&E. The T&E is extended meter tails, although split with protection, meter tails.

....well no - the 16mm twin & earth does not run from the meter it is fed from the small RCD board /63a mcb ( aka DB1) So why wouldnt you deem the 6mm CPC an earthing conductor ? I’d say it isn’t. No different than if the 16mm /6mm twin cable fed a garage CU is it ?

 

[John-SJW]

....well no - the 16mm twin & earth does not run from the meter it is fed from the small RCD board /63a mcb ( aka DB1) So why wouldnt you deem the 6mm CPC an earthing conductor ? I’d say it isn’t. No different than if the 16mm /6mm twin cable fed a garage CU is it ?

The 6mm T&E is still the prime route for earthing back to the DNOs main earth, irrespective what we might like to call it. The calculations will determine if it is big enough. .

 

[John-SJW]

This vid by John Ward focuses on the impedance. Combined with the first vid I posted by him, the size of the earth conductor can be calculated. It seems 0.3 ohms will be OK or the calc, but needs checking. his vid helps and does the calc.

If the earth is fine below 6mm then the T&E can remain. If not, it needs replacing. To save expense and hassle by keeping the T&E, if the 6mm earth size is fine, then use a 63A mcb and Smart RCD at the meter location. Then remove the RCDs in the CU. Bond the main earth terminal to the gas pipe, then if possible bond the CU to the water pipe.

The only caveat is that if a 10mm earth cable is run to the water pipe on the far side of the house, from the CU, the undersized 6mm conductor in the 16mm T&E may be suitable meaning the T&E can remain. I am not up on that. Others may help.

View: https://www.youtube.com/watch?v=HsWWKzjVUxE
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I just looked at John Ward's vids. By the figures he has used (typical figures) for the earth cable size, It looks like the 6mm earth conductor in the T&E is fine. The calcs give the correct size. The read off ballpark table says just keep the earth conductor size the same as the tails - they obviously side on caution in read off ballpark tables keeping the conductor size large, in case.

I changed some of the parameters here and there to represent changes in some systems, and always it came in under 6mm