EICR in Flat - cross-connected circuits, T&E tails, composite earth

[Slowbus]

I'm practising EICRs on friendly locations as I'm still in training - technically done my 2391-52 but frankly need loads more practise.
I've just done an EICR on a flat and a few things that I didn't know how to code/address came up:

1) How do you code (and look for) inter-connected circuits? I found 2 socket ring-finals had only 21ohms between them (L-L, despite each testing fine individually (both ring-continuity and IR). Finding it was pure chance; the sockets were almost randomly on one circuit or another, so I plugged into the wrong one and was a bit concerned - how do you normally spot issues like this? From a safe isolation perspective, does this warrant a C2?

2) How can you do live tests on an economy-7 CU? I can't think of a way to sensibly get a Ipf/Ze for it, let alone RCD

3) Can meter tails be composite? 100A DNO fuse, fused down to 80A after the meter, then up to each flat. Am I right in thinking that once it's been fused to 80A, it's no longer a meter tail, just a regular circuit - and so can be reduced to a 16mm cable with 10mm earth?
And, they'd used 16mm T&E with a 10mm bonding taped on the side which seems cheap, but should be ok thermally as it's fused down, right?

4) The 2nd CU (for Economy7), was also fed in the same way (80A fused switch, 16mm T&E), but it had no 10mm earthing strapped on the side - just the internal 6mm CPC.
However, it did have a 10mm link from the MET in the previous CU over to this one, so indirectly it had 16mm. Is that a compliant way of supplying adequate earth, via multiple conductors with different paths? All plastic so no eddy current issues.

 

[newfutile]

1. interconnected circuits would be a C2 mainly as there is not a direct connection ,so some type of fault ,21 ohms sounds like a circuit or load,
about 11 amps could flow though that.
2. operational limitation or come back after midnight (dead test only otherwise)
3.its a sub main after an 80 amp switch fuse , if PME ,TNC-S may need to check earth sizes ( I would imagine its equivalent to over 16mm)
4. good question I would think each supply would need its own 16mm eqivalent

 

[loz2754]

As a trainee, don't you have a mentor to go through this stuff with you? There's no way someone just starting out is in any way competent to conduct and sign off EICRs on their own with no supervision. It takes years to build up the necessary experience to do so.

 

[Pretty Mouth]

I read the OP's first paragraph as he is aware of his own inexperience, and is trying to improve his skills by carrying out mock EICRs.

4) The 2nd CU (for Economy7), was also fed in the same way (80A fused switch, 16mm T&E), but it had no 10mm earthing strapped on the side - just the internal 6mm CPC.
However, it did have a 10mm link from the MET in the previous CU over to this one, so indirectly it had 16mm. Is that a compliant way of supplying adequate earth, via multiple conductors with different paths? All plastic so no eddy current issues.

Good question, but you're actually asking quite a few things there, and it's the wrong question to start with. To start, the question is: What size earthing conductor does the installation need? Try to answer that and explain why, as best you can, and we'll take it from there.

 

[Slowbus]

Good question, but you're actually asking quite a few things there, and it's the wrong question to start with. To start, the question is: What size earthing conductor does the installation need? Try to answer that and explain why, as best you can, and we'll take it from there.

Thanks for your patience - that's right, I'm trying to get practice in whilst looking for work, so no mentor to ask but still trying to learn/stay active. And yep it was a late night garble of questions, sorry!

We were taught very little about meter tails, other than they "should" be 25mm with 16mm earth (without explanation), but obviously you see 16mm tails on older installs/sub-100A cutouts everywhere.
For sizing CPCs (and, I'm assuming this extends to the Earth conductor) I'd either use table 54.7's "equal to line conductor" for cables 16mm or below - or I'd do an adiabatic.
Ze was quite high for PME (measured at the DB - not the fused switch, possibly wrongly) at 0.33ohm - so adiabatic suggests even a 2.5mm would suffice!
And, with the 10mm that was stuck on the side, total CSA of the earth conductor is 16mm, matching the line and satisfying 54.7's minimum CSA rules.

 

[Slowbus]

As a trainee, don't you have a mentor to go through this stuff with you? There's no way someone just starting out is in any way competent to conduct and sign off EICRs on their own with no supervision. It takes years to build up the necessary experience to do so.

No, I don't have a mentor unfortunately - self-funding as an older retrainee, finished my 2391-52 and it was out the door, same with all the courses. Just trying to stay active by practising on friends' places whilst looking for work; it's tough getting started.
I do appreciate that EICRs require years of experience - but even to my inexperienced eyes I'm seeing plenty of awful/noncompliant work by both cowboys and CPS members, and that's before even thinking about drive-by fake-result EICRs for those renting. I know I don't have all the answers and I'm pretty slow - but I still think I'm being useful and learning plenty (of course, with the help of experienced folks here when I run into new situations! )

 

[davesparks]

2) How can you do live tests on an economy-7 CU? I can't think of a way to sensibly get a Ipf/Ze for it, let alone RCD

This can usually be done using the alternative Zs test method, this involves a temporary L-E link at the far end of the circuit and testing at the CU. This method was used to test RCD protected circuits before low current tests became commonly available, the method allows you to test without tripping the RCD.

3) Can meter tails be composite?

What exactly do you mean by composite?

'Meter tails' simply means the cables connected to a meter, it is not a specific type of cable and can be a few different types. Insulated and sheathed single core cables are the most common type you will encounter, especially in domestic work, this is the type cable which generally gets referred to as meter tails.

100A DNO fuse, fused down to 80A after the meter, then up to each flat. Am I right in thinking that once it's been fused to 80A, it's no longer a meter tail, just a regular circuit - and so can be reduced to a 16mm cable with 10mm earth?

After the switchfuse the cable is no longer meter tails, it's just cable.
The outgoing cable forms the distribution circuit which feeds the CU in the flat. Distribution circuits are often referred to as submains, the DNO's distrbution cable being the 'main' and further distribution circuits being 'sub-main'
The size of the cable would be calculated the same as for any other circuit, type of cable would be selected based on installation conditions and current regulations.

And, they'd used 16mm T&E with a 10mm bonding taped on the side which seems cheap, but should be ok thermally as it's fused down, right?

It is cheap yes, and was very common for a while but is less likely to be installed that way now with increased requirements for RCD protection of buried cables.
Also with a general reduction of knowledge and skill within the industry the bizarre idea that all distribution circuits must be armoured cable is appearing.

4) The 2nd CU (for Economy7), was also fed in the same way (80A fused switch, 16mm T&E), but it had no 10mm earthing strapped on the side - just the internal 6mm CPC.

It probably doesn't need the 10mm as a CPC, the 6mm is likely adequate (16mm T&E is manufactured with a 6mm earth as that is generally adequate for a circuit of that size)
The 10mm is likely a bonding conductor.

However, it did have a 10mm link from the MET in the previous CU over to this one, so indirectly it had 16mm.

That isn't an MET, an installation only has one MET which is connected to the means of earthing via the earthing conductor. Any other earth terminal after that point is just an earth terminal, also known as an earth bar, earth block, earth marshalling bar, earth marshalling terminal etc etc

There's no requirement for a CU to have a 16mm earth.

Is that a compliant way of supplying adequate earth, via multiple conductors with different paths? All plastic so no eddy current issues.

It may well have been designed with the intention that the 6mm is the CPC for each distribution circuit and the 10mm is a main bonding conductor.
But the fact we give them different names doesn't change the way electricity flows through them.
Linking the earth terminals of both CUs in the way you describe is just a sensible thing to do in that setup.

Also the regulations allow a single CPC to be used for multiple circuits, as long as it meets the requirements for the worst possible fault scenario.

 

[davesparks]

2. operational limitation or come back after midnight (dead test only otherwise)

Usually the alternative Zs test method can be used for off peak circuits.

4. good question I would think each supply would need its own 16mm eqivalent

No, as long as they are fed from the same DNO supply then the only requirement is for a 16mm earthing conductor (if standard domestic PME) connecting the MET to the means of earthing.
Everything after that point is just subject to the normal regulations for circuit design. The distribution circuits need a CPC which satisfies the adiabatic equation (6mm probably does) and the main bonding will need to be 10mm (again assuming standard domestic PME)

 

[Slowbus]

Thanks for the in-depth explanations and patience!

What exactly do you mean by composite?

By composite, I mean comprised of multiple separate cables performing the same role. I suppose it's not really that different to concentric cables, provided they take the same path - or parallel paths created by bonded conduit/SWA armour/etc, but the whole thing seemed a bit weird when I realised the 10mm wasn't bonding.

'Meter tails' simply means the cables connected to a meter, it is not a specific type of cable and can be a few different types. Insulated and sheathed single core cables are the most common type you will encounter, especially in domestic work, this is the type cable which generally gets referred to as meter tails.

Good to know, thanks for clarifying.

It probably doesn't need the 10mm as a CPC, the 6mm is likely adequate (16mm T&E is manufactured with a 6mm earth as that is generally adequate for a circuit of that size)
The 10mm is likely a bonding conductor.

My first assumption was that the 10mm was gas bonding as the meters were nearby, but this cable only went between the switchfuse and the flat's CU - hence thinking it was an attempt to bolster the 16mm T&E CPC.

That isn't an MET, an installation only has one MET which is connected to the means of earthing via the earthing conductor. Any other earth terminal after that point is just an earth terminal, also known as an earth bar, earth block, earth marshalling bar, earth marshalling terminal etc etc

Yep, makes sense. I've treated the earth bar in the "primary" CU as the MET as it has all the bonding on it - but obviously the earthing conductor to it isn't continuous, and is on a sub-main due to switchfuse. Plus the second CU gets a separate earth cable from the Ryfield board. It's more spider-web than tree. Now I've done the adiabatics I don't think it's a safety issue, but the labelling of what's what doesn't feel clean.

 

[Pretty Mouth]

No, as long as they are fed from the same DNO supply then the only requirement is for a 16mm earthing conductor (if standard domestic PME) connecting the MET to the means of earthing.

Just want to query this Dave. My understanding was that the earthing conductor for standard domestic PME was a minimum 10mm, based on the requirements for bonding conductors for PME installations, table 54.8. Is the 16mm necessary if adiabatic will allow 10mm?

 

[davesparks]

Just want to query this Dave. My understanding was that the earthing conductor for standard domestic PME was a minimum 10mm, based on the requirements for bonding conductors for PME installations, table 54.8. Is the 16mm necessary if adiabatic will allow 10mm?

You are probably right, but for some reason I have it stuck in my head that the earthing conductor needs to be 16mm with PME.
But it has been a good few years since I had anything to do with domestic and smaller supplies.

 

[7029 dave]

You are probably right, but for some reason I have it stuck in my head that the earthing conductor needs to be 16mm with PME.
But it has been a good few years since I had anything to do with domestic and smaller supplies.

In the 80's I was taught that with PME earthing arrangements we used 16mm for the main earthing conductor, some went a bit crazy and were bonding gas, water, even cross bonding with 16mm.