Discuss DEICReport and Main Earth Question in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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Hi there.

PME and/or TNS install; 16mm tails; 16mm main earth(s); BS1361 Type IIb 100A main fuse.

On a recent EICR i came across this:

Issue Number 1:

There's a mains incomer in the basement that has a 16mm earth run from the PME block on the mains head. There is also a 16mm connected to the mains incomer sheath. The Ze is 0.51ohms at the first consumer unit by the mains head. The Zdb on the top floor is 0.67ohms. PME and/or TNS? I''ve read about distributors leaving sheath connections on when they install PME. Obviously the figure is too high for a PME installation. Should I contact Western Power to get a definite on the means of earthing? That's one issue!

Issue Number 2:
There's a 16mm 6242Y split from the mains tails running up to the top floor of this very large house. The top floor is for 'the kids only' but looks like it used to be a separate dwelling for lodgers. It has its own consumer unit which is fed by the 16mm running up to it from the basement. The only main earth up at the top floor is the 6mm in the 6242Y. I realise from table 54 that PME requires 16mm main earth + 10mm MEBonding, so would I would put this as a Code 2 on the EICR. But doing the adiabatic equation ..... BS1361 main fuse, Zdb of 0.67ohms, measures PSCC of 1.45kA, t=0.2secs ...... I'm getting an S/mm value of 4.53mm. So should it be a Code 3?

Many thanks for any advice.
Hope you all have a good couple of weeks of eating and drinking!
 
Hi there.

PME and/or TNS install; 16mm tails; 16mm main earth(s); BS1361 Type IIb 100A main fuse.

On a recent EICR i came across this:

Issue Number 1:

There's a mains incomer in the basement that has a 16mm earth run from the PME block on the mains head. There is also a 16mm connected to the mains incomer sheath. The Ze is 0.51ohms at the first consumer unit by the mains head. The Zdb on the top floor is 0.67ohms. PME and/or TNS? I''ve read about distributors leaving sheath connections on when they install PME. Obviously the figure is too high for a PME installation. Should I contact Western Power to get a definite on the means of earthing? That's one issue! Why don't you use your meter and do a few tests to determine if it is PME or not.

Issue Number 2:
There's a 16mm 6242Y split from the mains tails running up to the top floor of this very large house. The top floor is for 'the kids only' but looks like it used to be a separate dwelling for lodgers. It has its own consumer unit which is fed by the 16mm running up to it from the basement. The only main earth up at the top floor is the 6mm in the 6242Y. I realise from table 54 that PME requires 16mm main earth + 10mm MEBonding, I doubt if this part of the house would require any main bonding if the services have already been bonded at their point of entry into the building. so would I would put this as a Code 2 on the EICR. But doing the adiabatic equation ..... BS1361 main fuse, Zdb of 0.67ohms, measures PSCC of 1.45kA, t=0.2secs ...... I'm getting an S/mm value of 4.53mm. So should it be a Code 3? The adiabatic equation should be fine to work out the size as this is a cpc in a sub main.

Many thanks for any advice.
Hope you all have a good couple of weeks of eating and drinking!

My comments in bold
 
Thanks Sintra.

I see what you mean, but this 16mm is coming straight from the link block, not via a switchfuse like it should (it must be 20m long). That's why I am approaching it - the 6mm in the 6242Y - like a main earth.

I'm not aware of how to determine an earthing system non-visually .... are you getting at the continuity between the neutral and the main earth?

Thanks
 
The adiabatic calculation can be used for your main earth

I don't think so; not if it is a PME. Main earthing conductor for PME is deemed to be a main bonding conductor and is sized from the table according to size of suppliers incoming neutral.
 
I'm not aware of how to determine an earthing system non-visually .... are you getting at the continuity between the neutral and the main earth?

Thanks


You can do a PEFC test and a PSCC test; if both results are the same then they are using the same conductors (So Line and 'CNE') and you have a TN-C-S system
 
Last edited:
Get your test kit out and start investigating....

I'm just wondering if this PME connection is an official one. Is there a DNO PME warning sticker on the service head?? If not get the DNO to visit the premises to confirm/test or otherwise. A Ze of 0.51 ohm (TNC-S) isn't unheard of on some of the older TN-S PILC distribution cables that are in the process of being converted to PME....


It may of course just be a PME service head with an external MET, that hasn't internally been connected to the neutral conductor.... So the earthing is in fact TN-S, and quite a good one too with a Ze of 0.51 ohm..
 
I don't think so; not if it is a PME. Main earthing conductor for PME is deemed to be a main bonding conductor and is sized from the table according to size of suppliers incoming neutral.

Main earthing and bonding are two totally different things.

Size of main earthing is dictated by adaibatic equation or table 54.7

A PME installation doesn't necessarily have to have bonding (if no extraneous parts). Agree that when there does need to be bonding then size of bonding is as per Table 54.8
 
I can never understand why there is always so much confusion on here with main earthing size and bonding size requirements whenever PME is involved.
 

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