120v between cpc and earth bar !

[pete.willerton]

I was carrying out an r1+r2 today and when disconnecting the cpc out of the earth bar to link worth the line it sparked on removal from the earth bar....
I tested between the cpc and the earth bar and got 120v
Tncs earthing arrangement
I've carried out insulation resistance test and that was fine .
R1+r2 was fine
This fault is on 80% of the circuits

Help and if any more info is required

 

[Darkwood]

You need to shut all power down before disconnecting main earth or bonding conductors regardless of supply arrangement.... with the installation isolated this will eliminate the possibility of a dangerous situation arising if any circuit have high earth currents by design or if a fault occurs during your testing. That aside if all power has been isolated to the premises by the main switch and you still have this voltage present then its possible your install may have common earth with other supplies where design leakage is present or a network fault with the TNCS supply could see network circulating currents flowing around equipotentially bonded services.

What current do you measure and what is the nature of the building ...IE Office block, Factory, Domestic etc etc

 

[pete.willerton]

Haven't checked the current but it was in a kitchen / restaurant with everything unplugged ...

 

[Darkwood]

Regardless of unplugging everything the main earth and equip' bonding is common to all circuits and should not broken unless you power fully down, if you have any faults, any high earth currents circuits you may put yourself and others at risk or damage sensitive equipment like computers, timeclocks, digital/lcd screens which filter the noise to earth. Power fully down ensure everyone is clear of earthed metal work and see if voltage still exists.. if it does it may be external issue to premises if it has gone then re-introduce circuits one by one until you see the voltage - then investigate further... This is est done when premises are empty so to minimise risk of shock from a floating voltge on any earthed equipment.

 

[pete.willerton]

Thanks and I realise the danger with the cpc's and earths being disconnected... The premises where empty...
Think my next move is a full EICR as it's had a lot of mods ect ..

 

[Darkwood]

Have you got dimmed lighting? ....

 

[pete.willerton]

Yes but this fault was there before the dimmer was installed ... Just seemed strange to have it on the lights and sockets but then the oven circuit was ok :s

 

[Darkwood]

No-one has said it is a fault yet.. that's to be established, on bigger installs its common to have quite high currents on the earth especially when large no's of computers and/or electronic controlled lighting is present but equally this could be a fault as well which is worrying.

 

[pete.willerton]

What made me worry was nothing was plugged in any of the s/o's so basically just the installation

 

[Darkwood]

There maybe N/E fault within the installation or other related N issues or earth issues, also an external network fault may be to blame as you have a TNCS so circulating network currents may exist on the installation earthing system... this is why TNCS bonding requirements can require significantly larger earthing arrangement within the install because of the possibility of high currents from the local network. Ive just recently bonded a 50mm earth to a 16mm incoming water pipe in a factory as crazy as it sounds.

 

[pete.willerton]

At my real job we have a building with a high voltage on the earth due to the massive amount of IT equipment ... We had some harmonic equipment installed but it is still the same .
But thank you I shall investigate further ...

 

[GT1]

Induced currents in CPCs can easily reach 100v or so. But with tiny levels of current..
Do you have a high resolution clamp meter to measure the current. ?

 

[Darkwood]

If you read his first post it sparks when disconnecting this would suggest it not induction and most testers would drain the voltage down within seconds if just induced.

 

[IzzyS]

I was carrying out an r1+r2 today and when disconnecting the cpc out of the earth bar to link worth the line it sparked on removal from the earth bar....

You should disconnect the line from the MCB and/or link it to the earth bar - that's the way GN3 shows it, and it's much safer if for some reason you are unable to isolate the whole board.

EDIT: Although obviously that doesn't remove any parallel paths.