Induced voltage?

[sparky351355]

i recentley carried out an ecr to a domestic dwelling and found the main earth was sparking when touched to the earth bar, all test result were ok , all ir's to all ccts were +299Mohm , main protective bonding conductors all verified , split board installed (16th) split with 30 mA additional protection and rcbos for lights. TNC-s earthing system. basically no problems, however i seem to be getting 72v from the earth bar to the main earth at the consumer unit. this voltage decreases as i start to turn off each cct all the way down to 0v, ive had the supply checked by DNO all ok. im using a megger mft 1720 test kit , totally bemused by what is happening,> im just wondering wether this is an induced voltage but from where ? any suggestion would be greatly welcome , many thanks.

 

[telectrix]

do a pscc and pefc. see if they're the same.

measure voltage with analogue meter.

 

[sparky351355]

these rsults were both 4.6 Ka with a Ze of 0.07ohm.

 

[SPARTYKUS]

i recentley carried out an ecr to a domestic dwelling and found the main earth was sparking when touched to the earth bar, all test result were ok , all ir's to all ccts were +299Mohm , main protective bonding conductors all verified , split board installed (16th) split with 30 mA additional protection and rcbos for lights. TNC-s earthing system. basically no problems, however i seem to be getting 72v from the earth bar to the main earth at the consumer unit. this voltage decreases as i start to turn off each cct all the way down to 0v, ive had the supply checked by DNO all ok. im using a megger mft 1720 test kit , totally bemused by what is happening,> im just wondering wether this is an induced voltage but from where ? any suggestion would be greatly welcome , many thanks.

You've given yourself the clue already.

as Tel says try it with a proper (!) meter, that isn't affected by induced voltages.

As for "from where" you've probably answered that too, from the live circuits running next to the cpcs .....inducing.......

 

[Marvo]

Firstly you should never have the installation live without the earth connected, if this was a commercial or industrial installation there could be a large enough earth current to cause injury or death.

The earth is functional as well as protective so not all earth currents are fault currents.

 

[sparky351355]

this was an empty property , im well aware of energised ccts and having the main earth disonnected ,the main protective bonding conductors were still connected and the functions of earthing!! there was no current passing so assumed this to be a "phantom voltage"!

 

[sparky351355]

will try a analogue meter and see what that gives me., many thanks people.

 

[Darkwood]

Yes! repeating Marvo.... why were circuits still live when you were disconnecting the main earth, this is not how you were taught to test; empty property or not.. and in certain set-ups can be potentially lethal to users if not a risk to yourself.
Even when you isolate the circuits you still have to show caution when disconnecting earth as network fault currents can exist, I have come across this twice in 20yrs, once where a neighbouring fault was making metal work live in the property I was in, it was traced to a faulty microwave combined with a breakdown of the DNO earth in the road so using our very good bonding system in part to return as an high impedance fault.
This is why you Isolate circuits and still heed caution as to dangerous voltages.

 

[HandySparks]

...there was no current passing so assumed this to be a "phantom voltage"!

Did you try measuring the current?

Try a clamp meter on the earthing conductor with all connections in place. See what the current is, with the system loaded and unloaded.

 

[Deleted member 72666]

this was an empty property , im well aware of energised ccts and having the main earth disonnected ,the main protective bonding conductors were still connected and the functions of earthing!! there was no current passing so assumed this to be a "phantom voltage"!

Are you sure about that? How do you account for the 'sparking' you mentioned in your opening post?

 

[Deleted member 72666]

Yes! repeating Marvo.... why were circuits still live when you were disconnecting the main earth, this is not how you were taught to test; empty property or not.. and in certain set-ups can be potentially lethal to users if not a risk to yourself.
Even when you isolate the circuits you still have to show caution when disconnecting earth as network fault currents can exist, I have come across this twice in 20yrs, once where a neighbouring fault was making metal work live in the property I was in, it was traced to a faulty microwave combined with a breakdown of the DNO earth in the road so using our very good bonding system in part to return as an high impedance fault.
This is why you Isolate circuits and still heed caution as to dangerous voltages.

Tell me about it! I've actually experienced this myself in fact this ground network current was still present when we had a power outage when the sub or part of the sub was down.