Domestic Earth Wiring

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My house is relatively New, Built 2001 by a National Building Company and wired by professional building contractors. No Electrical modifications have been undertaken and all electrics operate as built.
I recently loosened a Double socket live, and noticed one of the earth leads was loose albeit still located in the earth terminal. The earth wire came out of the terminal and I noticed a small electrical spark at the point of disconnection. There is an RCD installed in the socket ring main and there is No Earth Leakage detected as the RCD has not tripped.
I was puzzled by this spark so I connected a Multi-Meter between the disconnected Earth wire and the other Earth wire of the ring main and measured 90v ac between these two Ring Main earth wires. Before re-establishing the earth ring main, I measured the current through the earth ring and this was 4mA. Clearly there is a high impedance involved in this earth ring presumably associated with the RCD, but my understanding is that the RCD compares Line and Neutral currents and determines any difference would be earth leakage and trip if over 30mA is detected, but there is no direct connection of the RCD to the Earth. Also my understanding is that the Neutral is earthed at the Star Point of the Three Phase supply at the substation. So this 90v ac and 4mA current I am measuring is confusing me and may be normal.
Has any Sparky got any comment on my finding and thoughts, if so, please Educate me ?
 
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There is probably at least one more bad earth connection for you to see the '90V' measurement (probably capacitive coupling from a floating earth wire). Suggest you go round all sockets and check the connections - but switch off the breaker first. The 4mA could easily be leakage in one or more EMC filter circuit(s) in plugged in items.
 
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There is probably at least one more bad earth connection for you to see the '90V' measurement (probably capacitive coupling from a floating earth wire). Suggest you go round all sockets and check the connections - but switch off the breaker first. The 4mA could easily be leakage in one or more EMC filter circuit(s) in plugged in items.
Thanks for your suggestion, My thoughts were that 90V was rather high for a capacitive or inductive coupling as both of these earth wires are on a ring circuit and will be earthed at the consumer unit and as such would be equipotential, but of course if the earth ring is interrupted by another bad earth, this could be the result as part of the ring could be floating if there are two 'Bad' earths. By re-establishing the bad earth I have already identified then a good earth will then be re-established if there are no other bad earths.
Sounds like your suggestion is a good one. I will check all of the earths on this ring.
Thanks again
 
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My thoughts were that 90V was rather high for a capacitive or inductive coupling as both of these earth wires are on a ring circuit and will be earthed at the consumer unit and as such would be equipotential

The 90v you measured, will depend on the Ohms per volt of the meter you checked it with, but it does suggest you had something plugged into a socket which lacked an earth, but had a filter installed.
 
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The 90v you measured, will depend on the Ohms per volt of the meter you checked it with, but it does suggest you had something plugged into a socket which lacked an earth, but had a filter installed.
I used two different meters, the first was a Digital Meter and the second was an older 'Steam Driven' AVO Multi Minor, both gave a very similar result. There are couple of 'phone chargers plugged in which will be earth free. Not sure how that would effect the earth circuit characteristics though.
 
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With the power switched off (main switch on CU) you could check the resistance between the 2 earth wires, it should be <2Ω
 
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Also with the power switched off and an insulated screwdriver I would be tightening all terminals (except the incoming from the tails) as they may be loose.
 

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