tt earth voltage

[midgetman]

Hi


If I disconnect earthing conductor and put a voltmeter between the earthing conductor and the earth rod how many volts should I expect?
None I would have thought.
However I get 60V with nothing plugged in and 110V with computer plugged in.

Caravan on tt system fed from tncs property - ( no earth exported )
Only 1 radial circuit with 4 sockets.

ze = 20 ohms
insulation resistance l - e, l - l = 999 M ohms +
lead supplying caravan insulation resistance as above
proven polarity
proven continuity of all conductors

Am I being dull ?

thanks

 

[Marvo]

You should never disconnect a TT earth whilst the installation is live. If you're measuring 110v on the CPC then the chassis of every appliance is also at 110v which is more than double the touch voltage allowable even under fault conditions.

You're likely to injure someone or even worse.

Okay, with the warnings out of the way, the voltage you're seeing is also not a useful test result if you're using a digital voltmeter with a high input impedance. Rather test the leakage in mA with the earth wire connected, this would be a result that may indicate whether remedial work is required.

Secondly any voltage you're seeing with the earth disconnected is comprised of two separate things; it's the result of any low insulation problems with the installation plus the standing leakage introduced by the surge arresters in the computer power supply. Neither of these things may be a problem in reality so rather stick to the official methods of testing the installation to establish what's what.

 

[Lucien Nunes]

You could get any voltage between 0 and 230 without a fault although typically it will be between 50 and 150, it only depends on the relative leakage, stray capacitance and EMC filter reactance from L to CPC versus N to CPC. As Marvo says, not only is it dangerous, a floating CPC tells you nothing, you must measure the current not the voltage.

FWIW, if I had no test results to go on except the figures you mention, I would say the insulation resistance to CPC is probably OK. Without the computer, the CPC floated to a voltage much less than half mains voltage, but with the computer (which probably contains equal value Y-capacitors to L & N) it was close to half. Therefore, the AC leakage impedance of the system as a whole is much higher than that of the EMI filter in the PC PSU.