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High earthing conductor current at a house

Chris Pearson

In the discussion on a similar topic concerning a bridge, JP kindly wrote:

Chris

An urgent investigation is required.

You don't say how much current is flowing in your protective conductors?

The symptoms of a lost neutral,  or a high resistance neutral, on a PME installation are ...

I can only measure to the nearest 10 - 20 mA. The current in the earthing conductor, which matches the difference between line and neutral, is highly variable, but seems to be between 10% and 15% of the line current. There are currents of a few tens of mA in the gas and water main bonds. They do not sum to the value in the earthing conductor. Even though the water supply is plastic, there may be a connexion between gas and water in the CH boiler.

Turning off the main switch reduces the current in the earthing conductor to zero. Turning off all the breakers also reduces it to zero. No one MCB abolishes the current: instead, the greater the number of closed breakers, the greater the current.

Stainless steel garden fork to kitchen sink or gas cooker gives about 0.15 V. Met to garden fork gives about 0.3 V.

Zs to a convenient socket was 0.57 Ω.

If there is a N-E fault in the installation, I suspect that it is in a ≈ 10 m length of SWA which goes from a switch-fuse adjacent to the meter, under the floorboards, to the main DB.

Investigations will continue tomorrow.

Parents

  • Above from the COP for In-service I&T of Electrical Equipment. The 5mA seems liberal for an individual appliance. If I recall correctly, earlier versions of the COP had tables indicating allowable leakage for class 1 equipment of 1mA and higher for certain heating equipment. Perhaps the increased use of filtering on much equipment put paid to those values.

    It would seem that even a small number of such appliances could quickly increase earth leakage. I recently measured things in my own home which is TT. Everything off 3mA, everything on but not necessarily drawing normal load current, 11.4mA. Reasonable spread across circuits. Used two separate Megger earth leakage devices for comparison. Didn't inspire me with confidence that the results are an accurate reflection of reality but I guess a reasonable degree of allowance needs to be made for field measurements.

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  • Above from the COP for In-service I&T of Electrical Equipment. The 5mA seems liberal for an individual appliance. If I recall correctly, earlier versions of the COP had tables indicating allowable leakage for class 1 equipment of 1mA and higher for certain heating equipment. Perhaps the increased use of filtering on much equipment put paid to those values.

    It would seem that even a small number of such appliances could quickly increase earth leakage. I recently measured things in my own home which is TT. Everything off 3mA, everything on but not necessarily drawing normal load current, 11.4mA. Reasonable spread across circuits. Used two separate Megger earth leakage devices for comparison. Didn't inspire me with confidence that the results are an accurate reflection of reality but I guess a reasonable degree of allowance needs to be made for field measurements.

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