5 amp earth leakage?

[Richard Measures]

Hi all

Suggestions please. I’m working on a 3 phase shop install with a bunch of typical circuits. As each cct is energised the earth leakage rises to around 5 amps! It’s a PME supply via SWA with main earth coming from the armour. I’m pretty sure it’s not the individual ccts or appliances, even the lights add leakage!

So - main earth problem maybe?

 

[pc1966]

Firstly how are you measuring the leakage?

If you have a clamp ammeter around 3P+N then 5A is indeed quite a concern and probably indicates an insulation fault such as N-E short.

However, if you are clamping the whole SWA cable, or the main earth conductor, then quite probably it is just PME currents flowing to bonded extraneous parts such as service pipes or extensive buried steelworks that present a very low parallel impedance for the neutral current in the PEN conductors.

 

[pc1966]

Secondly how big is this installation? I.e. total current and types of loads used?

Some things like large LCD displays or computer servers have a few mA or leakage anyway, so if your installation has a thousand of them you might have a simple answer!

 

[Rockingit]

Secondly how big is this installation? I.e. total current and types of loads used?

Some things like large LCD displays or computer servers have a few mA or leakage anyway, so if your installation has a thousand of them you might have a simple answer!

Absolutely. Have this problem in my world on occasions, 7A cumulative is my record so far.

This is where a soft wire loop type current meter wins out over a clamp everytime - strap around all four conductors and see what you've got, as well as purely checking the cpc with bonding disconnected.

 

[timhoward]

The level of my concern (and degree of care I'd take with any further steps) would depend on the loading of the installation at the time of this test. If it was doing this with a really light load I'd be very concerned and would stop and consider actions very carefully.

Otherwise if I wanted to reassure myself I think my approach would be isolating loads, carefully disconnecting main earth and Loop testing
1) Line-Neutral (without bonding)
2) Line- main earth (without bonding, i.e. Ze) which will probably match 1 if PME.
3) Line - bonding

I'd be wanting to see if without the bonding connected, the (presumed) PEN conductor has acceptably low impedance, and we are also interested in whether the bonding happens to have an exceptionally low impedance. If so you could be just measuring the results of electricity doing it's thing and following two parallel paths. (As @pc1966 said above)

If it was doing this with a really light loading, suspicions would begin that the bonding has a lower impedance than the PEN conductor, which is of course both very unlikely and a big issue if true. Great care would then need to be taken in case other nearby installations are using the local bonding as part of a Neutral path.

 

[Richard Measures]

Thanks guys.
I’m measuring with a clamp meter around the main earth.
This problem came to light while installing an RCD in the incomers to the DB that tripped as soon as load was introduced. This is pre-EICR. The panel has approx 20 ccts; lights, power rings, cooker, fan heaters.

 

[pc1966]

Thanks guys.
I’m measuring with a clamp meter around the main earth.

Not a reliable indication of a fault!

This problem came to light while installing an RCD in the incomers to the DB that tripped as soon as load was introduced. This is pre-EICR. The panel has approx 20 ccts; lights, power rings, cooker, fan heaters.

What RCD characteristics (and also why is an incomer needed here)?

Have you done a global L+N IR test to see if there is anything like a N-E fault?