Finding earth leakage on 3 phase supply cables?

[hoover]

Hi. I've got a high earth leakage value on a main db supplying many 3phase cables. What's the best way to find the leakage values on each cable? Some of them can't be turned off. If I clamp around all 4 live conductors as they enter the db would this give me the earth leakage?

 

[telectrix]

use an earth leakage clamp meter.

 

[i=p/u]

thats it if you cant disconnect

 

[hoover]

And clamp what exactly mate?

 

[i=p/u]

the green and yellow one

 

[telectrix]

earthing conductors. measure the leakage current in mA. either that or isolate and IR test each phase conductor.

 

[i=p/u]

would that be to earth telectrix? might only happen when the machine is switched on and opens another switch when running

 

[telectrix]

earth leakage is a current flowing to earth via earthing /bonding conductors. this can be measured with and without loads being applied. measure first with no loads connected. then add loads one at a time to see if the reading, if any, increases. any sudden increase in the reading will show which load is causing the problem.

 

[HandySparks]

Hi. I've got a high earth leakage value on a main db supplying many 3phase cables. What's the best way to find the leakage values on each cable? Some of them can't be turned off. If I clamp around all 4 live conductors as they enter the db would this give me the earth leakage?

Yes, if you can clamp all the live conductors in a circuit (including neutral), any current detected must be the leakage to earth. (With a ring final you'll have to clamp the lives on both legs, of course.)

The difficulty will be getting a reliable leakage figure in the presence of large load currents and the magnetic influence of current in adjacent circuits.

 

[Knobhead]

The only way you will test for leakage in this situation is by clamping the cable cores. You will have so many parallel paths to earth that clamping a CPC won’t give a meaningful reading.

 

[plugsandsparks]

How big is your leakage, i had 14amps going down the earth from the DB, all caused by a large number of old fluorescent fittings. Thought i had a cross over somewhere but did a load of pulling and tugging of wires and all checked out, except between fittings, which are at high level i have not checked

 

[Engineer54]

The only way you will test for leakage in this situation is by clamping the cable cores. You will have so many parallel paths to earth that clamping a CPC won’t give a meaningful reading.

Very true, in fact on many switchboards and/or bigger DB panels that incorporate E L protection relays on out-goers, it will be via a CT that encompasses all the cores of that cable being monitored...

Clamping the CPC is only really suitable for final circuits, or for comparison purposes on larger earthing cables. In an industrial/factory installation, as Tony stated, your not going to see any meaningful or particularly accurate values clamping only the CPC's ....