Earth Leakage Faults

[scragger]

I have been given the enviable? task of tracing several earth leakage faults in several medium to large shops, basically the bulk of the fault current (measured in the switchroom on an ammeter which I cannot confirm as to the accuracy. It is over 30 years old) is on the lighting circuits, the bulk of which are fluorescents with starters and ballasts although these are not that old probably 8-12 years. This is confirmed by literally turning off two of the fuseboards and the fault drops by about 5 amps.

We have carried out a random sample earth leakage test using a PAT tester and gained readings of approx 0.1 to 0.2 milliamps over 3 ballasts,however this does not allow for the control gear cooling down after isolation before testing, we have also "meggered" the fuseboard with the lowest reading being 4 megohms which although low is acceptable and this one circuit has no more of an impact than any of the others.

As you are aware this type of equipment gets extremely warm and I feel this is causing a breakdown in the insulation inside the gear, I have been told that we as a firm need to prove this to the client probably to allow the responsibilty to be passed to the lighting maintenance contractors.

I have found the info on this subject to be a little sparse?
and wonder if anyone has experience in this field and may have some useful insights as to the possible cause, also it only appears to be on older equipment, and do harmonics play a part in increasing the readings obtained at the sitchroom meter.

Thanks

 

[Graeme38]

It is worth speaking to the light fitting manufacturer to get their opinion, or contact the IEE who I am sure would be very helpful.

 

[dpc]

Conventional magnetic ballasts will produce a fair amount of third harmonics. These will sum at the neutral point of a wye-connected system, and may appear as "ground" current, depending on how your ammeter is connected. A standard ammeter will probably respond in some fashion to third harmonics.

These are not really ground currents, since they will return in the neutral conductor to the transformer. But if your neutral conductor has become open-circuited for some reason, these third-harmonic currents may be able to find an earth path back to the transformer neutral, depending on your grounding.

 

[lyledunn]

Scragger,
I investigated a similar problem. In the fluorescents both fixed wiring and the internal wiring of the fluorescent itself passed close to the ballasts and were literally squeezed between the ballast and the wall of the fluorescent enclosure. In use the ballasts warmed up and caused low insulation resistance values between live conductors and earth ( measured with a low range clamp meter). These low readings were, like yours, not apparent when the circuits were tested with the supply disconnected.
I never PAT tested individual units but protective conductor currents of 0.1ma from 1 fluorescent, would seem rather unacceptable. Regards,

Lyledunn