Dead Short

OK 500Watts is 2A hot, more like 10-20A inruch when cold, so 10 - 20 ohms per lamp cold.  Even one is very much off the bottom of the meg ohms range.  A contimuity meter with a low ohms range might see it more accurately.

  Now 42 posts of 2 lamps is 84 lamps, so  expect between 10 ohms/84  to 20 ohms/84 . That will probably look like  a short even  to the continuity meter.

We expect all phases and N to be almost shorted to all but the highest test current lowest ohms range.


So The oddone out is E - we do not expect either L or N to short to E !!

But it works without blowing anything, so L-E short is not present.


My money is on an N-E fault.

Genset will not see that, but  somewhere -perhaps a  pinched cable in a lampholder, or at the base of the pole or underground cable spiked.


Divide and conquer ( make an all cores break in the centre - expect N-E fault in only  left or only right side only. Divide faulty half in to to quarters etc.) until it is one or two poles or the link between them,


Let us know how it goes.

Ah but... the OP said "the photo cell was removed" - so we're more in a 'lamps in switches off' kind of situation - so the lamps shouldn't appear as connected loads (just with the proviso that the L wire after the photocell is tested as it it were part of the N side).


   - Andy.

I was assuming that the testing was for the circuits on the load side of the photocell and any associated contactors or whatever, and that it was fully isolated from the supply side on all 3 lives and the neutral as well, prior to test - if not then it may be really measuring the  supply N-E link due to current flowing backwards out to the supply side of things. 

I'd presumed a photocell on the head of each lamppost - so no extra control gear needed - but that might not be right, of course.

   - Andy.

Full setup is 4 pole switch fuse from busbar fed to contactor which then feeds a 4 way mem cartridge fuse db.

Photo-cell is fed from incoming supply to contactor then stepped down to a 6 amp mcb. 

This then feeds the photo-cell and override 20amp rotary  isolator.

The system used to be fed from a high voltage step down transformer which leaked b4ecause of age and was removed from service.

The whole site was left abandon for 10 to 15 years.

It is now going to be fed from a generator.

Sorry but no 1 photo-cell controlling the contactor.

Ok, so where in that system did you take your readings?

Is the 4 pole contactor breaking L1, L2 L3 and N; or is N solid, and there are  groups of lights on  L1 or something - you say 4 way MEM fuses surely not fused neutral ?



84 lamps at 500W each, even accross 3 phases  is 15 kW (50 plus amps, say 10 times that on inrush ) per phase - it is a fairly bosky set up, especially by modern LED standards.