Part 2: When the Light seems at Fault, but Isn’t Faulty

Two lights. Two circuits. Both apparently causing their respective breakers to trip.  The initial fault-finding seemed straightforward – something must be wrong with the lights.

The lights were unplugged, the breaker reset, and the fittings progressively reconnected. When connecting a particular light caused the circuit to trip, that light was unplugged again.

Problem identified?
Not quite.

What we found

When the lights themselves were tested everything was OK.  Megger testing of the circuit showed there was current leakage so further investigation was required.

Some plugs may have just not been pushed fully home when plugged in. Others more likely had been left unplugged for a period during their history before the lighting upgrade (which didn’t involve unplugging lights above the chiller ceiling.) 

That dust supported the buildup of a little extra condensation in cold weather, and the dust and moisture together created a leakage path between terminals.  Just a small leakage, but in the worst-affected sockets this had gone on long enough to cause visible deterioration in the insulation and corrosion on the plug pins.  (See photo)

With the new RCBO protection installed as part of the lighting upgrade, that leakage was now enough to trip the circuits.

Why unplugging the light worked

This was the interesting part.

On one circuit, a single socket may have been contributing enough leakage to trip the breaker.  On the other, the more likely cause was cumulative leakage across several sockets.

So the test appeared to prove that the light was faulty, when in reality removing it had simply removed enough leakage for the circuit to hold – we had removed the straw that was breaking the camel’s back. But that didn’t mean the camel was fit to go on!

The fix

We cleaned the socket faces and plug pins throughout the ceiling space.  The two worst connections had both the plugs and sockets replaced. Megger testing confirmed the problem was solved.

The takeaway

This was a rare scenario. But it showed that while fault finding by elimination is useful, the last item connected isn’t always the source of the problem. Especially when current leakage is causing the breaker trips.