Downstairs lighting issue

Andrew Parsons · Mar 13, 2019

A customers downstairs lights keep tripping (they have advised me that this has been happening for the last 7 years that they have been living there). All lights are standard type lamps and there are no low voltage transformers used on these lights.

The lighting circuit has 2 x wall lights, 4 ceiling lights and a outside light, there seems to be a 300+ volts shown at the kitchen switch (which operates one light with 4 x LED GU10 lamps). When this light is switched on the voltage drops down the 238 volts.

The lounge switch which controls 2 x ceiling lights shown 238volts.

Does anybody have any ideas ?

SparkyChick · Mar 13, 2019

Yes, you need to be very specific with your voltage measurements... is that 300v line to neutral, 300v line to earth, 300v neutral to earth?

What type of supply is it? I'm guessing based on your location that it's not a rural location on the end of a pole transformer.

You'll also need to expand on 'tripping'. Is there an RCD present? Does it trip or is it a circuit breaker that's tripping? Important difference which will guide an investigation.

Seven years is a long time... how often does it trip? Standard lamps... are you talking standard filament lamps? Have they noticed a pattern such as trip, power back on to find a bulb blown?

Have you carried out any testing other than measuring some voltages? IR testing for example?

telectrix · Mar 13, 2019

my money's on cheap bulbs (lamps for the pedantic farts) blowing and taking out the MCB.

Andrew Parsons · Mar 13, 2019

The tripping is the circuit breaker and there is no RCD on the lighting circuit as the board split between lights and then everything else is a RCD

It seems to trip whether all lights are on or not.

The voltage reading at the switch was live to earth.

The house age is approx 25 years

I do not know the distance of the transfornmer.

Lamps are a mixture of filament and LED's

I hope these answers help.

SparkyChick · Mar 13, 2019

What I was getting at with the comment about pole transformer... if it's a rural location and they are supplied by a small local pole mounted transformer, there is scope for the voltage to fluctuate more, but 300v is somewhat excessive. What's the voltage at the incomer? What are you measuring the voltage with?

So you need to be doing some dead testing... insulation resistance at the very minimum, specifically L-E and L-N to check for possible faults. I'm assuming it's a digital meter... when you test for IR, keep the test running for a while and look at the display. For example, my Megger 1731 has a virtual needle and a digital read out. Intermittent faults can show up on an extended test as a high value that suddenly dips down and then rises back up to around the same high value. This will occur randomly during a long test, but you need to be able to keep the test running for a while to see it happen.

It's been happening for 7 years... how frequently? One a year, once a month, once a week? If the client is unsure, I tend to test and check for obvious possible faults and then advise them to keep a diary of when it happens, include in this what the weather was doing (could be there is an outside light somewhere and this only happens on a rainy day with a westerly breeze because the lamp fills up with water), also what was on when it happened? Could be associated with a particular light fitting.

But as the absolute minimum, if you haven't already done so, you need to carry out a fairly thorough IR test of the lighting circuit, exercising any two way strappers etc. (i.e. remove all the bulbs, turn all the lights on and test, switch the two way switching and test again).