discharge lighting circuit tripping MCB periodically. Why?

[timmy123]

I have been working on a circuit which comprises of 10 x 70 watt high pressure sodium lights. The circuit is protected with a type "C" 10 amp MCB. The MCB is tripping out periodically. I have changed all the "Bulbs" plus the MCB. I have isolated each light fitting in turn and then checked the circuit after each isolation. The MCB is still tripping. The circuit is wired in SWA and is for lights in a pub carpark. The tripping does not seem to follow any pattern in respect of timings of trippings. I have not conducted any insulation tests yet primarily due to difficult access to the light fittings which are on 20 foot lighting poles.
Any suggestions/ ideas about my next steps would be greatly appreciated as I need to solve this problem asap due to the customer getting impatient with me!!

Many thanks

 

[aaelectric]

Remove all bulbs and do an ir test you will soon find a short of vrey low reading!

 

[Deleted member 9648]

You dont need to remove the bulbs as if carrying out a L-N IR test you will still get a reading through the gear.....just join L and N together and then IR test to earth at the source of the circuit,do not test L-N as this may damage control gear.
As above,an IR test to earth should have been your first step in solving this problem.....you state you havent done this due to difficult access and yet you have replaced bulbs and disconnected fittings??

 

[truckster]

I had a similar problem with tennis ct lights, tested the things to hell and back, eventually tracked it down to a chafed cable in the pole with was shorting to earth when the wind was blowing and rocking the poles.

 

[timmy123]

Many thanks wirepuller. I will do this testing tomorrow. I think I lost sight of the wood for the trees! I should have done exactly what you said straight away. Many thanks for getting me back on track. Cheers.

 

[timmy123]

Many thanks truckster for the information. I reckon it's going to turn out to be something pretty elusive like the problem you had. Thanks for the information. Cheers again.