Flood lights occassionally trip MCB

[ewillday]

Hi all, any help appreciated...

The short story is that I built a new lighting circuit that has one switch and 5 already installed flood lights around a sports changing room. 6amp MCB in conjunction with a 30mA RCD. Everything tested out sweet when I installed it. Oh, the floods are rated at 500W but three have 330W bulbs and two have 130W bulbs. Total 1250W so current draw a bit under 5.5A.

Currently (no pun intended), every now and then the MCB trips the RCD doesn't. If you reset the MCB it doesn't trip again that evening no matter how many times you turn them off and on, it only seems to do it the first time and even then not always. I've taken the circuit apart and tested all the lights and each piece of cable, continuity, IR, visual, praying to the gods.

I'm now persuing a war of attrition and removed one light from the circuit to see if I still get the problem and will work my way around and hopefully find an errant light. As it only fails occassionally this could take a while to fault find.

Does anyone have a clue what could be causing this?

Cheers!

 

[Richard Burns]

This is probably down to the initial current draw of a cold lamp which can be high (up to 10 times) because you are using incandescent lamps close to the current rating. If the lamps draw over 5 times the quiescent current then the MCB will trip in 0.1s. Once they have been slightly heated then they will come on OK thereafter, until they cool down completely again.
If the cable will take it (unless you are covered in thermal insulation) you could try going for a 10A MCB or if not try a type C, but a bit OTT and need good Zs.

 

[ian.settle1]

Are you talking about normal Tungsten Halogen lamps?

If you are where did you get 330w and 130w lamps from as I have only seen 300w and 120w

 

[telectrix]

and 150 watt are common. you may find a damaged/burnt connection on the end of a lamp. this could arc and cause the problem.

 

[ewillday]

Are you talking about normal Tungsten Halogen lamps?

If you are where did you get 330w and 130w lamps from as I have only seen 300w and 120w

They are R7S lights which I got from Rexel Senate. They are lower wattage that give a higher incandescence.

 

[ewillday]

This is probably down to the initial current draw of a cold lamp which can be high (up to 10 times) because you are using incandescent lamps close to the current rating. If the lamps draw over 5 times the quiescent current then the MCB will trip in 0.1s. Once they have been slightly heated then they will come on OK thereafter, until they cool down completely again.
If the cable will take it (unless you are covered in thermal insulation) you could try going for a 10A MCB or if not try a type C, but a bit OTT and need good Zs.

Thanks Richard, I'm inclined to think that you may have hit the nail on the head. I don't think there is a problem with bulbs or their connections as I've replaced all the bulbs and checked the connections (thanks for the suggestion Telectrix).

The cable isn't running through any insulation so I'll try the 10A MCB option first and investigate the second option only if that fails. Cheers All!