florescent kitchen light tripping mcb?

[panther]

Friend of mine has a florescent kitchen light (2 way switching) and every now & then when come to switch on the light there's a bang & a blue flash seen around the switch area and trips the mcb lighting circuit. Once reset he said it can go several weeks before doing the same again. I replaced one of the switches and after another few weeks of working ok it did the same & tripped the mcb - only this time when he reset the mcb it blew again as soon as he went to turn on the light (this time using the other switch on the opposite side of the room). For now he's using lighting from the kitchen sockets until he gets the problem sorted.

Sounds like it's shorting out somewhere but I couldn't see any damage to the cable / insulation when I replaced one of the switches. I don't know if this is just coincidence but he said it started to happen after changing the starter in the light so not sure if it could be a problem with the starter, ballast, light fitting or wiring. Any pointers welcome.

 

[TicklyT]

I have seen an unconventional 2-way switching setup where both switches had their 'com' terminal connected to either side of the light fitting, and commutated between live and neutral connected to L1 and L2. That behaved the way you describe (as well as connecting either two live or two neutral conductors to the fitting when it was turned off). Any arcing at the switch contacts ended up as an arc directly between live and neutral = big flash and bang, then no lights.

 

[PrenticeBoyofDerry]

So the mcb only trips when this light is switched on?
Is this part of a circuit with other lights on, or other electrical equipment?
If so, do you know what the output watts are in total on the circuit and the size of the mcb?

 

[bernardgreen]

I have seen an unconventional 2-way switching setup where both switches had their 'com' terminal connected to either side of the light fitting, and commutated between live and neutral connected to L1 and L2.

I have also seen that method in a house where Live and neutral had been looped around all the swtches. It worked with older switches as the gaps between contacts in those switches were much more than 3mm so an arc would have a hard job to strike. But it is a NO NO with modern switches and the danger of both sides of the lamp fitting being possibly live when the lamp is "OFF".

 

[panther]

Yes it only trips out (flash bang) when you go to switch on the kitchen light. Mainly only use one of the light switches (the one I replaced) but after resetting the mcb he tried again using the other light switch but another bang and mcb trip. I couldn't say for sure if other lights / electrics are connected to the same circuit but all other lights are working ok, only turning on the kitchen light is causing the problems. There's 2 x 5amp mcb's for the lighting circuit. 1 for upstairs & 1 down. I'm told it happens even when no other lights are on.

 

[aptsys]

I would be looking at the light fitting rather than the switch. Intermittent short at the fitting or damaged cable run from switch to light fitting.

 

[mikhailfaradayski]

If you are in a position where the problem is now happening frequently, is it possible to remove the feed to the fitting, chocky block it, restore the power and flick the switches a number of times?

 

[Aragorn84]

Mainly only use one of the light switches (the one I replaced) but after resetting the mcb he tried again using the other light switch but another bang and mcb trip.

Does this mean the kitchen light has two switches?

If so, you need to read the first reply again, and see if it applies here.

 

[panther]

I got it wrong when I said it only tripped when you switched on the light, he said on all previous occasions it tripped out when you came to switch OFF the light. Any thoughts as to why it only blew when switching off the power and not when turning on? After the last trip it blew again when you switched on the light but that was very soon after resetting the mcb so the fault must still have been present when switching on. However, after leaving it off for a while he tried it again yesterday and said everything was working ok again!!

I had another look last night - replaced the starter, re-installed the tube to make sure it's seated on the contacts ok and checked the wiring at both switches. I used a digi meter & wrote down some figures but not got them with me at the mo so will post later. A couple of screw connection terminals on one of the switches seemed a bit loose. All appears to be working ok again & I tried many times switching on / off but no bang & trips - but there again he did say it can go many weeks working ok before doing it again!!

 

[Spark123]

I have seen it before when the back box (with 4 lugs) has come into contact with a terminal in the switch.
Have also seen wires caught up in the back box screws.
Couple of ideas to look at, also worth looking at what TicklyT said above to see if it is a DIY nightmare.

 

[bernardgreen]

Any thoughts as to why it only blew when switching off the power and not when turning on?

If the lamp has a wound choke in it then when switching off there will be a small arc at the switch across the opening contacts as the stored energy in the choke disapates. In a correctly wired circuit this arc breaks rapidly.

But if the switch is a change over with Live on one fixed contact ( L1 and L2 are fixed ) and Neutral on the other fixed then the small arc may trigger a larger arc cuasing a short between the Live and the Neutral via the moving ( common ) contact as it moves from one to the other fixed contacts