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Topic Title: Mcb tripping now and again
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Created On: 02 April 2013 10:31 PM
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 02 April 2013 10:31 PM
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thomo2525

Posts: 7
Joined: 05 March 2013

I have recently come across a lighting circuit that is tripping the mcb now and again. This only happens when one of the 2 way light switches is switched on in a specific room. But it doesn't trip all the time. I can here the mcb clicking when the switch is operated to turn the light on. The light fitting is a 5 light halogen on a transformer. The other 2 rooms on the circuit are fine and only have pendant fittings so the circuit isn't overloaded. Reckon the strappers maybe giving me trouble..any ideas



Watch out for the sun...it's hot
 02 April 2013 11:54 PM
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peteTLM

Posts: 3137
Joined: 31 March 2005

dismantle enough to isolate the strappers on their own and IR test.
500v should stress it enough to show up.

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 03 April 2013 08:05 AM
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thomo2525

Posts: 7
Joined: 05 March 2013

Cheers. Could it be a switch that has a high resistance across it causing a draw on the current?
 03 April 2013 09:06 AM
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AJJewsbury

Posts: 11349
Joined: 13 August 2003

Sounds more like switch-on surge current drawn by the transformer & halogens to me. The peak current is probably different each time depending on when in the a.c. waveform the switch is closed . MCB clicking but not quite tripping is probably the magnetic trip mechanism starting to move, but there not being quite enough current to push the mechanism all the way.

Could it be a switch that has a high resistance across it causing a draw on the current?

Unlikely - Ohm's Law says that high resistances reduce current, not increase it.

Suggestions would be: swap the 20th Century incandescent technology & (wound?) transformer for something a bit less demanding (mains LEDs say); failing that, swap the (wound?) transformer for an electronic version with a soft-start feature; failing that (and provided Zs and conductor c.s.a.s are all OK), swap the B-type MCB for a C-type.

- Andy.
 03 April 2013 09:24 AM
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Jaymack

Posts: 4622
Joined: 07 April 2004

Originally posted by: thomo2525
I have recently come across a lighting circuit that is tripping the mcb now and again.

If a leakage to earth or partial short circuit, this would progressively get worse. It's most probably mis-rating of the transformer w.r.t. total wattage, and it would be more likely to trip on start up. Check that the individual lamp wattages are not being exceeded.

Regards
 03 April 2013 09:45 AM
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daveparry1

Posts: 6151
Joined: 04 July 2007

If it's a toroidal transformer it's probably the inrush current that's causing it. Either change the fitting or replace transformer with an electronic type, or subject to Zs change breaker to 10 amp or fit a c type. (all much as Andy said really)

Dave.
 03 April 2013 10:26 AM
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thomo2525

Posts: 7
Joined: 05 March 2013

Cheers guys, I'll do a Zs and possibly change the mcb to a type c 6 amp
 03 April 2013 04:12 PM
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mikejumper

Posts: 1703
Joined: 14 December 2006

Also worth looking behind all switches on the circuit to check that nothing is shorting out.
It's what I found the last time I came across this problem.
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