mcb trips when downlights are turned off

[maudy]

HI Guys

I've had a call back to a job. It seems that a 6A light circuit mbc trips when some kitchen downlights are turned off and occasionally on. Its an intermittent fault. I replaced a light switch on this part of a circuit and looked a a poor contact on a 12v fitting (its a return to fit with parts). The circuit isn't rcd protected but surely a bad 12v contact on a downlight wouldn't trip a 6a mcb?

any ideas

 

[telectrix]

sure it's not a lamp blowing that's tripping the MCB?

 

[maudy]

sure it's not a lamp blowing that's tripping the MCB?

nope, there are some blown lamps in the kitchen downlights but no new ones have gone since I started work there. I have changed a few other light fittings on the circuit but it seems to be the kitched downlights that cause the problems.

When I was there I Zs'ed each light fitting that I changed but didn't ir the circuit (just because of the number of downlights and lamps to remove.

Think I'm going to have to go back today and do some more testing but I was just seeing if this was a known problem with 12volters or trannies or something else.

 

[Farmelectrics]

try ir test method 2 link ln and test to earth

 

[Farmelectrics]

check for any loose connections

 

[maudy]

try ir test method 2 link ln and test to earth

l and n to earth?

yea - i will when i go back today, thanks.

What are the signs that an mcb is failing, this one's really old and probably got switched more times yesterday than in the las 10 years.

 

[kingeri]

Deffo need to IR test that like farmelectrics says, but do check every termination first. Last time I had this it turned out to be a melted cable just above one of the downlighters. I actually hate downlighters. What's wrong with freakin pendants?

 

[telectrix]

so, replace the MCB. costs peanuts and takes 10 minutes. then that's 1 possibility eliminated.

 

[kingeri]

so, replace the MCB. costs peanuts and takes 10 minutes. then that's 1 possibility eliminated.

That's too logical tel! :tounge_smile:

But a good idea.

 

[Farmelectrics]

change breaker get one for under a fiver

 

[maudy]

so, replace the MCB. costs peanuts and takes 10 minutes. then that's 1 possibility eliminated.

So, here's the update.

Tested circuit dead and found no faults. Tested ir up to the point of the spotlights with no problem. Introduced the spots one by one and after 6 of them, the problem recurred. Changed breaker for a 10a one and the fault will not recur.

2 questions

Does an mcb fail safe. (I recon there's 4 amps on the circuit) or do older transformers use more juice than their attached lamps

 

[Inteificio]

All transformers use more juice than the lamps, they also have a power factor which should be taken in to account.

On that regard most modern transformers only drop a few percent.

Simple check is what is electricity normal wasted as, heat, hot transformer means inefficiency.

What do you mean by MCB fail safe? If you are asking about what currents they knock of at, there is a graph in BS7671 that shows all.

 

[spark 68]

I wonder if replacing the MCB with a C6 type may have fixed the problem rather than replacing it with a 10A type, assuming it was originally on a B6 that is, lol

 

[telectrix]

probably a tired MCB and a new 6A type B would have cured the fault just as well.

 

[Mr.Ian]

Faulty transformer ? do the lighting transformers have smoothing circuits in to cut out electrical noise?? if so they'll have capacitors and if faulty or breaking down could cause problems on switch on or even switch off.
I've had appliances that'll run fine with a duff capacitor but when you switch off they can blow the 13a plug fuse