mcb trips when downlights are turned off

[maudy]

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?

no melted cables mate but def too much load for the breaker. I brought the downlight sub circuit back on 3 at a time and when I got to 9 lights the breaker went.

Curious that it always tripped when switching the lights off though.

Other than that, yea - downlights are the work of Satan. Halolight my a&*

 

[maudy]

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.

Thanks for the reply

these are pretty old trannies - the cylinder ones.

what i mean by fail safe (and I probably use the wrong term) is that do they start to trip a lower amps as they get towards the end of their life. As with an RCD which rather than not tripping - trips too easily

 

[telectrix]

no melted cables mate but def too much load for the breaker. I brought the downlight sub circuit back on 3 at a time and when I got to 9 lights the breaker went.

Curious that it always tripped when switching the lights off though.

Other than that, yea - downlights are the work of Satan. Halolight my a&*

have you measured the actual current drawn, using a clamp meter?

 

[maudy]

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

to be honest that probably would have been better - or a delay type (what are they S- type?). But I was in ruralish essex with one local electrical wholesaler - needing a once common, but now less so common, old style breaker. Also the circuits are 1.5 wired singles in steel pipe so I knew 10A would be fine.

 

[maudy]

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

Hi Mr Ian

I wouldn't have known whether the Txs had smoothing circuits - is there a way to tell? They were heavy cylinder shaped transformers - looked at least 10 years old- probably weighed 1 kilo or so.

What you say about the duff cap is a toughie to find and fix eh?

 

[maudy]

have you measured the actual current drawn, using a clamp meter?

no - I dont have one - that did occur to me - but I also thought that if the problem was caused by a very brief spike in the current as the txs powered down i might not see it on the meter anyway.

 

[maudy]

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

I suspect you're right.

It was my conclusion, however the load was getting close to 6A. ANd the 9 kitchen lights looked like they had been installed since the board (i.e. the circuit design). And 1.5mm in steel conduit would still work fine with a 10A mcb.

 

[spark 68]

I have seen similar in the past,
where the 'designer spark' has calculated the load, or the amount of DL's by X amount of lamps times the wattage divided by the voltage (also on the amount of 230V DL lamps too), and come up with too many lamps on a given MCB, leading to erratic tripping, and then scatched his head wondering why ?

What he has failed to allow for, is the lower resistance of filament lamps when cold, or for the inrush of the transformers (not the electronic ballast types) leading to a much higher start up current than the MCB can cope with,
and the cure was to either first try a C type MCB, or in extreme cases having to split the circuits up on to seperate MCB's

 

[maudy]

I have seen similar in the past,
where the 'designer spark' has calculated the load, or the amount of DL's by X amount of lamps times the wattage divided by the voltage (also on the amount of 230V DL lamps too), and come up with too many lamps on a given MCB, leading to erratic tripping, and then scatched his head wondering why ?

What he has failed to allow for, is the lower resistance of filament lamps when cold, or for the inrush of the transformers (not the electronic ballast types) leading to a much higher start up current than the MCB can cope with,
and the cure was to either first try a C type MCB, or in extreme cases having to split the circuits up on to seperate MCB's[/QUOT

All true.

But why does the mcb trip when turning OFF the lights?

 

[spark 68]

But why does the mcb trip when turning OFF the lights?

I can only speculate here, and may be wrong, but here goes,lol

it may be back EMF from the collapsing magnetic fields of the transformers, that is if they are just good old fashioned transformers with no electronic components, combined with possibly borderline too many transformers on one circuit.

This is just speculation on my part of course, and if anyone else provides another reason I would be more than willing to listen.

 

[Mr.Ian]

Hi Mr Ian

I wouldn't have known whether the Txs had smoothing circuits - is there a way to tell? They were heavy cylinder shaped transformers - looked at least 10 years old- probably weighed 1 kilo or so.

What you say about the duff cap is a toughie to find and fix eh?

If you can get into the transformers (I know a lot a sealed) you'll be able to see any caps that are there, if they have blown or are breaking down they might have a small burn hole in them or even be split, they'll also smell of electrical burning, the normal fail sign for the cylinder type caps is to bulge on the end.
It might not be that but i'm sure the transformers will have a smoothing circuit to stop interference.
Might be worth a quick look if you can get into them.

Also a duff cap can cause a trip on switch off as I said earlier suppression caps in appliances can overload a 13a plug fuse on switch off but it will switch on and run fine.