Transformer tripping MCB

[rebuke]

Hello,

I've been trying to help a friend out today who's been doing up his house (yes he's notified the electrical work to the LABC etc).

In his kitchen, he's put up two sets of these - http://www.wickes.co.uk/Zepher-5-Light-Cable-Track-Kit/invt/195289 - each set has a 175W transformer. He's put them both on the same switch. The problem is if he switches them on, quite often the MCB trips out. If he uses a dimmer switch and turns them on below full, then up to full it works fine every time.

Now, the initial thought was to try going for a C-type MCB, so after checking the EFLI etc was suitable, we tried that, but it still trips. We've also done an IR test etc to make sure there isn't a fault anywhere.

I'm essentially out of ideas - the only thing I can think is could it be worth putting a capacitor across the L&N at the transformers, as I seem to remember fluorescent fittings often have one, presumably to try and reduce the startup surge?

Does anybody know if my capacitor idea is worth a try, or is there anything else we could try?

 

[electronicsuk]

A few things worth mentioning. The capacitor you're referring to on fluorescent fittings is generally there to provide power factor correction, so it's unlikely to fix your problem.

Secondly, are the transformers intended to be dimmed? If not, this could be, at least in part, to blame for your problems.

Finally, it could simply be the inrush current is too high when switching on two transformers at once, but I suspect there is more than that at play here - it seems very unlikely that they should be capable of tripping a C type, unless the load on the circuit is already very high elsewhere.

 

[rebuke]

They are designed to be dimmed, and since it happens also with a switch, I don't think that's the issue.

I also thought it was very odd that it tripped a C-type, the circuit has a few other lights and a bathroom fan on it, but all the lights are energy saving bulbs (and it trips with them all turned off), so it doesn't appear to be load related.

I might try one transformer at a time, and see if one is capable of tripping the breaker on its own, as it could be a dodgy transformer I suppose. If it is simply that both produce two much of an initial inrush then I wonder if there's a way to achieve some sort of time delay so they don't both turn on at once?

 

[electronicsuk]

If it is simply that both produce two much of an initial inrush then I wonder if there's a way to achieve some sort of time delay so they don't both turn on at once?

Simple - fit a 2 gang switch!

 

[RF Lighting]

Get a 2 way soft start dimmer. Wire it into the circuit, turn it up to full and leave it.

Turn the light on and off with the existing switch

You now have a soft start light switch.

simples

 

[tim west]

sounds like you need a choke

 

[RF Lighting]

hey, steady. He only asked a question.

 

[tim west]

 

[ricicle]

Transformers will not always trip the MCB but will do when energised at certain places on the AC sinewave (on the up iirc) Even type D's will trip if the inrush is too high.

 

[sparkyspike]

Get a 2 way soft start dimmer. Wire it into the circuit, turn it up to full and leave it.

That's a good solution. It doesn't have to be wired as a 2 way - you can still use it to replace the 1 way switch. As long as it's a push dimmer (which it will be if it's 2 way) you will have a good solution. Toroidal dimmers can be a problem, but a C curve normally cures it.

 

[RF Lighting]

True, you could put a 1 way in series with the switch if you wanted.

 

[rebuke]

My mate wants a dimmer on the circuit, so a soft start dimmer sounds like it would be perfect - I didn't know such things existed...