LV light tripping problem

[John]

Having trawled the Google and the FAQs I could find no definitive statement on this, so I hope this is not a much asked question.

- I installed a new light in our kitchen which is a low-voltage luminaire type with in-built transformer (toroidal? circular type). Since then, the MCB has been tripping once a day or so, which is annoying to say the least. Looking through the FAQ suggested that this may be resolved by fitting a soft-start switch to allow the unit to ramp up over a period of a second or so. Is this right? If so, where can I find such a thing? If not, any suggestions as to what may be the cause?

Additionally, when it trips, it also causes problems with the alarm system, which seems to be wired into the same breaker. Is that normal? I would have though it strange for anything non-lighting to take power from the lighting ring, but then I'm (clearly!) no electrician :-)

Thanks for any suggestions (as I, and especially my wife) are going nuts.

[Andrew Gabriel]

Torroidal transformers have quite a large switch-on surge. I made a case for one (not wanting the giant ceiling rose to accomodate it which came with the lamp). I included a PTC resistor in series to prevent this effect, although I hadn't actually had any such incidents.

The alarm running from the lighting circuit is normally a good idea. You will quickly notice if the alarm's supply is dead. However, it sounds like your alarm has a dead battery if the mains failure causes a problem, unless it's simply alerting you to the failure.

[Andy Hall]

Is it defintely an MCB or an RCBO or RCD which is being tripped?

The latter two trip on earth leakage and this may therefore indicate a faulty fitting. Take a look at the model number on the breaker. More than likely it is an MCB, but it's worth checking.

If an MCB does it trip when you switch on the light or at some point thereafter? An MCB will trip on an overload condition after a period of time depending on what the overload is and the type of MCB.

What is the value of the MCB? Normally lighting circuit ones are 6A.

Does the tripping depend on how much other lighting is on? Have you tried seeing whether it is just related to this new light?

That's just laziness on the part of whoever installed it. As a minimum, it really ought to wired to a fused spur from a ring or other power circuit, and ideally have a dedicated circuit on the consumer unit.

To email, substitute .nospam with .gl

[Frisket]

Is the trip happening when you've turned the LV light on/off/on a few times in quick succession? Is the tripping MCB a B curve? If the answer to both is yes then it'll help if you fit a C curve breaker. Then you can move the lights to the new breaker and leave the alarm on it's own dedicated circuit. HTH, Richard

[ARWadsworth]

If the MCB never tripped before you changed the light then your wiring on the new light is probably the main suspect. Check it again for loose connections and strands of cable that may be outside the connection block.

As for alarms running off the lighting circuit, this is quite normal for both smoke and intruder alarms. I prefer this than a dedicated circuit on a CU as you will soon know when the MCB is down.

-- Adam

adamwadsworth@(REMOVETHIS)blueyonder.co.uk

[Rob Whitton]

My brother had the same problem. Toroidal transformers are notorious for taking a huge surge when you close the switch - depending what the voltage is at the point the switch is closed. A different breaker - type C may help. My brother couldn't get such a breaker for his ancient instalation and so changed the toroidal transformer for an electronic transformer instead.

Rob

[The Natural Philosopher]

Toroids really hammer things when they start up. May be worth replacing the fitting with a decent toroid with a soft start on it.

Mmm. No comment :-)

[The Natural Philosopher]

Thats a good idea. Source of such?

[Andrew Gabriel]

Actually, that should have said NTC resistor, of course...

Can't remember for sure, but CPC seem to have ones which look like the one I used, probably SN35042 for 100W transformer...

needs to be mounted such that if it overheats and forms an arc or explodes throwing out burning parts, it can't set light to anything. I used a large aluminium box which also includes the transformer, terminal block, and fuse.

[John]

This doesn't necessarily seem to be a switch-on surge, it just sometimes trips out, not necessarily when the lamp is on. However it had never done this prior to the installation.

It's simply an alert - it flashes a "no power" warning signal.

Thanks!

[John]

I take it a C-curve breaker is tripped by different characteristics, i.e. more related to the problems caused by a toroidal transformer?

This is not something I'm likely to tackle myself at this time to resolve, but I want to gather as much knowledge as I can so I'll know the next time!

Thanks John

[Andrew Gabriel]

Oh, in that case it's almost certainly a plain wiring fault. Most likely in the new connections or related wiring disturbed during installation.

[Frisket]

Hi John, yes a C curve breaker has different characteristics. As a rough guide B type are the most sensitive then C then D (which we occasionally have to fit to motor circuits 'cos their inrush is even worse). Incidentally, I've fitted C curve to all my lighting circuits 'cos I got p'eed off resetting the B curve every time a lamp failed. HTH, Richard

[Andy Wade]

NTC, surely?

[derek]

I've got one to hand right here. RS part No 210-673 it's a 2 amp one but they do a complete range and if you do a search on the RS website it will be revealed. They need to be sized and rated properly. They run very hot even in normal operation.

DG