Downstairs lighting circuit MCB randomly tripping since electrician's visit last week.

[ceecjc1]

Six months ago I had downlighter units professionally installed in two rooms and everything has been working fine up until a month ago when about half the time turning on the downlighters in one particular room they would flicker and sometimes fail to start. It took 2-3 flicks of the switch to get them to come on. Once they were successfully on they remained on, no flickering. I suspected a faulty downlighter so called the electrician back.

He arrived and said there was a loose fitting in the light switch and tightened it up - the lights no longer flicker when turning on.

Unfortunately the day he fixed the flickering the entire lighting circuit MCB now trips randomly. It happens when the lights are on or off. I've turned everything off (that I know of) that's connected to that circuit but it still randomly trips.

We've lived here for three years with no problems and this MCB only started tripping the same day the electrician visited.

He's not been keen to visit, although I'm happy to pay him to come back and check it, so I wondered if some experts would agree this looks like a wiring fault triggered by his recent visit?

All input appreciated!

Cheers,
Chris

 

[ban-all-sheds]

Maybe instead of being happy to pay him to come back and check it, you should start trying to get your money back for the last visit - that might focus his mind a bit.

You are sure that it's an MCB tripping, not an RCBO?

If you have a fault tripping an MCB then somewhere you have danger.

 

[ceecjc1]

Hi ban-all-sheds,

Thanks for the response!

It's definitely the MCB. It's a single B6 MCB that's tripping and I've left it switched off now until I diagnose the fault, just in case. There is only a single RCD covering the entire house and it hasn't tripped once.

I agree with your point about paying but when he originally installed the downlighters he didn't touch the wiring in the switches, so when he visited last week to repair the flickering and told me the switch wiring was loose, I figured he was really fixing a problem which wasn't his making. That's why I assumed I would have to pay for a subsequent visit unless the problem was proven to be in his original installation.

I re-checked the wiring in the switches and it's all solid. My argument is it's possible the problem which caused the flickering is now causing the MCB to trip.

Thanks,
Chris

 

[sparkwright]

Check, with the power off, at the switch again.

Sometimes a wire can get 'pinched' against a faceplate screw, or against one of the fixing lugs.

 

[ceecjc1]

Hi Sparkwright,

I checked that nothing was pinched. There is a reasonable amount of space in the light switch back box and nothing seems pinched or caught.

Thanks,
Chris

 

[ceecjc1]

Update:

When I reset the MCB it works for five minutes then trips again, tripping immediately upon subsequent reset for another few minutes. Could it be that something is heating up and then cooling down again? Again, nothing is switched on that I know of. There is a light/fan combination in the utility room of the type which remains on for a few minutes after being switched off, which I initially suspected but even with this switched off the same thing happens.

I don't want to continue turning the MCB on just in case there is a dangerous issue somewhere but for the sake of investigation...

 

[JohnW2]

When I reset the MCB it works for five minutes then trips again, tripping immediately upon subsequent reset for another few minutes. Could it be that something is heating up and then cooling down again? Again, nothing is switched on that I know of. There is a light/fan combination in the utility room of the type which remains on for a few minutes after being switched off, which I initially suspected but even with this switched off the same thing happens.

If you are saying that the MCB trips even when absolutely nothing is 'switched on' on the circuit, then either there is something wrong within the wiring somewhere or else the MCB itself has become faulty. I may have overlooked something, but I can't think of anything else that could cause that to happen.

It takes something in excess of 10A to trip a correctly functioning B6 in 5 minutes. That's over 2.3kW being dissipated somewhere for those 5 minutes, and I would have expected that to show itself as smoke, smells or fire if it were actually happening. I would therefore probably be more inclined to point my first finger at the MCB itself.

Kind Regards, John

 

[ceecjc1]

Thanks John, that's a useful point. The MCB actually trips after only 2 minutes or so and there's no smoke or smell anywhere. There is one outdoor light and sensor on the circuit but I can't see anything wrong with it, and it's not on fire.

It could all be a coincidence that the problems started after the electrician visited, or it could be that the faulty MCB was somehow triggering the flickering lights in the first place and the loose wiring in the light switches was a red herring.

I'll get him in to take a look at it.

Cheers,
Chris

 

[JohnW2]

Thanks John, that's a useful point. The MCB actually trips after only 2 minutes or so and there's no smoke or smell anywhere. There is one outdoor light and sensor on the circuit but I can't see anything wrong with it, and it's not on fire.

To trip after two minutes would require even more power to be dissipated somewhere - so, again, I would certainly expect that to 'show itself' - imagine gripping the (dry) element of a switched-on electric kettle for a minute or two!

It could all be a coincidence that the problems started after the electrician visited, or it could be that the faulty MCB was somehow triggering the flickering lights in the first place and the loose wiring in the light switches was a red herring.

It is not impossible (although not a very common issue) that a faulty MCB could cause such flickering.

I may, of course, be completely wrong in what I'm suggesting but, as you will presumably understand, there's a degree of logic in what I'm saying.

I'll get him in to take a look at it.

It would be good if you could let us know what he discovers.

Kind Regards, John

 

[ceecjc1]

To trip after two minutes would require even more power to be dissipated somewhere

You're right, I just flicked on the MCB for the lights and the blinking LED on the meter went from flashing once every 10 seconds to once every second. All lights are off, so something with a substantial load is attached to that circuit. Given I've found nothing apart from the downstairs lights that has stopped working with the MCB tripping I guess there must be a fault somewhere! I'm gonna stop turning it back on now just in case...

It would be good if you could let us know what he discovers

Will do.

Cheers,
Chris

 

[SUNRAY]

Update:

When I reset the MCB it works for five minutes then trips again, tripping immediately upon subsequent reset for another few minutes.

No one else picked up on this point?
Classic symptoms of overload heating the MCB.

One assumption could be the high current was causing the bad joint to arc causing the flickering and limiting the current to that below the tripping level. Tightening the lose joint removed this fault and the 'high current' fault has now become apparent.

IF and I stress IF this is the case then I don't think it's too unreasonable to have been missed by the electrician. If that was me I'd have no hesitation to take it on the chin and return and I'd like to think most of the regulars here would do the same.

 

[ban-all-sheds]

there's no smoke or smell anywhere.

When was the last time you went into the loft?

[DISCLAIMER]

IHNI if that circuit goes through the loft, but I hope you take my point - there could be a fault somewhere where any smoke/smell might not be noticed in a timely manner.​

[/DISCLAIMER]

 

[ceecjc1]

Thanks for all your replies guys, much appreciated.

SOLVED: It took a while to find but we eventually found a transformer for a 12V halogen that had somehow moved in the ceiling void and was sitting right on top of the bulb. Everything was functioning but it was slowly getting fried. Presumably once it got to a certain temperature something inside the transformer was drawing a massive amount of current and tripping the MCB. Since we replaced that everything has been working fine, no more trips.

Chris

 

[JohnW2]

SOLVED: It took a while to find but we eventually found a transformer for a 12V halogen that had somehow moved in the ceiling void and was sitting right on top of the bulb. Everything was functioning but it was slowly getting fried. Presumably once it got to a certain temperature something inside the transformer was drawing a massive amount of current and tripping the MCB. Since we replaced that everything has been working fine, no more trips.

I'm pleased for you if that has solved the problem, but I thought you said that the MCB was tripping when absolutely nothing was switched on, and I would have thought that 'not switched on' would have included that light and its transformer?

Kind Regards John

 

[ceecjc1]

I'm pleased for you if that has solved the problem, but I thought you said that the MCB was tripping when absolutely nothing was switched on, and I would have thought that 'not switched on' would have included that light and its transformer?

Honestly, you're right and I am still confused and amazed that this has fixed the problem, but I wanted to keep the "solved" post simple and I can't argue with the fact that the lights haven't gone out since he replaced that transformer.

Two things still concern me:
1) The MCB was tripping when all of the lights (including this light and its transformer) were switched off, suggesting the transformer wasn't the problem
2) The lighting circuit is still drawing a large amount of current at random intervals even though everything apparently attached to it is switched off.

For point (2) the electrician wasn't really concerned and didn't quite see my point, so I may be wrong. To explain further, there's a flashing red LED on the meter which flashes faster the more electricity you're using. I guess everyone is familiar with this. The speed varies throughout the day as kettles, ovens, washing machines, electric underfloor heating, etc. are turned on and off and more or less power is used. However, at some "quiet" points of the day, with all lights off and when little power is being used, the LED goes from flashing once every 17-20 seconds to once every two seconds, suggesting a large load has been switched on, but at this point when I flick the 6A lighting circuit MCB off, the load also disappears and the LED goes back to its 17-20 second cycle. Turning back on the MCB returns the LED to flashing every 2 seconds. This only happens for a short while at a time and without watching the meter there's no way to know how often or for how long this happens, I've only just noticed it because I've been in the meter cupboard frequently the past few days.

It seems to me like something is attached to the lighting circuit which has a large load and is switching on and off periodically. This would explain why an MCB might trip. Other than the relevant lights I can't see anything in the house which isn't working when the lighting circuit is off, and nothing should be cycling on and off like this unless there's a faulty wire arcing or otherwise shorting occasionally.

It is probably unrelated but when the electrician was doing his final checks he did an RCD test and noticed the RCD does not trip under test, even if the "test" button is pressed. We need a new RCD then, and may potentially be why a fault allowed the MCB to trip before the RCD.