RCD/MCB trip

[Ghummy]

Hello wonderful people, just wanted some advice if that is ok.

3 days ago, around 9pm, we were sitting in the living room watching tv, all of the sudden we hear an MCB trip. when checked it was for the "sockets" circuit in the kitchen.

we have a separate consumer unit for the kitchen as it was refurbished and re-wired last year. at the time, there were no appliances running other than the fridge/freezer and chest freezer.

turned it back on and all has been fine, until this afternoon, lost all power in the kitchen, when checked only the RCD had tripped and NOT and MCB.

unable to turn on the mcb even after turning off all the other MCB's. call to me trusty electrician as my attempts to diagnose end there. he advised me to unplug everything and turning the RCD back on, as this will tell him if the fault is with the wiring or one of the appliances.

we duly did this, i unplugged all appliances and presto, the RCD was able to turn back on. my electrician stated to me over the phone that the problem is one of the appliances and likely not the wiring.

i slowly plugged in appliances one by one, the washing machine now has finished its cycle as it stopped mid way previously, the fridges are on and the t'other half has even had a shower (as the 10mm cable is on the new board as it was a new feed)

am i being naive here thinking something would have tripped straight away or is it a waiting game to see whats what?

also, are both the above incidences linked? as both devices provide a complete different function?

any help would be greatly appreciated.

 

[ericmark]

MCB trip is a concern, the RCD trip is not really as they do tend to trip from time to time.

So only thing running is fridge/freezer and chest freezer, so since now running the only thing I can think of is the auto defrost, so if you find frost building up then you will know why, but unless it trips again there is really nothing you can do, you can only wait and see if it happens again.

 

[JohnW2]

turned it back on and all has been fine, until this afternoon, lost all power in the kitchen, when checked only the RCD had tripped and NOT and MCB. .... unable to turn on the mcb even after turning off all the other MCB's.

Something there doesn't seem to add up - if the MCB had not tripped, why would you be trying to switch it back on? Could you clarify?

If/when an MCB (or RCD) does trip, it is sometimes necessary to ('firmly') push its lever 'right down' before it can be reset by 'pushing it up'.

Kind Regards, John

 

[Ghummy]

Something there doesn't seem to add up - if the MCB had not tripped, why would you be trying to switch it back on? Could you clarify?

If/when an MCB (or RCD) does trip, it is sometimes necessary to ('firmly') push its lever 'right down' before it can be reset by 'pushing it up'.

Kind Regards, John

thank you, please allow me to clarify

3 days ago, only the MCB tripped, the RCD remained on so i only lost power to sockets. other circuits for lighting, shower and oven remained powered.

the only appliances at the time being powered was the fridge freezer and chest freezer

today, only the RCD tripped, but not the mcb, but lost power to all electric in the kitchen. appliances on was fridge and freezer and extractor fan as we were using the gas hob

the RCD was firmly stuck in the off position, i tried to push it down as you say but it remained stuck.

on the advice of my electrician, he advised me to turn off all of the MCB and unplug everything before turning back on one by one then plugging in things one by one.

 

[JohnW2]

... today, only the RCD tripped, but not the mcb, but lost power to all electric in the kitchen. appliances on was fridge and freezer and extractor fan as we were using the gas hob ... the RCD was firmly stuck in the off position, i tried to push it down as you say but it remained stuck.

OK. So, when you said you couldn't reset the MCB, you meant the RCD?

on the advice of my electrician, he advised me to turn off all of the MCB and unplug everything before turning back on one by one then plugging in things one by one.

If you cannot turn the RCD back on with all of the MCBs turned off then either (a) the RCD is faulty or (b) there is a "neutral-to-earth" fault on one of the circuits (standard MCBs don't switch the neutral). In either case, you probably would need an electrician to investigate but, in the meantime, you could try unplugging everything in the house (or switching it off with a local 'isolator' if one is present (e.g. for a cooker or boiler), and see if you can then reset the RCD (and, if you can, start plugging things back in, one at a time, in an attempt to identify the culprit).

KInd Regards, John

 

[Ghummy]

OK. So, when you said you couldn't reset the MCB, you meant the RCD?
If you cannot turn the RCD back on with all of the MCBs turned off then either (a) the RCD is faulty or (b) there is a "neutral-to-earth" fault on one of the circuits (standard MCBs don't switch the neutral). In either case, you probably would need an electrician to investigate but, in the meantime, you could try unplugging everything in the house (or switching it off with a local 'isolator' if one is present (e.g. for a cooker or boiler), and see if you can then reset the RCD (and, if you can, start plugging things back in, one at a time, in an attempt to identify the culprit).

KInd Regards, John

yes i meant RCD was in the jammed off position today, any more force attempting to turn it on would have physically broken it. the cosumer unit is seperate from the rest of the house so unplugging the whole house makes no sense to me?

as soon as we unplugged everything in the kitchen, we were able to turn the RCD on with east, in fact the switch had no resistance at all unlike before unplugging.

we plugged in appliances one by one and since posting this PM, all is holding and working as should.

 

[JohnW2]

yes i meant RCD was in the jammed off position today, any more force attempting to turn it on would have physically broken it. the cosumer unit is seperate from the rest of the house so unplugging the whole house makes no sense to me?

Oh, fair enough - yes, all that needs to be unplugged/switched off are things supplied by that consumer unit (and protected by the RCD in question).

as soon as we unplugged everything in the kitchen, we were able to turn the RCD on with east, in fact the switch had no resistance at all unlike before unplugging. we plugged in appliances one by one and since posting this PM, all is holding and working as should.

OK - so I suppose /fear that the most likely explanation is that one of the appliances (or, conceivably, the wiring/accessories) probably has an intermittent fault. I can but suggest that if the RCD trips again, you unplug things (in the kitchen) one at a time and determine if/when you can reset the RCD.

Kind Regards, John

 

[Ghummy]

Hello, just an update regarding this. RCD went again today, washing machine on aswell as fridge and seperate freezer. It tripped when the washing machine has 5 mins left of spin cycle.

Bingo we though . Unplugged but rcd not turning back on. All other items unplugged and the same occured

My regular Electrician called again, he came over the started at the consumer unit. Below is what he found. Bear in mind that this was newly installed last year and i have the paperwork

He firstly tested the legs of the live, earth and neutral and said he cant see anything wrong with the wiring and then discussed testing the rcd. We had a long chat about about why the mcb tripped the other day and now rcd

He went to terminate the cables and in the b32 mcb for the sockets he found the bottom screw more or less completely undone. He removed the offending mcb from the board and examined it. He showed it to me and showed me where it had overheated and discoloured the copper.

He also said that the board is new and someone has thrown in a random old mcb. He said it was a type 2 and it needs to be a type b
He also said he he thinks the one that was installed was meant for a 3 phase board and not this.

He initially tightened it then put a meter on it and said he was not happy as the voltage was fluctuating. I did ask what this means, he explained but it went over my head.

He went back to his van and replaced the mcb, asked me to put all appliances on in the kitchen whilst he was there. We duly did and it was fine.

He has left saying that he cant see any issues with the wiring and he strongly feels it was the loose mcb causing this. He left confidently saying he doesnt even think its the appliances.

I dont understand why though a loose connection on the bottom of the mcb would cause either the mcb or rcd to go? I would have thought only the mcb due to overheating? Or is there some other wizzardary to RCDs

 

[Ghummy]

i have attached pictures of the MCB, he asked me to open it and see for myself as i was in disbeleif, yes, it is burned at the contacts

 

[JohnD]

An MCB can usually trip from an substantial overload or short circuit (fast) or with a thermal trip (overheating) or magnetic, slower.

In your case, it's likely that the poor connection from the loose terminal screw was overheating, and this eventually raised the MCB enough for a thermal trip.

The "loose screw" is a bugbear that people try hard to avoid, using techniques like checking the tightness of all screws before putting the cover on, or marking each one after tightening with a touch of chalk or something.

 

[Ghummy]

An MCB can usually trip from an substantial overload or short circuit (fast) or with a thermal trip (overheating) or magnetic, slower.

In your case, it's likely that the poor connection from the loose terminal screw was overheating, and this eventually raised the MCB enough for a thermal trip.

The "loose screw" is a bugbear that people try hard to avoid, using techniques like checking the tightness of all screws before putting the cover on, or marking each one after tightening with a touch of chalk or something.

thank you, it wasnt even like it was tightening, the screw was literally pi$$ing in the wind so to speak. i understand the mcb tripping from overheating as a result of a loose screw, my curiousity wants to learn how this would make the RCD to trip more than the actual MCB. the electrican said was there would have been an inbalance but this is where i got confused.

 

[flameport]

It's definitely an older one, BS3871 was replaced by BS EN 60898 in 1991.

The blackening inside is exactly what you would expect to see in a 30+ year old circuit breaker that's been tripped on overload/short circuit a number of times.

Loose connections will cause overheating which can trip the MCB. That won't trip an RCD, as that requires an imbalance between L&N.

 

[JohnD]

RCDs sometimes trip oddly. Sometimes for no reason you can find. Sometimes electronic equipment can cause it. I don't know if the poor connection was sparking or something that could cause it. I used to have an RCBO that tripped several times, until I unplugged the digital timer socket on a table lamp with a CFL. It's now got an LED in it, and has tripped, I think, once in about five years. If it happens again I'll bin it.

 

[securespark]

I have had RCDs trip due to a loose connection and subsequent buildup of heat.
But I cannot understand why a fault that caused the RCD to trip would affect the physical resistance of the switch mechanism.

 

[JohnD]

Is it one of the brands that has to be pushed further "off" before it will go to "on?"