MCB tripping for no apparent reason

[KennyB]

I am an amateur so would like some guidance if you would be so kind. I have an RCD with 4 MCB’s. One of these MCB’s has started tripping for no reason, It could happen while watching TV, working on the computer or while we are asleep. I found a loose earth on a socket and tightened that, but 6 hours later it tripped again. I went around the house with a Draper 13 amp socket tester and found one double socket initially gave a reading of left light on only: live/neutral reversed. So I checked out the wiring, which was fine but screws needed tightening. However, the RDC and MCB both tripped immediately. Once reset the socket tester gave a different reading, left and middle lights on: No earth! and sometimes flickered on the middle light. Could this be the cause and if it is wired correctly, could it be the socket is faulty.... There is only a single wire feeding this socket so I assume this is a spur?

I remember not too long ago, we had several power cuts in a short period..... total power loss for 30 seconds or so then back on, only to repeat again another couple of times until it went off for ages, so we went to bed. Could this have caused a fault or problem with the MCB? I thought about swapping to MCB’s around to see if it transfered it to the one next door, but it doesn’t look that easy when I had a closer look..... hence turning to some professional advice!

Thanks in advance.

KennyB

 

[PrenticeBoyofDerry]

I assume you have no test equipment, such as continuity and insulation resistance testers.
It would best to have this circuit tested out.
You could check all socket outlets on the circuits for loose connections for a start, this would include the connection back at the CU.
Isolate circuit before doing this and also mains isolate when inspecting at CU.
Have you had any work done in the house that could have damaged cables, do you have visits from mice like creatures?

 

[stem]

One of these MCB’s has started tripping for no reason.

You could check all socket outlets on the circuits for loose connections for a start, this would include the connection back at the CU.

Would a loose wire cause an MCB to trip? I thought they just detected an overload.

 

[KennyB]

Hi PBofDerry, I did think of the mouse issue. I know my neighbor has caught two mice this winter. But if it was mice chewing cables wouldn't it be less irregular or is it possible that it has to ark to trip out? I did reconnect an outside armored cable to my new workshop. I did all the wiring inside myself and have been using power-tools with no problems. I actually Isolated this cable to the workshop by pulling the fuse, which is a fuse in switch type. But still it occurs.

I have checked wiring behind any sockets that are in use and did think about doing them all, but I thought the 13 amp socket tester would speed things up? Guess I better start checking! Any ideas about the results on the faulty switch has indicates 'no earth' on the 13 amp tester?

Hi Stem, Isn't it possible an overload could be caused by a loose wire getting hot. Also, I read that power goes up the L and down the N, wouldn't a loose wire have an effect on this also? I don't know I'm a golf course manager by trade...... I can tell you anything about managing grass!

KB

 

[sherlockholmes]

Sounds like you need to hire someone with the proper test equipment to check the cables. No earth is a wee bit of a problem.

Are you sure there's no link on the timing of the trips - central heating pump kicking in for example?

 

[KennyB]

High Sherlockholms,
Definitely no link to anything else as that was part of my investigation from the start. There was no particular time it happened and could go 10 hours or ten minutes or ten seconds between trips. The MBC in question only feeds the following: half the sockets in the conservatory, all the lounge, and the sockets in the atic room. There was a spur taken off to operate my workshop but I have isolated that and besides I have never had a trip when using lights and sockets in the workshop. The one socket in the conservatory that was coming up with a 'No Earth' signal on the 13 amp tester had a standard lamp attached, which was permantly switched on at the socket and turned on at the lamp. When I found the 'No Earth' fault, I pulled the plug this morning at 10 a.m and have not had a trip since. Could this be the problem? My plan is to replace the double socket with a new one, since I can borrow one from the maintenance guy at the hotel where I work.

Can I take this opportunity to thank you guy's for the quick and helpful response.

Cheers

KB

 

[PrenticeBoyofDerry]

Have you opened the the socket outlet up that is showing no earth for a looky inside, to see if the earth is connected up? and if it is a spur check the outlet that is feeding the spur too.
I would also try the lamp in another socket outlet and see if you get a trip, could be faulty lamp!

 

[KennyB]

Hi PBof Derry, Yes, I checked inside the socket earlier today when I discovered the fault when using the 13 amp tester. there was a single wire (which made me assume it was a spur) and all 3 wires were correctly connected. My ealier post explains this in mor detail.

I cannot locate where the spur originates and assume it is from the lounge because they are both isolated when I turn off the one MBC. The conservatory is quarry tiled so difficult to trace where it originates.

Good idea, I will try the lamp in another socket tomorrow, but was holding fast on trying anything else to see if the tripping stops! If it does stop then the lamp or socket must be the reason.

KB

 

[sherlockholmes]

Hi PBof Derry, Yes, I checked inside the socket earlier today when I discovered the fault when using the 13 amp tester. there was a single wire (which made me assume it was a spur) and all 3 wires were correctly connected. My ealier post explains this in mor detail.

I cannot locate where the spur originates and assume it is from the lounge because they are both isolated when I turn off the one MBC. The conservatory is quarry tiled so difficult to trace where it originates.

Good idea, I will try the lamp in another socket tomorrow, but was holding fast on trying anything else to see if the tripping stops! If it does stop then the lamp or socket must be the reason.

KB

The spur may originate in one of the nearby outlets on the lounge ring. Have a look inside each for one that has three cables instead of two, and check those connections. Regardless of whether or not the tripping stops you should solve the missing earth issue, which may just be a bad connection at the point it is spurred from.

 

[KennyB]

Thats sounds easy enough ... Thank you, I will try that.

Bringing the problem up to date.... I had no more trips throughout the day and evening and went to bed at 10pm. Glancing at my bedside clock at 1am (because I heard the church clock chime 1 am) I noticed the power was off, which meant the MBC and RDC had tripped. I had to go and turn the RCD back on straight away because of the the fridge & freezer defrosting. I left the MCB in the tripped position until this morning. When I turned on MCB back on, it stayed in the on position but the RCD would not and kept tripping out! This is not the usual pattern and only occurred now and again. So what would have caused this change of events. I believe the night-time trip had occurred shortly after we went to bed, since the dish-washer had not finished its cycle and no this appliance is not connected to the problem MCB. I did isolate this switch from the system this morning and try resetting the RCD, but it made no difference. I plan to explore the 'No Earth' problem further and also plan to borrow a meter to do some testing from the hotel electrician..... The only problem is, I don't know how to use one, so any advice would be much appreciated.
KB

 

[KennyB]

UPDATE: It took ages to get the RCD to reset. I eventually did it by unplugging and switching off everything in the house, including te isolation switches for the emersion and power in the loft. Then went back and turned everything back on and the RCD refused to trip even if I deliberately pushed the RCD tester and reset it several times. Having power to the socket with the 'No Earth' reading it allowed me to play around with this. I started by changing to a new double socket and the 13 amp tester gave a single left light signal.... 'Live/Neutral Reverse'. I changed it back to the old double socket and got the same reading..... 'Live/Neutral Reverse'?

So I traced the spur from this socket to one across the conservatory. There were two sets of wires, which wired up correctly. I identified which one was the faulty spur and isolated it.

So it is now a question of waiting to see if either the RCD trips or this problem MBC.

Can anyone give me a quick lesson on the tester I borrowed from the spark at work? It is a Mastech. Do I set it to the right where there is the letter A with some squiggly lines beside it? If so what setting 200u, 2000u, 20m or 200m? and what tolerance of reading should I be hoping for and what do I not want to see?

Thanks Guy's
KennyB

 

[PrenticeBoyofDerry]

20Ω setting
Looking for a low resistance reading.

 

[KennyB]

There are five options in that section 2000k, 200k, 20k 2000 or 200.... I assume it is 20k?

When you say look for low resistance, what range of numbers am I hoping for? and where do I stick the pronged leads... actually in the 13 amp socket or open the back a place each one on L & N connections?

Sorry to be a pain!

KennyB

 

[KennyB]

There are five options in that section 2000k, 200k, 20k 2000 or 200.... I assume it is 20k?

When you say look for low resistance, what range of numbers am I hoping for? and where do I stick the pronged leads... actually in the 13 amp socket or open the back a place each one on L & N connections?

Sorry to be a pain!

Like the latin by the way!

KennyB

 

[PrenticeBoyofDerry]

What model is the MM?
The test process is a little more complicated than just sticking the probs in the socket outlets, TBH I am surprized that you did not ask the owner of the MM how it worked and the test procedures.
I will try find a good link to testing.
Here is a link read it carefully, it may take some sinking in but it's the best I have been able find that explains not only in diagrams but text.
http://www.tlc-direct.co.uk/Book/8.4.2.htm