3 phase RCCB tripped, won't reset

[naylorpd]

Hi, in my garage I have a 3 phase RCCB that has tripped and I just can't get it to reset, even with all the downstream MCBs switched off. Bit of a nuisance really as mother in law has no power to her flat above the garage and the garage door and front gate don't work.

I tried uploading photo but "The uploaded file was not an image as expected." so had to make do with PDF...sorry.

I've tried switching off every MCB that is downstream of the RCCB but even if I can get the RCCB to reset for a minute or so as soon as I switch anything on (just MCBs, not load) it trips again. And once or twice I can reset the RCCB for a minute or so, with no other MCBs on, and it will trip again.

Clearly there is a damp problem in the wall, but actually we are well past winter and ambient temperatures are rising - already around 18-20C. Everything inside the CU looks dry. Cables in the wall are in plastic pipe (UK T&E), except the incoming 3 phase which is SWA, and I guess the 3P link to the upstairs flat is also SWA - can't remember...

Just not sure where to look or what to check. Normally I would turn on the MCBs one at a time until I found which one was causing the trip - but this time it seems to be any of them, or none of them. Actually I've never really had electrical problems here.

I'm in Azerbaijan, I don't have access to a qualified electrician. Apart from design and first fix I did most of the installation myself, about 5 years ago.

Thanks

 

[Julie.]

When you trip the mcbs, the neutral is still connected, so any stray current could pass via the neutral connection through the rccb, causing the trip.

If you disconnect the neutral cable so it cannot pass any current through the rccb, and try resetting the rccb, if it cannot be reset then it's a faulty rccb, if it is ok, then you most likely have a connection between the neutral and earth.

 

[naylorpd]

The MCB to the upstairs flat is 4 pole, so neutral is also switched. And in the upstairs flat there is also a 4 pole MCB before the flat CU.

You mean disconnect the neutral on the downstream side of the RCCB? The two that go to the bus bar and the 4 pole MCB?

How would a connection between neutral and earth arise, in the absence of any work being done?

BTW, not sure what you call it, but the incoming mains cable is 3 phase & neutral only, and I have earth rods buried in the garden, with good tested earth resistance.

 

[pc1966]

If you only have phase(s) & neutral with a separate earth rod it is called TT (from the French for Earth-Earth).

Do you have any test equipment? If you feel you are safe enough to disconnect the neutral and measure the resistance (ideally a high voltage insulation test, but as a start a continuity tester or multimeter on ohms will do) to the Earth cable it ought to be completely isolated. If not then somehow/somewhere you have a fault to Earth.

What kind of cable it is? Usually a fault is the result of work or other obvious physical damage events, but it can be from rodents chewing cables or a combination of a bad installation (e.g. cable touching metal box edge) and vibration or thermal cycling causing it to gradually bite through the insulation.
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Looking at the main CU and your diagram, the 4P MCB would isolate the flat so if that made no difference to the fault it would have to be either a faulty RCCB or there is a neutral fault on one of the 3 single-phase circuits taken from the neutral before the 4P MCB.

It looks as if you can get to those 3 circuit's neutrals at the top-left busbar, so you could switch off those MCBs and remove their neutrals (ideally with ALL power off!) and see if it then works and the feed to the flat (4P MCB) are now OK.

If so replace one neutral at a time, try it with that and related 1P MCB closed, and see if any show up. If you can do a measurement/test then all the better!

 

[naylorpd]

What kind of cable it is? Usually a fault is the result of work or other obvious physical damage events, but it can be from rodents chewing cables or a combination of a bad installation (e.g. cable touching metal box edge) and vibration or thermal cycling causing it to gradually bite through the insulation.

It's a long time ago, but I have a feeling that I acquired some 5 core SWA from the main property CU to the garage. It's entirely buried and we don't have rodents. The main CU is all three phase, SWA in and out with glands etc. The CU with the problem RCCB is plastic, cables buried in the wall so no movement possible. There hasn't been any work undertaken that could have caused a damage event. I guess I could try tightening all the connections, haven't done that yet.

 

[pc1966]

Also you have it down the is a 1P RCCB for the 2nd CU (outside lights / gate / garage lights), is it really just single pole isolation?

If DP have you tried opening it?
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Try the removing 3 neutrals you can access and see if it rules out the RCCB & 4P MCB + flat supplied.

 

[naylorpd]

Also you have it down the is a 1P RCCB for the 2nd CU (outside lights / gate / garage lights), is it really just single pole isolation?

If DP have you tried opening it?
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Try the removing 3 neutrals you can access and see if it rules out the RCCB & 4P MCB + flat supplied.

I meant it was single phase, not single pole, sorry. Not 100% on the terminology!
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Anyway, 11.30pm here so time for bed! Some helpful ideas on isolating the fault for me to try tomorrow. Thanks a lot.

 

[pc1966]

Try opening it, see if it changes the main 4P incoming RCCB behaviour.
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Anyway, 11.30pm here so time for bed! Some helpful ideas on isolating the fault for me to try tomorrow. Thanks a lot.

Good luck! Stay safe!

 

[pc1966]

It is confusing because with a single phase you might have single pole switching or double pole. I have no idea of the regulations in Azerbaijan but your set-up looks not too dissimilar to what might be found here.

In the UK is it common to have only protective fuses or single-pole MCB in the phase (live) wire, but for safety switching/isolation but the general public it has to be all-pole (so 2-pole for single phase + neutral, or 4-pole for three-phase + neutral).

Tomorrow the first thing I would try is to open the RCCB in the smaller box and see if that has cleared the fault. It is possible the two RCD are similar so it is not quite tripping fast enough to stop the main three-phase 4P RCCB tripping.

If that fails, it is probably one of the 3 circuits out of the main CU so as suggested try opening the accessible neutrals with the live off at the MCBs to see if it helps identify where the fault might be.

 

[Pete999]

Hi, in my garage I have a 3 phase RCCB that has tripped and I just can't get it to reset, even with all the downstream MCBs switched off. Bit of a nuisance really as mother in law has no power to her flat above the garage and the garage door and front gate don't work.

I tried uploading photo but "The uploaded file was not an image as expected." so had to make do with PDF...sorry.

I've tried switching off every MCB that is downstream of the RCCB but even if I can get the RCCB to reset for a minute or so as soon as I switch anything on (just MCBs, not load) it trips again. And once or twice I can reset the RCCB for a minute or so, with no other MCBs on, and it will trip again.

Clearly there is a damp problem in the wall, but actually we are well past winter and ambient temperatures are rising - already around 18-20C. Everything inside the CU looks dry. Cables in the wall are in plastic pipe (UK T&E), except the incoming 3 phase which is SWA, and I guess the 3P link to the upstairs flat is also SWA - can't remember...

Just not sure where to look or what to check. Normally I would turn on the MCBs one at a time until I found which one was causing the trip - but this time it seems to be any of them, or none of them. Actually I've never really had electrical problems here.

I'm in Azerbaijan, I don't have access to a qualified electrician. Apart from design and first fix I did most of the installation myself, about 5 years ago.

Thanks

Try switching off all the outgoing circuits from the 3 phase board, resetting the RCCB, and switching the outgoing circuits back on until the RCCB trips, not very scientific but it may just enable you to get the other supplies back on, albeit not the one that trips the RCCB. hope this helps good luck.

 

[pc1966]

Thinking about your problem, it is possible you have had a fault for some time but only recently has an external change made it apparent.

Let me explain: With your sort of TT system you have an Earth rod with, say, 100 ohm resistance and to trip your main RCCB you need 15-30mA, so if you have a N-E fault in your system it will only trip if the N-E voltage is greater than 1.5-3V here so you can get that sort of residual current down your Earth rod.

Now it the ideal three-phase case the N voltage is the same as E and all are at 0V. However, the practical case of three-phase distribution will not have a perfect balance between the three phases (and maybe harmonic currents that don't cancel) so there is now a current flowing in the neutral conductor and due to its impedance you get a voltage drop along it.

So the N-E voltage seen at your home could depend on local changes to the network (route back to the distribution transformer), or to how others are using it (altering the balance and thus the neutral conductor current).

It might work fine during the day and fail early evening when more folk use ovens, etc!

 

[naylorpd]

So, tonight's testing, the RCCB did actually behave itself and not trip with everything off, and allowed itself to be reset after tripping. I didn't disconnect anything but tried to identify which circuits caused the trip. I managed to get either the outdoor circuit or the upstairs flat working without tripping for 10 minutes or more. In the apartment CU and there were some circuits more likely to cause a trip, notably kitchen sockets. But the main ring (with TV) and lights would also work for a while before tripping. Lights on their own have been working for at least 45 minutes with no trip.

• The earth resistance was pretty good, I think it was in single digits when installed.
• The N-E voltage in the garage is hovering around 3V, probably reflecting that the mains transformer is around 800m away
• there is a voltage difference between the three phases, 238V, 222V and 231V

Immediate plan is to leave just the flat lights on, and try some more circuits tomorrow. MIL spends most of the day in the main house anyway. I suspect that she may have got water on something, either in the flat or hosed down the front gate junction box

 

[pc1966]

If still tripping then something is wrong. If you are seeing 3V between N-E it is not a dead short (or you would see practically zero with the RCCB closed) but there may be another issue somewhere or an intermittent fault. Maybe even a L-N leak of some sort.

Getting hold of a Megger style of insulation tester would be great - you could check to see if there is a very high resistance from N-E at 250V or similar with the main RCCB open.

 

[naylorpd]

I vaguely recall reading something from (maybe) @Lucien Nunes that this kind of N-E voltage was normal when the neutral was grounded at the transformer a long way away. I definitely read something that made me not worry about it. I think it's been there a long time and doesn't trip anything, I have the same 3V N-E pretty much everywhere.

 

[pc1966]

The 3V is no problem at all, that is perfectly reasonable.

My point is if you measure N-E at your property with the system on (RCCB, etc, closed) then seeing 3V is a good thing as it implies E and N are separate.

It might not be a E-N fault, it could be something like an oven element being leaky, but the fact it still happened with all of the breakers opened really suggests N-E.

But it might be intermittent, and one reason for the HV sort of test is to pick up on almost-touching contacts.