RCD tripping

[ian cotts]

Hi, can anyone advise on where next to go please. I was installing a lighting circuit in a Village Hall. There is 2 C/U's, the one I was working on is not tripping, but no. 2 C/U has now started to trip on the rcd. It has 6 quartz halogen heaters on 3 separate mcb's , ring main and a burglar alarm. I isolated all circuits, turned off heaters, unplugged all plugs and reset the rcd. Stayed in, plug in radio, ok. Turned on kettle and trips. Unplug everything and reset again. Turned on heater and tripped. Seems to only trip when load is above a few amps. The only thing that I can remember doing that might have caused the rcd to trip is that I was pulling out the old lighting circuit cable in the storage loft. All boards are up so access is not a problem. Checked connections in C/U 2 and all okay. Had a crawl around area I was working in and can not visually see a fault.I have access to a multi function tester at work to test the C/U 2 circuits. Any pointers on where to go now would be gratefully received. Both C/U's have all circuits rcd protected. Supply is a 3 phase PME with a phase for each C/U.
Many thanks in anticipation

 

[Space cat]

Obvious question: are you sure the fault wasn't there before you started?

The symptoms you describe suggest a leak from CU2's neutral to earth. This would explain the current dependent nature of the fault and the fact that it doesn't matter which circuit carries it. Was CU1 there already or is it new? How was the old lighting connected?

Silly question: have you kept the two sets of neutrals separate all the way back through their respective CUs, joining only on the supply side?

 

[ian cotts]

Thanks for reply. C/U 1 & 2 were wired at the same time, in 1998 I believe.
The lighting circuits are all in C/U2 and seperate to C/U1. The main earth is not connected to a MET but looped across each C/U. Now you mentioned that "was the fault there before", when carrying out various other repairs this rcd has tripped, I just reset and all ok. This is now now 8 - 12 months down the line and I believe it was purely coincidental to my work. During this project I have come across some village idiot wiring, so can only assume I have disturbed another of his unusual styles. My problem is how to locate fault, I do lack fault finding experience as I have only carried out small additions in the past. I have thought about using the ramp facility on the works tester to check circuit leakage. Any other tips would be greatfully received.

 

[Space cat]

I would definitely look for a neutral-earth leak first because it fits all the symptoms. This could be inside a heater if they're permanently wired and switched only on their live sides. As a first test I would turn off the main switch and all the breakers in CU2. Turn off all the heaters and unplug everything. Now check the resistance from neutral to earth; it should be infinite.

Note that an AC leakage test will show some capacitive coupling. A ball park figure would be about 100 pF per metre of cable. Include all the cable in all the circuits on the CU. 100 pF is about 32 megohms at 50 Hz. Divide this by the cable length to get a reasonable figure for AC leakage resistance.

 

[ian cotts]

Thanks for that space cat, 1 of the wiring faults I found was the lighting circuit had the switched live connected to the loop live in the 2 flo tubes in the kitchen which I corrected yesterday, checked polarity in the lights tested and then powered up ok. I have carried out a ramp test on the rcd with no loads it did not trip at the 30mA range but went out at 60mA when 100mA selected. checked leakage on the 3 heater circuits and did not trip at all on 100mA range. operated manual test and failed to trip. I will replace rcd tomorrow. Heaters 7 & 8 on C/U2 when switched on tripped the rcd on C/U1. Have got a bit baffled by it now. Will hope rcd replacement clears fault 2morrow. I also insulation tested C/U1 and all above 299 Mohms. Have been advised to remove heater elements and disconnect burglar alarm, check sockets polarity(were ok when I did PIR 8 months ago) and power up to see if trips. Any other tips greatfully received. This has got me baffled how a circuit on another C/U can trip rcd on the other.

 

[Space cat]

This has got me baffled how a circuit on another C/U can trip rcd on the other.

That's easy. It will happen if current going out through the live side of one CU returns through the neutral side of the other one. An RCD doesn't measure earth leakage directly. It compares the live and neutral currents and expects them to be the same. If they don't match, the 'missing' current must have gone somewhere. Did it go to earth or to the wrong CU? The RCD doesn't know - or care! Meanwhile, the RCD in the other CU sees more current coming back than went out. The race is on as to which will trip first.

I would expect a straightforward wrong connection to trip an RCD on the lightest of loads. Since this isn't happening you should be looking for an unwanted link between the two sets of neutrals. With their main switches off, there should be infinite resistance between the neutral bars of the two CUs.

 

[ian cotts]

Space Cat, first thing this morning I purchesd a replacement RCCB and fitted to C/U1. Carried out the ramp check and 28mA, powered up heater circuits one by one and rest of circuits, hey presto no tripping and very small amout of leakage. Reinstated the hall and switched everything on and all ok now. Thanks for taking the trouble to reply and for all the tips. Learnt a lot of new skills on this one.

 

[Space cat]

Thanks for taking the trouble to reply and for all the tips. Learnt a lot of new skills on this one.

You're welcome.

 

[shortcircuit48]

Ian

So you have really tested nothing, other than you no the RCD trips at 28mA.

Would suggest you have not identified the problem and a bit more testing is required rather than "we will try this then try that" and hope the fault disappears?

 

[Space cat]

I really hate it when I 'fix' something without really knowing why. I nearly posted that but decided not to bother. However, since shortcircuit48 posted this:

Would suggest you have not identified the problem and a bit more testing is required rather than "we will try this then try that" and hope the fault disappears?

I have to say that I agree.