Nuisance RCD tripping

[AMEdinburgh]

Back in 1995, we rewired and have three consumer units one with RCD protection for power and two smaller non RCD boards mainly for lighting but also powering the alarm and a single socket (with it's own built in RCD) supplying a distribution board for the phone system and fax machine.

We've had our share of nuisance tripping from faults in fridges, washing machines etc, either picked up by elimination and also at the PAT testing we do.

However we still have the occasional trip which occurs when we switch on or off a light switch which are on the non RCD boards. We've a lot (about 120) of fluorescents in 4x36W suspended ceiling luminaires and typically a switch would switch two i.e 8 tubes but there are a couple where four i.e. 16 tubes are switched. There's no pattern to it but it definitely coincides with switching on or off.

My 30 year old physics A level is not allowing me to understand why this might occur. Is there anyone who could give me an explanation I might understand.

Thanks

 

[AMEdinburgh]

hello again. Anyone online this afternoon who could offer an explanation?

 

[holmslaw]

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[AMEdinburgh]

Thank you, holmslaw.

The tripping can occur at any time, even first thing in the morning when the first light is switched on with little drain overall.

We did have a PIR a few years ago and found no fault with the circuits and with adding an extension at the moment, we shall do another when all is finished at the request of the building control officer.

We have 9PCs running all the time (screens off at night) on various UPSs and two lab machines on various RCD protected circuits. We also have a sophisticated and monitored alarm with secret video capture if any burglars are reading the thread (off one of the non RCD boxes)

All the lights are non RCD protected.

The tripping current is 30MA and last time the PIR was done the RCD was deemed faulty but tested and certified OK by the manufacturer. At that time a fridge was the cause of frequent tripping and that has been replaced and the tripping seems to go in cycles where we might have a week of a couple of times a day then weeks with no problem. Very frustrating.

 

[holmslaw]

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[AMEdinburgh]

Camera at home, at the moment, so instead of a single picture here goes the thousand works

The meter tails go into two L and N contact blocks and from there there are two 100A mains switches each in an enclosure and three consumer units.

One of the 100A goes straight to my X-ray machine which draws a very big current for a very short time but causes no problem

The other 100A goes via a SWA to my hydrotherapy room and a RCD protected consumer unit there and this is unaffected.

Consumer unit 1 has a 30MA RCD and 13 MCB. They run all of the "power" circuits, sockets round the rooms, external lights, one external socket (rarely used) an autoclave (10mm T&E for 9KW). The computers are plugged in to local UPSs in these rings

Consumer unit 2 has a 80A main switch and 8 MCB. It feeds the lights and with one way for the alarm and one way for a RCD single socket that maintains the phone fax and answering machines and one way for the door bell.

Consumer unit 3 has a 80A main switch and 6 MCB. It feeds the remaining lights

As yet we've not popped the main fuse although the meter can become disturbingly warm to the touch when we're in full swing. We did ask the electricity board to survey to see if we could have a larger fuse which can sometimes be done but an extremely objectionable man appeared whilst I was out and wanted to pull the fuse without us having time to switch everything off and wouldn't wait the few minutes for me to get back to do it for him so we're still running on a domestic supply.

 

[holmslaw]

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[AMEdinburgh]

Thanks again. With some of the other threads about RCBOs I had wondered whether it would be worth exchanging the MCB for RCBOs and changing the main switch to a standard one which I have a spare one left over anyway.

All the PCs passed their PAT test last time but I didn't record the earth leakage currents which are displayed on the tester. I'll do it next time out of interest and see what the total is and also whether individual bits of kit deteriorate year to year.

It has got a little worse since I added another PC in my office and am still running my old box whilst I bring things across in an orderly fashion leaving the junk behind. I've also got 4 lasers, and inkjet and a dot matrix presumably possibly leaking slightly as well.

We'll decide what to do soon and again thanks for the advice.

 

[holmslaw]

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[AMEdinburgh]

Thanks again.

If I went down the new CU route, I would perhaps look at one of the large double, say 32 way boards and populate it in sections, replacing at least two of the exisiting CUs . That way I would reduce the number of tails coming from my blocks and neaten up the meter cupboard. I see TLC Direct have a new range of Contactum CUs but I've seen disparaging remarks about Contactum elsewhere on the site.

Is there a professional's favourite for making large boards at reasonable cost with good reliaiblity?

Finally, I still don't undetand why switching on a non RCD protecterd light coming from a differet CU causes the tripping. Is that something to do with the fluorescent starters?