Options to reduce RCD tripping during power spikes

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datahawk

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#236091 17-May-2018 11:51

I have two RCD's in the main switchboard.

They rarely trip and I am confident that there are no wiring or leakage issues, however, one does trip when we experience abnormal power spikes.

I have a standby generator that will kick in automatically when there is a distinct power outage after more than 2-3 seconds and in those circumstances, the RCD is fine.

The most recent event was this week. My house/office was drawing 5-6 Amps only, no major appliances were running, only my office PCs, servers and other equipment mostly all on UPS. A quick 'double cut' outage and surge tripped the RCD again.

I'm trying to avoid this happening because the standby generator (which is there to keep the office running 24/7) becomes useless if the switchboard RCD has popped and there is no-one nearby to manually reset it.

Does anyone know of solutions to this issue or maybe other options for mains power buffering or filtering?

Lastman

302 posts

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#2017610 17-May-2018 12:48

The current through a capacitor is proportional to the rate of change of voltage. When you get large voltage spikes you can, apparently, get quite large leakage to ground in this way and hence an RCD trip.

Some expert advice on filtering etc might do the trick.

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datahawk

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#2017617 17-May-2018 13:01

Lastman: The current through a capacitor is proportional to the rate of change of voltage. When you get large voltage spikes you can, apparently, get quite large leakage to ground in this way and hence an RCD trip.

Yep. If this issue was related to one of my many DC voltage projects I would simply be fitting a huge CAP on the main supply to smooth out the Vcc.

I'm not game to try that on a 40Amp AC Mains Supply :-)

Goosey

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#2017621 17-May-2018 13:14

Have you advised the supply company? Potentially something a miss in power factor and feed to your location?

solutionz

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#2017629 17-May-2018 13:26

Any surge protectors (MOV) on that circuit that could be clamping to ground when voltages fluctuate over?

datahawk

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#2017650 17-May-2018 14:21

solutionz: Any surge protectors (MOV) on that circuit that could be clamping to ground when voltages fluctuate over?

Thanks, that is a good question. The aspect of surge protectors grounding had not occurred to me.

I'm not sure if any of the UPS units I am using contain protection but I will check the specs and I'll be doing a search of all the circuit sockets looking for any stand alone units.

Scott3

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#2017954 17-May-2018 21:05

You can get auto reset units for RCD's (no idea on NZ specific compliance). Could be a quick fix.

http://www.legrand.com.au/uploads/tx_sbdownloader/Legrand_Stop___Go_Automatic_Resetting_Device_02.pdf

If uptime / reliability is a big concern (must be if you have shelled out for a auto starting standby generator), then another thing you could consider is going with a dedicated RCD for each circuit (i.e. dedicate the existing RCD to one circuit, and replace the remaining breakers with slimline RCBO's (combined MCB/RCD). Not a cheap option, but will mean if a bit of equipment faults, it will only take down only that circuit. (plus your loads are split over more RCD's, so you are less likely for leakage to exceed 30mA). If tripping continues to be an issue, at least the circuit causing the problem will be identified.

https://www.clipsal.com/Trade/Products/ProductDetail?CatNo=RCBE232/30S&c=6&ms=7&s=35&mg=12062

datahawk

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#2018053 17-May-2018 21:40

Scott3:

You can get auto reset units for RCD's (no idea on NZ specific compliance). Could be a quick fix.

Thanks. That's the sort of solution I am looking for if there are no line surge or filtering options that can be installed just after at the meter box. I'll do some study.

Yes, zero power outage down time is important for my business, and my sanity, so I have always owned backup generators to keep servers, internet and equipment alive. I gave up on portable 'manual connect' generators more than 10 years ago and installed a full auto start, auto switching unit that runs the entire office (and house).

Best money that I ever spent. We have experienced more than 100 hours of power outage time since it was installed including an 18 hour stretch during the recent Auckland area storm where we had no down time at all.

On a side note, I was impressed that our rural Chorus cabinet and my VDSL stayed alive during those 18 hours also so it was business as usual.

andrewNZ

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#2018072 17-May-2018 22:12

It will almost certainly be mov's causing this. They're in nearly everything, including lighting power supplies.

The best option here is probably installing more RCDs so there are less circuits and therefore less leakage per RCD. Modern rules state max 3 circuits per RCD. This situation could be one of the reasons.

There is provision in the rules for 100mA RCDs for equipment protection. depending on the environment it might be an option if you have critical equipment, but in a home/office situation it may be tricky to find a compliant way of using one.

richms

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#2018074 17-May-2018 22:14

Try just replacing the RCD. My old one was bad, new one is good at not falsing.

Richard rich.ms

datahawk

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#2018078 17-May-2018 22:42

andrewNZ: It will almost certainly be mov's causing this. They're in nearly everything, including lighting power supplies.

The best option here is probably installing more RCDs so there are less circuits and therefore less leakage per RCD. Modern rules state max 3 circuits per RCD. This situation could be one of the reasons.

There is provision in the rules for 100mA RCDs for equipment protection. depending on the environment it might be an option if you have critical equipment, but in a home/office situation it may be tricky to find a compliant way of using one.

After some basic research on my various model UPS's they all appear to have MOV's in them also so I fully agree with your theory.

That particular RCD is servicing six separate socket circuits and three lighting circuits so it is certainly 'loaded'. I'll arrange for a sparky to squeeze a few more RCD's in there.

The second RCD is also handling 6 socket and 3 lighting circuits but that one has never 'popped' (which leads me to the next answer)

Cheers.

datahawk

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#2018079 17-May-2018 22:48

richms:

Try just replacing the RCD. My old one was bad, new one is good at not falsing.

I also agree that this could be the solution. Simply because the other RCD has never popped and it has the same number of circuits (9 total which is way above the latest recommendations of 3 per RCD)

Thanks Again.

richms

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#2018080 17-May-2018 22:54

The only movs that matter are ones between live conductors and the earth. They seem to have gone out of favour in cheap nasty powerstrip surge protection for this exact reason. Rules now are max 3 circuits per RCD, they used to be really expensive when there were many more put on an RCD, and I never figured how that was legit since they are all only good for 40 amps thru them. My sparky mentioned something about maximum demand calculations but it still doesnt seem right to have something rated 40 amps with 60 amps worth of breakers on the output (3 20A power circuits)

When I get the board moved once I can afford to relocate the front door I will getting it all swapped out for RCBOs in a bigger board, but who knows what the regs will be by then?

Richard rich.ms

Aredwood

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#2018086 17-May-2018 23:28

Run your office equipment via an isolating transformer. And get the sparky to hard wire the input side of the transformer. So hopefully the rules will allow you to connect it without a RCD. But be aware of any possible secondary earth paths. Mainly between different items where some are on the transformer output, and some are not. Check if the UPS can be hard wired maybe, if that can also get you out of having to have an RCD.

How many circuits do you have that run your critical equipment? Put each circuit on a dedicated RCD. And consider relocating the RCDs next to the equipment. As capacitive coupling on long power cables makes RCDs more likely to trip.

And supposedly the latest proposed electrical code will require RCDs on all residential circuits. Including stove and hot water circuits. If you think you have problems with RCDs tripping now, that will be far worse.

Edit

If so, might have to hurry and get the switchboard upgraded before the new code gets cited.

Edit 2

Interference filter circuits can also cause RCDs to trip. As such circuits contain small capacitors between all 3 conductors. On a clean 50Hz sinewave, only an extremely small amount of current flows between phase and earth. But add in some harmonics, and that current increases.

And every single switchmode power supply contains these filter circuits. Put too many On the same RCD, and you then get tripping problems. And UPS also contain those filter circuits.

Worse case, you can relocate your equipment to a commercial building. So then you can have power point circuits without RCDs. Or buy a house that was built before the rules requiring RCDs came in, and which still has an older switchboard.

mclean

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#2018246 18-May-2018 11:05

richms: ….it still doesnt seem right to have something rated 40 amps with 60 amps worth of breakers on the output (3 20A power circuits)

Having three 20A breakers doesn't mean the load will be 60A - it just means that each circuit is wired with cable that's good for 20A without overheating. The load current rating of the RCD needs to be more than the maximum demand. For residential socket outlet circuits that's usually calculated by a formula based on the number of outlets.

The load current rating of the RCD also needs to be more than the rating of any one of the downstream circuit breakers - in your case more than 20A.