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Day after day, between 13:30 and 14:30, most frequently a few minutes after 14:00 we're getting power outages in our firm. One (random) or both of RCDs protecting circuits to whole wing of the building activates, switching running computers off, powering tested devices off, and so on.

We don't have any device to activate at that time, and the fact two different circuits are affected (and there doesn't seem to be any pattern which of the two, or both), suggests some external source. We sought an internal source (using a power strip with milliamp meter plugged into the ground line) and measuring leakage from different devices shows we're approaching rated leakage amperage of the RCDs, but still staying good 30% below the threshold (50mA).

Now, for more info:

Our firm is located practically under two high-voltage lines leading to a power substation barely some 200m away, supplying power to iron works - a massive industrial complex of roughly 10km^2, involving some extremely heavy industrial machinery. It seems 2PM is the end of the first shift in the iron works. I don't know how much these things can influence us, just giving them as possible avenues of research.

Can you suggest what can be done about that? What can be the source of such failures? Getting UPS for the whole firm would be rather expensive.

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  • \$\begingroup\$ As a first step I would have a professional electrical engineer look at your wiring with a special focus on the earthing arrangements. \$\endgroup\$ – JIm Dearden Nov 18 '13 at 13:51
  • \$\begingroup\$ Take the entire staff on a team building session, away from the office. Then if the RCD's trip when everybody is away, at least you know the cause is not in your internal wiring. You catch my drift, maybe investigate a more practical way to accomplish the same. \$\endgroup\$ – jippie Nov 18 '13 at 14:01
  • \$\begingroup\$ Oh and start keeping a detailed log of day, time, holiday or not and which RCD tripped. Probably enrich the log with production runs you have, ... Also in my opinion the 30% of 50mA means that you have 35mA leakage current to ground. I'd investigate where that current comes from, it is quite a lot (a small light bulb)! \$\endgroup\$ – jippie Nov 18 '13 at 14:07
  • \$\begingroup\$ @jippie: One PC produces some 5mA leakage. We have at least 6 PCs switched on at the same time, sometimes more. There are a few other devices with lesser leakage. Our trip with the power strip and multimeter has shown there is no single source of the leakage, it simply adds up from many small leakages from multiple devices. The firm is rather small, there are no mysterious forgotten mainframes in the basement. \$\endgroup\$ – SF. Nov 18 '13 at 14:20
  • \$\begingroup\$ Someone I know used to have tripping RCD's and it turned out there was a greenhouse down the road, illegally tapping electricity. So I guess there are situations that cause this, though I'm not sure what the mechanics behind them are. It would be a good start to check the various voltages between line, neutral and ground. \$\endgroup\$ – jippie Nov 18 '13 at 16:19
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Very interesting. My guess would be that when some large piece of machinery switches off (or on), it causes a huge shift in ground current that extends through your property, and this impulse is what's tripping your RCDs. You probably wouldn't see this on a multimeter; you'd need a datalogger with a reasonably high sample rate to catch it.

I'm thinking that to solve the problem within your firm, you'll need to carefully isolate all of the ground connections for every piece of equipment downstream of the RCDs and tie them back to a single point, like they do in hospitals for sensitive equipment (usually indicated by an orange-colored outlet or other marking). However, depending on how extensive your facilities are, this could get to be as expensive as an UPS (but would be lower maintenance, long-term).

The other approach would be to get the power company involved and see whether they can make any adjustments at the substation or have any other advice for you.

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