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RCD circuit breaker / freezer / power cut / surge

Postby Bluto » Fri Nov 18, 2011 3:15 pm

My meter set-up contains a 100A 30mA RCD protected circuit breaker, which I know works because, last year, when a fault occurred with my boiler, the breaker kicked in even before the boiler's 2A fuse had chance to blow. However, after a recent holiday, I found that the RCD had tripped in my absence, even though the only appliance I had left switched on was the fridge freezer. As a consequence, all the food it contained had been compromised and, unfortunately, the value of the contents was less than the excess on my house insurance. When I re-set the RCD the fridge freezer came back on straight away and operated as normal. I asked a neighbour if there had been any disturbances in the force and he commented that one day he had had to re-set his radio alarm clock that was flashing. Do external power cuts / surges cause RCD units to trip and, if so, am I right to assume that there is no way of "protecting" a freezer etc. (other than running it from a diesel generator in the garden)?
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Postby ericmark » Fri Nov 18, 2011 10:19 pm

There are a number of issues here.
1) Electric storms can trip a RCD.
2) You can get auto-resetting RCD's but as far as I know only made by GEWISS see part number GW90901 and cost around £350 each plus likely you will need to change consumer unit as well.
3) A dedicated socket feeding a fridge or freezer supplied with suitable cable like Ali-tube does not need to be RCD protected under current regulations BS7671:2008.
4) Using RCBO's will also allow segregation of circuits and likely reduce the chances of it tripping.
5) There are now RCD's which also give an indication when the leakage is approaching the trip level and instead of tripping at between 50% and 100% of rated value will trip at 90% to 100% of rated value so less likely to trip. The Eaton Xpole try to google "Xpole entering the digital age" it should find page.
6) Apart from two specials modern RCD's are better than older type for not tripping due to spikes on the supply.
7) To test a RCD we use a special meter (not just press test button) these will measure time as well as current and normally we test 6 times. This is 3 tests each with positive and negative half cycle. So two prove it will not trip at 1/2 rated value. Two test it will trip at rated value. Two test it will trip within 40ms at 5 times rated value. It may be worth getting it tested.
8) The UPS will power anything when there is a power cut but there will be a time limit as to how long it will run for. Also the freezer will keep food safe for some time without power. The cheap option is to get someone to check. If lights run on same RCD only a case of leaving a light on and some one driving past can see if alls well.

What you have to decide is what it is worth although Moeller make the Xpole I have not found where to buy them from. I would consider fitting the GEWISS for my mother who is in a wheelchair so can't access the consumer unit to re-set a standard type but my excess is I think £50 so would need to have 7 holidays and for the RCD to trip on every holiday to be worth the change. They are really designed for non attended sites like water pumping stations not the domestic installation.

The problem is to get short lengths of Ali-tube cable. One would only likely need 10 meters so to buy 100 meters is a bit OTT. So in the past I have used steel wire armoured cable around outside of house to feed something important like a fish tank. People will pay silly money to look after their fish.

The socket would need marking "Freezer only" to comply.
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