RCD Fused Spur That Does not Trip RCD on Main Consumer Unit


Postby collectors » Sat Jun 17, 2017 9:45 am

Hi, i'm having a senior moment here on a name/type of RCD.
I need a RCD switched fused spur. But i need the one that wont trip the RCD on the main consumer unit if the RCD spur trips. There is a name for this type of spur that wont let the fault past the spurs RCD & i cant remember the name/type of spur. Any ideas.

Thanks
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Postby ericmark » Sat Jun 17, 2017 8:59 pm

We have two variables on a RCD, unbalance current and time, for modern domestic time is fixed at 40 mS and the maximum current 0.030 amp or 30 mA older installations had S type (delayed) and 100 mA but these are now rare.

So we lift current by factor of 3, so you can have a main RCD at 100 mA and sub RCD's at 30 mA but where the main one is already 30 mA the sub one needs to be 10 mA, MK K6231WHI is a double 10 mA socket, but you normally need to order them not a standard stock item in most places and although if the leakage slowly rises it will trip first, if there is a fault likely all RCD;s will trip anyway.

Normal is to use a consumer unit with so many slots for RCBO's and have a dedicated circuit either for bits likely to cause a trip or for bits where tripping could cause problems,
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Postby collectors » Sun Jun 18, 2017 3:22 pm

Ah, thanks for getting back. You think they would of built this facility in to save tripping the main board.Im sure a couple of diodes & a small bit of electronic wizardry wound make one that had it built in for just a pound or so.

Cheers.
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Postby ericmark » Mon Jun 19, 2017 8:00 am

314.1 Every installation shall be divided into circuits, as necessary, to:
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit.
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.

There are other bits to the regulation, however the two bits shown have been argued about many times, some do not feel RCD's actually split the supply into circuits, and the water has been muddied by the Part P regulations seeming to have its own definition of a circuit, IET/BSi wise fitting a fused connection unit forms a new circuit, but not with Part P it would seem.

If the consumer unit has all RBCO's installed then clearly it must comply, but when we only fit two RCD's for whole house, then it is questionable if it complies.

(i) avoid hazards and minimize inconvenience in the event of a fault.

It is all well and good saying this, but when a house is wired or re-wired the electrician often has no idea what will be fitted inside the house, he simply does what he has done many times before. But if you have 10 RCBO's then you can have a leakage of 300 mA without the RCD's built into the RCBO's tripping, so having all RCBO's is the best option, however there are others, not all RCD's are equal, most trip between 50% and 100% of rated value, this is standard test, but some are designed to trip between 90% and 100% and have warning lights to tell you before they trip your getting near to point when they may fail.

Consumer units have a isolator and RCD of same physical size, so easy enough to swap a RCD for an isolator, then fit RCBO's instead of the MCB's, however some consumer units simply don't have enough room for RCBO's, also with a TT earth really one should use double pole RCBO's and only some makes do single width RCBO's with double pole switching, the MCB bit still only monitors the line, but both line and neutral are switched.

If you can't get double pole then instead of a isolator you can fit a delayed (S type) 100 mA or 300 mA RCD which will switch neutral however although stipulated with caravans and boats, it is unclear if required with a house.

When we fit a RCD the device has to be tested, and that is with proper tester not simply pressing button, the tester is expensive, so really this is not a DIY job, but there are ways around the problem.
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Postby kbrownie » Mon Jun 19, 2017 6:31 pm

Why are you concerned that the equipment connected to the spurs is going to trip?
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Postby ericmark » Wed Jun 21, 2017 12:09 am

In my house two 30 mA trips supply two fuse boxes now converted to MCB's and every so often we have a spate of trips, then maybe a few years before it trips again. I was installed 1992 so rather old RCD's.

Mothers house two RCD's supply most of house, a MCB supplies a sub board in kitchen with 4 RCBO's so in all 6 RCD's in the house, to date never had one trip apart from when doing the annual test with the RCD tester.
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