by ericmark »
Mon Jun 19, 2017 9:00 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.
