by ericmark »
Fri Aug 13, 2010 12:19 pm
There are two regulations controlling type and number of RCD's
314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(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
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.
415.1.1 The use of RCDs with a rated residual operating current (I∆n) not exceeding 30 mA and an operating time not exceeding 40 ms at a residual current of 5 I∆n is recognised in a.c. systems as additional protection in the event of failure of the provision for basic protection and/or the provision for fault protection or carelessness by users.
These two regulations mean the only way to stop tripping of an RCD is to split it into more circuits so the the cumulative effect of many items all leaking a little to earth does not trip it. I did see a report of a new RCD which had warning lamps to tell you when it was near to tripping stage and also only tripped at 90% of the 30ma where most trip anywhere between 50% and 100% of the 30ma. Use of plug in 10ma RCD's can isolate suspect items. There is also an auto resetting RCD but at about £350 each about the only time I would use them is remote pumping stations and like. Maybe for the disabled if they can't reset it themselves.
The common method is to use RCBO's instead of MCB's in the consumer unit and with a TT system use a 100ma main RCD and with TN-C-S an isolator instead of the main RCD. At about £30 each this is not cheap.
Because of the 314.1 regulation many consumer units have two 30ma RCD's splitting the house into two circuits this is not as good as the RCBO method but does help to avoid tripping. You can even mix the RCBO method and double RCD method with some circuits using RCBO's and others grouped together with the two RCD's.
It is considered due to 314.1 not to comply with BS7671:2008 with only one 30ma RCD feeding whole house. However all caravans and boats are fed with a single 30ma RCD so it is not cut and dried and if it can be shown it only trips with genuine fault than a single may be allowed with for example a very small flat.
Since changing a consumer unit come under Part P and the LABC charges are so high over £100 the only real option is to get a registered electrician to change the consumer unit.
I did find a garden centre on internet selling 10ma RCD adaptors I think called The Garden Lighting Shop called the ELEC070 RCD Safety Adaptor at £13 which would help if it is tripping due to a faulty bit of equipment in identifying which unit is faulty. But most of the adaptors are 30ma so with the main RCD being also 30ma are about as much use as a chocolate fire guard.
