by ericmark »
Tue Jun 17, 2008 8:41 pm
From the way you describe your fuse box/consumer unit it seems unlikely that RBCO’s can be fitted. The RBCO needs connection to the neutral and the way the MCB’s are fitted into the old Wylex board will not allow this connection. Even with more modern boards because RBCO’s are either longer or wider often they still can’t be fitted.
As to price a MCB for the old Wylex board is about £7.25 each but for more modern board £4.45 each a RBCO for Wylex board is about £40 but other makes of board will use RBCO’s costing as little as £12.5 although I am not sure I would want to use unknown brands with such cheap items. Well know makes are still available at £33. So even if parts were available it could still be cheaper to renew whole board.
Now to answer your general question on RCD’s. It has be determined that a shock of less than 30milliamp is unlikely to cause death and is therefore selected as the threshold for RCD’s to protect personnel but all actions take time and two whole cycles of a 50Hz supply are allowed to pass through the device before it operates i.e. 40 milliseconds this is enough to give a nasty shock but still unlikely to kill. As a result all sockets under 20 amp in new installations except where they are for designated equipment have to be protected by a 30ma RCD. Not all RCD’s are for the protection of personnel 300ma versions are used for fire protection. Of course a 30ma RCD will also protect against fire.
Now for the down side. All electrical equipment leaks some current and filters used to stop one piece of electrical or electronic equipment interfering with another piece of electrical or electronic equipment allow a few milliamp to leak into the earth. If too many items on a single circuit allow this current to leak it builds up and causes to RCD to trip even when there is no real fault. Also items like cookers can be hydroscopic and absorb moisture which also will cause the RCD to trip although within minuets of switching on the moisture would have been driven out and would have caused not problems. As you may have already realised if freezers, fridges, smoke alarms, lights etc are supplied through the same RCD you can lose critical services so supplying everything through one RCD is not recommended. Also faults may have accrued over the years which have gone on unnoticed and changing a consumer unit can highlight such faults.
Conclusion looking at price the fewer RCD devices fitted the better but looking at inconvenience and safety the more RCD devices fitted the better. Using a consumer unit with all RBCO’s will cause the least inconvenience and best safety but fitting a duel RCD consumer unit is cheaper. With 16 way consumer units the difference in price is very high but with a 4 way consumer unit it would not be that great.
The biggest problem arises just after the RCD’s are fitted. Mine were fitted some 25 years ago and they will trip out about once every 6 months for no apparent reason. But when my son-in-law brought a faulty computer into the house we realised exactly what was faulty as it was just one bit of equipment and the power supply was changed but when RCD’s are first fitted there may be half a dozen bits of faulty equipment and items like washing machines with timers which may only trip out the RCD as it starts it spin cycle or an outside light which only switches on as it gets dark can be hard to find and the replacement of faulty items even harder to finance and in the past when only sockets likely to supply items used outside needed RCD’s it was common to return to a house and reduce the protection. From the 1st July when the new regulations come in this option will be no longer open to us.
Appendix a RCBO is a RCD and MCB combined but often they only switch the Line not the neutral so are not permitted in caravans.
Hope that answers your question. Eric
