DIY Doctor

Types of RCD

Postby ericmark » Fri Apr 25, 2008 4:01 pm

I understand most of the aspects of RCD protection.
The milliamp range of 10 (garden use where you want discrimination between garden and main 30ma consumer unit mounted unit.
30ma for personal protection
300ma for fire protection
500ma for sockets over 32 amp on building sites.
And the times with 40 milliseconds at 5 times rated ma value in most cases and 2 times ma value for TT systems.
Also that the Amps rating of some RCD’s even with same ma rating means one designed for a plug top is a lot cheaper than one designed to fit in a consumer unit.
Also that where combined with a MCB i.e. a RBCO that normally they are only single pole and as a result can’t be used in caravans.
Even the S type or time delay I under stand where decimation is required and even the combined RCD and RCM where a bar graph shows earth leakage giving a warning before it trips.
Active and passive where they drop out or hold in when the supply fails the active being popular to stop motor restart on power failure i.e. plugs supplying lawn mowers ant the like. Also auto resetting RCDs which test the circuit and resume supply when tripped through atmospheric conditions etc.
But when we come to different types of wave form and how likely they are to trip with positive or negative cycle and how the different electronic systems within the RCD are more or less susceptible to wave form distortion then I am out of my depth.
I know because the way they sense some 30ma RCD’s are less susceptible than others at erroneous tripping. I know it is all to do with wave form and uses electronics in a clever way to avoid tripping where switch mode power supplies and the like distort the wave form but as to selecting the types which are more or less susceptible to this wave form distortion with 3rd and 5th Harmonics I really do get lost. In layman’s terms I only have a foundation degree not an honours what do I look for to select the type less likely to trip.


Simply Build It

Postby sparx » Fri Apr 25, 2008 5:13 pm

as a humble sparkie I don't have time or inclination to spend time analysing details of design, however after far too many years in the wider aspects of the trade my own observations tend to steer me away from 'electronic' rather than electromechanical rcd's ie avoid the ones needing an earth connected fly lead. It would be unwise of me to state makes to avoid but again I know which ones give most trouble!!
Probably every leckie has his 'pet hates', I keep a list of prefered makes of most readily used items of all sorts not just RCD's but switches, sockets, consumer units, dist. boards etc. it saves that one-site sinking feeling when you go to fit something & realise you've fitted this cr*p before and swore you'd never do so again!!
I do feel the current design of most single module RCBO's is poor in the way you have to connect the out going cores to deep buried terms, particularly rings with 2 cores in each term, another good reason for using radials!
Overall though I don't subscribe to the view of 'nuisance-tripping' as if an earth leakage device trips at all then there is a reason, be it cheap lamps,
low IR on a circuit or too many P.C.'s not being run via a UPS.
The sole rcd fitted at home some 16years ago works well on monthly instrument cal. checks but has never gone out without good reason, we do run all 3 PC's on UPS's tho'
regards SPARX
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Postby ericmark » Sat Apr 26, 2008 12:02 am

I have also had two RCD's protecting the whole house for years but with these new regulations I expect we will get problems. Where there is a fault which can be found that's OK but where it trips for no apparent reason I know in the past I have changed RCD's and found no further problems in spite of both old and new RCD testing out as A1. With most of my work being industrial we did not use cheap units they cost too much in down time and putting two cables in one hole was always frowned on and radials were the norm. Although I have seen din rail mounted junction blocks with link bars fitted. Although with larger cables they are also a pain un-clipping them selves. 10ma RCD plugs seem hard to get hold of we always stocked a couple but always had to order replacements never seemed to be available off the shelf and real handy to temp fit to suspect items then return latter to find which one it was.
Biggest problem I have had is thunder storms, computers etc only trip the RCD when people are using them so easy to reset but thunder storms can trip RCD while on holiday when your not there to reset it. And I have seen price for auto reset types £250 for a single RCD is to me a little OTT. But with everyone using RCD's the insurance can not longer quibble should one trip and I always run down freezer before I go on holiday.

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