Is My Masterplug RCD Safe? Please Help!

[JohnW2]

I would point out that in most cases* the RCD is the already the third layer of protection against electric shocks.

Very true. In fact, you probably could have gone further and pointed out that, in many situations, it's the fourth level - since OPD-mediated ADS should (in TN systems) remove some risks of electric shock due to faults. However, ...

The first layer of protection against electric shocks is the regular insulation ... The second level of protection against electric shocks is either the use of a second layer of insulation or a metal layer that is connected to earth.

True, but a substantial proportion of shocks probably result not from failure of those two levels of protection but because they have been circumnavigated by the victim (particularly if an electrical DIYer) who has exposed bare, uninsulated live conductors and then touched them. They have engineered a situation in which an RCD is the only level of (possible) 'protection'.

In any event, RCDs provide (as well as other things) a conceptually totally different type of protection. All the other levels of protection you mention (plus OPD-mediated ADS) seek to prevent electric shocks occurring. RCDs may also do that but they also, uniquely, offer some potential 'protection' even if (despite all the other measures) a shock does arise. In that respect, they are the one and only level of (possible) 'protection', so if a single RCD fails to operate, one has no 'protection' of that type.

As I've said before, it's down to a personal view of the risk situation. If one is happy with the various levels of measures designed to prevent shocks, plus a likelihood (be it 93%, 97.2%, 99% or whatever) that a single RCD will operate if, despite the preventative measures, a shock does arise, then that's fine. However, if one is less than happy with the probability (whatever it is!) of having (any) 'protection' once a shock has arisen, then one should consider the possibility of a second RCD. It's a personal decision, which unfortunately has to be taken without much useful data on which to base it.

Kind Regards, John

 

[EFLImpudence]

As for double RCD protection I can't argue with any of what you say but/so -

for ease of fitting and tidiness would you consider fitting four RCDs (2 x 2 in series) in a CU or a double RCD board with RCBOs or do you consider the failure rate such that two RCDs are only necessary on circuits where RCD protection is currently mandatory?

 

[JohnW2]

As for double RCD protection I can't argue with any of what you say but/so - for ease of fitting and tidiness would you consider fitting four RCDs (2 x 2 in series) in a CU or a double RCD board with RCBOs or do you consider the failure rate such that two RCDs are only necessary on circuits where RCD protection is currently mandatory?

At best, it would obviously only be a personal opinion, based on my own attitude to risk. However, even on that basis, I wish that I had a good enough clue about the true failure rate to forumulate a sensible view. I think we probably all feel that the 7% figure is far too high, but how much lower than that it really is has to be anyone's guess. It may even be so low as to mean that only the most risk-averse people would consider that there was any point in 'doubling up' on RCDs at all, in any situation.

As you imply, if the answer (about failure rate) was 'in middle ground' then one might conclude that it was worthwhile to only 'double up' on RCDs for certain circuits - maybe just sockets circuits. One problem with that, of course, is that it would not be ideal in the presence of 'electrical DIYers', who could be playing with any circuit. I don't think I would (personally) necessarily select all 'circuits where RCD protection is currently mandatory', since I (again personally) think that some of them are a bit OTT - particularly 'buried cables'.

As for the mechanics of any 'double protection', I suppose that having 'a pair' of identical RCDs-in-series side-by-side in a CU would not be ideal, since they would be subjected to nearly identical environmental conditions - so I suppose that 'RCD-protected RCBOs' would be better (and would also benefit from the two devices being different). Alternatively, one might consider using RCD sockets (as well as an RCD/RCBO in CU), particularly for those likely to be used for 'more risky' loads/locations - and that would also enable one to invoke the potential advantage of having an active RCD as the downstream one?

If we had some reasonable clue as to 'how unreliable' current RCDs are in service, then, if the answer was sufficiently 'bad', then maybe the devices themselves could be improved - perhaps by having two independent trip mechanisms within the same device?

However, unfortunately, unless/until we have some sensibe handle on the actual in-service 'failure' rate of RCDs, it's essentially impossible to formulate a view, let alone make decisions, about all this. Electricians, as a group, are theoretically in a position to at least partially address that deficiency in knowledge/data, by maintaining and collating records of all the tests they undertake on in-service RCDs.

Kind Regards, John

 

[ebee]

I`m not saying 2 x 2 RCDs in a board would be futile because it`s still possible to make that essential difference but if you consider each cascade would likely be 2 RCDs by the same manufacturer in possibly the same batch and in the same environment and of the same age then copared to the hassle and expense is it worth it?

an RCD board with RCBOs could be argued the same way and most common RCBOs tend to be single pole switch with a solid N . There are DP switching RCBOs (more on the continent but some here now) so discrimination problems could be a nuisance.

Take the camping scenario.

One Site RCD covering preferably one socket but could be more.
Hopefully regularly tested and inspected by the site owners staff/agents.
Open to frequent use and abuse by all and sundry.
Then in cascade (OK after the commando plug & flex) the campers own RCD of a different make age and length of service.

A bit more of a fighting chance.

RCDs still allow shocks but (hopefully) reduce the duration to a survivable level

 

[ebee]

"Whatever, it has IMO really been a case of over-reaction on his part. The point originally made by ebee was not really specifically about actual (numerical) failure rate of RCDs in service (which we really not have any reliable handle on) or the mathematical process. He was merely pointing out the general truth (which is surely obvious from common sense, without any maths) that the probability of both of a pair of RCDs failing is less than the probability of a single RCD failing. That is always going to be true to some extent (but the probability of both failing will never be lower than the figure ebee calculated, as the product of the two individual probabilities)."

Well put John.

I think BANs question was a reasonable one to ask.

 

[JohnW2]

...an RCD board with RCBOs could be argued the same way and most common RCDs [I presume you mean RCBOs] tend to be single pole switch with a solid N . There are DP switching RCBOs (more on the continent but some here now) so discrimination problems could be a nuisance.

If you had a standard 2-RCD CU with each RCD protecting a bank of (single pole) RCBOs, the 'nuisance discrimination problems' would be no greater (may even be slightly less) than if (as 'normal') one had two banks of MCBs, rather than RCBOs.

Kind Regards, John

 

[JohnW2]

Well put John.

Thanks.

I think BANs question was a reasonable one to ask.

Yes, if he wanted to 'rub in' riveralt's lack of knowledge/understanding of probability calculations. I would personally question whether that was either necessary or 'decent', given riveralt's demonstrated sensitivity about the matter.

Kind Regards, John

 

[ban-all-sheds]

Yes, if he wanted to 'rub in' riveralt's lack of knowledge/understanding of probability calculations. I would personally question whether that was either necessary or 'decent', given riveralt's demonstrated sensitivity about the matter.

I would personally say that it was both of those, as after saying it riveralt continued to criticise people for "bad maths".

 

[JohnW2]

Yes, if he wanted to 'rub in' riveralt's lack of knowledge/understanding of probability calculations. I would personally question whether that was either necessary or 'decent', given riveralt's demonstrated sensitivity about the matter.

I would personally say that it was both of those, as after saying it riveralt continued to criticise people for "bad maths".

Maybe you would. It's touching that you feel inclined to defend me (and to some extent ebee) against the criticisms of "trying to justify 'bad maths' ", but I'm really quite capable of looking after myself, just possibly in a slightly less provocative fashion than you are.

Kind Regards, John

 

[riveralt]

Yes, if he wanted to 'rub in' riveralt's lack of knowledge/understanding of probability calculations. I would personally question whether that was either necessary or 'decent', given riveralt's demonstrated sensitivity about the matter.

I would personally say that it was both of those, as after saying it riveralt continued to criticise people for "bad maths".

How many working electricians when installing full board RCBO installations tell their customers don't just rely on the RCBO's you also need to have plug in RCD's as well .....just in case?
That will be - zero - unless ebee is a working electrician in which case 1.

If the truth about someone is that their behaviour or personality are such that the only words which describe them have become terms of abuse, because their behaviour or personality are so objectionable, then it is inevitable that terms of abuse will have to be used to truthfully describe them.

 

[radioredcat]

To the OP's problem what about a UPS (uninterruptible power supply) which has surge protection and keeps power applied to the item plugged into it when the mains power goes off.

I have had one for many years to protect my PC as we used to get regular powercuts but fortunately not had any for some time touch wood.


Andy

 

[JohnW2]

How many working electricians when installing full board RCBO installations tell their customers don't just rely on the RCBO's you also need to have plug in RCD's as well .....just in case? That will be - zero - unless ebee is a working electrician in which case 1.

I'm sure it will be zero, even if ebee is a working electrician - he has never suggested that anyone should do that. The general issue which we've been trying to discuss is whether there is actually any evidence base for what we (essentially all) currently think, advise and do - and the answer is, AFAIAA, essentially 'no'. Although I think we all believe that the in-service failure rate of RCDs is nowhere near as high as 7%, I don't think anyone has got a clue as to what the true figure actually is. It may,or may not, be 'acceptable' as the reliability of the only device which offers any hope of protection once an electric shock has arisen.

As I recently wrote, this is a situation which electricians could remedy to a significant extent. There must be hundreds of RCDs tested every day, so it wouldn't take very long to generate some reasonable data (an awful lot more reasonable than we currently have) about in-service failure rates - and once we had some reasonable data to work from, we could have a sensible discussion. In the meantime, we can but speculate and guess.

Kind Regards, John

 

[JohnW2]

To the OP's problem what about a UPS (uninterruptible power supply) which has surge protection and keeps power applied to the item plugged into it when the mains power goes off

I may be wrong, but I would rather doubt that it would be necessary/appropriate to use a UPS for an electric organ. Even if one believed that 'surge protection devices' are useful, there are simpler/cheaper ways of getting it.

Kind Regards, John

 

[riveralt]

Although I think we all believe that the in-service failure rate of RCDs is nowhere near as high as 7%, I don't think anyone has got a clue as to what the true figure actually is. It may,or may not, be 'acceptable' as the reliability of the only device which offers any hope of protection once an electric shock has arisen.

As I said the ESC (2006) data puts it at 2.8% and zero caused by environmental conditions.

As I recently wrote, this is a situation which electricians could remedy to a significant extent. There must be hundreds of RCDs tested every day, so it wouldn't take very long to generate some reasonable data (an awful lot more reasonable than we currently have) about in-service failure rates - and once we had some reasonable data to work from, we could have a sensible discussion. In the meantime, we can but speculate and guess.

I'll start the ball rolling on that one - over a five year period up until Sept this year I have installed 112 RCD/RCBO's and none of them have ever failed the standard tests.
In the same period I have conducted 12 PIR/EICR and only once has an RCD failed the test - in fact it was the mechanical test button - it was only when I looked at the device did I discover it had been shorted out by a previous ????
So someone found the fault but because it was/is difficult to find a replacement they took the short cut.

 

[ebee]

Thanks John I have now corrected my RCD/RCBO typo.

Riveralt - Yes I am a working Electrician , no I do not advise all of my customers to use RCD adapters on such RCD protected circuits .
It`s just that I would not necessarily advise them to remove one if it`s not causing discrimination problems, equally I would not advise them to not replace a faulty one either.