Fitting two RCDs one after the other

[sparkwright]

On a recent thread, a recognised poster stated how silly and pointless it is to have two (30mA) RCDs 'in series' protecting a load.

Which is probably, certainly no requirement for this as such.

But is it recommended if you wanted the user to be extra safe - perhaps a loved one, or someone frail, etc.

Obviously RCDs can fail and go unnoticed - so if you wanted a specific person to be extra safe, would two 30mA RCDs be a bad thing?

 

[John D v2.0]

No it wouldn't. It's basically redundancy

 

[JohnW2]

On a recent thread, a recognised poster stated how silly and pointless it is to have two (30mA) RCDs 'in series' protecting a load. ... Which is probably, certainly no requirement for this as such. ... But is it recommended if you wanted the user to be extra safe - perhaps a loved one, or someone frail, etc. ... Obviously RCDs can fail and go unnoticed - so if you wanted a specific person to be extra safe, would two 30mA RCDs be a bad thing?

As has been say, it is not a bad thing - the very worst downside is really possible slight 'inconvenience'. If you look back, you'll see that I have said that many times when people have been maligning the use of two (or more!) RCDs in series.

As JohnD has said, although a second RCD services no purpose if the first one functions satisfactorily, there are suggestions that the in-service failure rate of RCDs is far from trivial, so the more risk-averse people might reasonably want the 'belt and braces' reassurance of RCD redundancy.

Having said that, one can at least test RCDs. That is essentially impossible with MCBs (or the over-current part of RCBOs), which also means that we don't really know anything about the in-service 'failure' rate. The risk-averse person may therefore feel that they ought to also 'double up' on MCBs, lest their frail loved ones perish in an electrically-initiated fire.

Kind Regards, John

 

[EFLImpudence]

If anyone decided intentionally to have two RCDs, for the above reasons, then I would suggest they both be fitted in the same place.

There isn't much point having one in the consumer unit and others on the outside wall and other locations.

 

[sparkwright]

If anyone decided intentionally to have two RCDs, for the above reasons, then I would suggest they both be fitted in the same place.

There isn't much point having one in the consumer unit and others on the outside wall and other locations.

I imagine a practical example would be an RCD or RCBO at the consumer unit, and an RCD socket outlet.

Not at all how you are suggesting, but seems logical to me.

 

[JohnW2]

If anyone decided intentionally to have two RCDs, for the above reasons, then I would suggest they both be fitted in the same place. There isn't much point having one in the consumer unit and others on the outside wall and other locations.

Yes, that would certainly make sense from the point-of-view of convenience.

Mind you, aircraft have come to grief because, although they had redundancy of (three) hydraulic systems, the pipes of all three systems (probably necessarily) come into close proximity in some places, so that a single event can damage or kill all three.

Kind Regards, John

 

[John D v2.0]

I have my line and neutral wires entering the house on opposite sides, and only meet at the middle.

 

[JohnW2]

I imagine a practical example would be an RCD or RCBO at the consumer unit, and an RCD socket outlet.

Indeed - but there are some rather different reasons (other than RCD protection) for considering RCD sockets on RCD-protected circuits - not because of the RCD protection, per se, but because they are usually active RCDs, and hence act as a 'NVR' switch (something not readily available as a domestic wiring accessory).

Kind Regards, John

 

[EFLImpudence]

Indeed - but there are some rather different reasons (other than RCD protection) for considering RCD sockets on RCD-protected circuits - not because of the RCD protection, per se, but because they are usually active RCDs, and hence act as a 'NVR' switch (something not readily available as a domestic wiring accessory).

That's a different consideration. The thread is about having two in series for extra safety and fear of failure.

I don't see how the aircraft analogy relates to an RCCB covering a few RCBOs in a CU.

 

[JohnW2]

That's a different consideration.

That's what I said.

The thread is about having two in series for extra safety and fear of failure.

It is, but I thought it work pointing out that, in the case of an RCD-socket (or, I think, RCD-FCU) there are reasons other than redundancy of RCD protection for considering using them on an already-RCD-protected circuit.

I don't see how the aircraft analogy relates to an RCCB covering a few RCBOs in a CU.

My comment was perhaps somewhat 'tongue-in-cheek', but I was just making the general point that when one duplicates/replicates 'safety'-related things in order to achieve redundancy, there is an argument for not having the replicates in the same place (just as with computer backups etc.). However, I agree that in the case of multiple RCDs, one would have to scrape the barrel to think of ways in which they could be simultaneously damaged.

Kind Regards, John

 

[ericmark]

There are two places where two 30 mA RCD's are fitted in series, one is boats the other is caravans. But RCD does not always need earth leakage to trip, spikes on the supply can cause them to trip, and if not corrected a RCD trip can cost a lot of food when the freezer fails.

In the main boats and caravans have 12 volt lighting, and fridge can work on gas, so losing power is not as critical as with a normal home. We are looking at risk assessment and although my freezer after a power cut displays the highest temperature before power was restored, not all do that and food which has thawed and refrozen must present a danger, plus loss of lighting must present a danger.

So since so easy to test a RCD no really point doubling up other than boats and caravans where we have no option.

 

[Simon35]

That's a different consideration. The thread is about having two in series for extra safety and fear of failure.

I don't see how the aircraft analogy relates to an RCCB covering a few RCBOs in a CU.

Putting my functional safety hat on, which I sometimes wear at work, using diverse routes or diverse locations, as was alluded to in the aircraft analogy, is a means of reducing common cause failure. As is the use of RCD's in different locations, or different types and/or manufacturers of device.

Measures to reduce common cause failure are a necessary part of achieving high reliability safety systems.

Which you might want to reduce the chances of electrocuting yer grandma. Or not.

 

[JohnW2]

Putting my functional safety hat on, which I sometimes wear at work, using diverse routes or diverse locations, as was alluded to in the aircraft analogy, is a means of reducing common cause failure. As is the use of RCD's in different locations, or different types and/or manufacturers of device.

Exactly my point - although, as I have conceded, one would have to scrape the barrel pretty deeply to think of scenarios in which two RCDs would both become non-functional because they were sitting next to one another.

Kind Regards, John

 

[John D v2.0]

Exactly my point - although, as I have conceded, one would have to scrape the barrel pretty deeply to think of scenarios in which two RCDs would both become non-functional because they were sitting next to one another.

Kind Regards, John

Depends on your barrel. Heat could be a failure mode, or humidity.
Also, how do you know when there's a latent failure? Both could fail and everyone be blissfully unaware until they are next tested. So you're just changing from the time until one faults, to the time until two fails.
My concern would be falling down the stairs when the lights go off though.

 

[Jackrae]

Classic example might be a large(ish) house with a main consumer unit in the vicinity of the incoming mains feeder/meter and a sub-distribution unit for lighting systems located elsewhere in the property. RCD in main unit protects against typical ground faults and RCBO in sub-DBunit protects against outgoing lighting circuit faults, particularly excessive current when a lamp fails. Yes it is possible that either RCBO or RCD might trip on an imbalance but my experience has been that the outgoing RCBO only trips on a lamp failure (as might be expected). Because lighting systems are 'distributed', only part of the lighting is lost. So, two RC.. units in series serving similar yet different purposes.