RCD feeding RCD thoughts.

[ericmark]

I have back in 1990's worked on construction sites with RCD's from 10 mA to 5 amp and 40 mS to 1 minute with up to 5 RCD's feeding each other. Standard was each three times rating of the last, and supply amps also varying from 13A to 1000A three phase.

However the nail through cable in portacabin took out all the RCD's even the one set at 1 amp at 1 minute. And in 2004 when we had a 10 mA RCD in the workshop, pressing test button also tripped the 100 mA S type.

As to types AC, A, F and B not a clue, but I know we fitted active RCD's to the bench grinder not for RCD protection be because it stopped an auto restart if we lost power. And with the narrow boat and caravan we have had two 30 mA RCD's for many years.

Using an energy monitor I know if a RCD has tripped as no reading, but with a garage and battery charger it is so easy for a RCD to trip and be unaware.

I have had the caravan RCD trip and the caravan site one remain energised, with what was assumed water ingress, but in the main in spite of being 30 mA off 100 mA S type a fault has taken out both, only once have I had a direct belt with a RCD protected circuit, I hacksawed through a cable which should not have been there, it did not follow the rules, even with 30 mA it knocked me out, it did trip, but think made really no difference as I fell away breaking contact, so even with no RCD result would have been the same.

So I can't think of a single time when a RCD has tripped when same fault would not have tripped a 100 mA RCD, In the main water ingress trips the RCD and the leakage is over 100 mA anyway.

So are we going OTT with RCD protection?

 

[AndyPRK]

I’m surprised with your findings with time delayed rcd. Mine are working ok so far. wylex 100ma

I had one time where a cooker ring was on the edge of failing. It took out a 30ma but not a 500ma. Sometimes both would go.

of course rcds don’t limit the shock. just trip sooner

 

[JohnW2]

So I can't think of a single time when a RCD has tripped when same fault would not have tripped a 100 mA RCD, In the main water ingress trips the RCD and the leakage is over 100 mA anyway.

The most obvious example that some people would cite would not be in relation to 'fault' currents but, rather, to 'shock currents' passing through a human (or animal) body - 12mA of such current should cause a 10mA one to trip, and 32 mA should cause a 30 mA one to trip - but neither of those currents should trip a 100 mA one.

So are we going OTT with RCD protection?

That is, of course, a question I've often posed, for a number of reasons (albeit not quite the ones you are mentioning)

I have had the caravan RCD trip and the caravan site one remain energised, with what was assumed water ingress, but in the main in spite of being 30 mA off 100 mA S type a fault has taken out both ....

For residual currents of IΔn and 2 x IΔn, the minimum operation time for time-delayed RCD ('Type S') is less than the maximum operation time for a standard (not time-0delayed) one, so it is not impossible that the Type S may operate as well as the non-delayed one. With a residual current of 5 x IΔn, the maximum time for a standard one and minimum time for a Type S one are the same (both 40ms), so it's pretty unlikley that the time-delayed one would operate if the non-delayed one has operated within it's maximum permitted time.

Kind Regards, John

 

[JohnW2]

I’m surprised with your findings with time delayed rcd. Mine are working ok so far. wylex 100ma

As I've just written to eric, for residual currents up to at least 2 x IΔn, the minimum trip time for a Type S is less than the maximum permitted trip time for a non-delayed one - so it's not impossible that, in some situations, the delayed one will trip even if the non-delayed one has tripped.

Kind Regards, John

 

[SUNRAY]

The 30mA downstream RCD should trip with the test button but that should never trip an upstream RCD

Where is the imbalance here?

Unles of course its one of the very old RCD's which used the earthwire for the test current.

 

[JohnW2]

The 30mA downstream RCD should trip with the test button but that should never trip an upstream RCD

Agreed - but I haven't noticed anyone suggesting otherwise. Have I missed something?

[Similarly, of course, pressing the test button on an upstream RCD should not cause a downstream one to trip]

Kind Regards, John

 

[SUNRAY]

Agreed - but I haven't noticed anyone suggesting otherwise. Have I missed something?
Kind Regards, John

Only in OP.

And in 2004 when we had a 10 mA RCD in the workshop, pressing test button also tripped the 100 mA S type.

 

[JohnW2]

Only in OP.

Whoops, sorry, I missed that However, having now seen it, another thought occurs to me. As you say, eric wrote ...

... And in 2004 when we had a 10 mA RCD in the workshop, pressing test button also tripped the 100 mA S type.

I may be wrong, but I'm pretty sure that when we previously discussed this, and I did some experiments, we discovered that some/many/most RCD sockets and RCD plugs/adapters (all generally 'active' RCDs, the latter often 10mA ones) did inject a leakage to earth when one pressed the test button - and I think we decided that the most likely reason for that functionality was that it meant that the test would 'fail' if there was an inadequate earth as well as if the RCD was faulty.

If the 10mA RCD eric referred to was of that persuasion, it could presumably explain what he reported?

Kind Regards, John

 

[ericmark]

Clearly the MK metal clad 10 mA RCD socket tests to earth not neutral. Unless it test the earth, you don't know if there is an earth to the socket, but 10 mA tripping 100 mA some one was not very selective in choosing the test resistor size. With the demo on RCD's not working due to being type AC rather than type A the test button is shown not to work, but had the current been 10 times the tripping current it would have done, I still have that socket some where, must see if I can find it, I had intended to use it for outside when my house only had two RCD's but never got around to fitting it, now with 14 RCBO's likely never will.

I know the test button was only to test the mechanical action, as clearly can't measure 40 mS with a stop watch, however I was not happy using the RCD tester on 400 volt, so where no neutral all I did was press the test button, which often did not trip either, as wired one phase to neutral, so would test phase to earth as no real option to do anything else. Common to find test button will not work when using a three phase and neutral RCD on single phase or three phase with no neutral.

I have seen the earth fly leads on RCBO's but not really worked out why they are there. Not sure how a RCD would work on an IT supply? Clearly since we must use RCD's in the bathroom and a shaver socket is an IT supply there is a conflict here. Seems 200 VA is OK in a bathroom but rest of house 30 mA is max? There seems to be an exception to every rule, and I know I am being pedantic and silly with the IT supply, however let us look at a real situation.

House with TT supply and 100 mA type AC RCD for whole of house, is it potentially dangerous?
Two issues one 100 mA not 30 mA and second type AC RCD, for the argument the home is unfurnished and there is no installed equipment which requires better than type AC RCD, clearly with an EICR one would highlight the problem and likely give Code C3, not worried about that, what I am asking is could one really issue a Code C2 either because 100 mA not 30 mA or type AC not type A?

Personally I would say no, all one can do is issue a Code C2.

I was looking at a Pro Elec in line RCD, with the thoughts it could be used for bathroom lighting circuit, but IEC61540 is for portable RCD's so technically would not comply in the bathroom as does not have the right BS EN number. Note it says nothing about type, it does not even say if active or passive so may be unsuitable, however I would prefer to debate it's use when not part of a my rental home as failed an EICR.

 

[JohnW2]

Clearly the MK metal clad 10 mA RCD socket tests to earth not neutral. Unless it test the earth, you don't know if there is an earth to the socket, but 10 mA tripping 100 mA some one was not very selective in choosing the test resistor size.

Yes, it's a big difference. I have no idea what test current the 10mA ones use, but I would not have expected it to be more than 20-30 mA (I think 30mA ones commonly use a test current of around 50mA).

Whatever, if (as I would certainly expect to be the case), that test current is less than 50mA, if it tripped a 100mA RCD thaen that RCD would have to be faulty ('out of spec') and ought to be replaced, wouldn't it?

I have seen the earth fly leads on RCBO's but not really worked out why they are there.

They seem to be much less common these days. They were 'functional earths' and the argument for them was apparently that in the case of a 'lost TN-C-S neutral' the electronics of the RCBO would not work. However, it would seem so incredibly improbable that there would be a 'lost neutral' at the very time when the RCBO was required to trip (becasue of a fault within the installation) that I think the idea may have been largely 'dropped'! I don't think they ever did it with RCDs, even those the same argument could have been used!

House with TT supply and 100 mA type AC RCD for whole of house, is it potentially dangerous?

I suppose as 'potentially dangerous' as a TN installation with only 100 mA RCD protection - and opinions about that will undoubtedly vary. I don't personally think that the 'Type AC' is a relevant issue in a standard domestic installation.

Two issues one 100 mA not 30 mA and second type AC RCD, for the argument the home is unfurnished and there is no installed equipment which requires better than type AC RCD, clearly with an EICR one would highlight the problem and likely give Code C3, not worried about that, what I am asking is could one really issue a Code C2 either because 100 mA not 30 mA or type AC not type A? .... Personally I would say no, all one can do is issue a Code C2.

As above, opinions will probably vary as to how the complete absence of 30mA RCD protection should be coded, but I suspect that a good few might give it a C2.

However, also as above, I don't personally regard 'Type AC' as being an issue in a standard domestic installation, and I think you may be one of the few who thinks as you appear to. I would certainly be very unhappy if (otherwise satisfactory - e.g. 30mA) RCDs in such an installation of mine were given C2s solely because they were 'Type AC', and I doubt that many people would contemplate doing that (and hope they don't!).

Kind Regards, John

 

[AndyPRK]

I don't think anyone would code an AC RCD as C2, but there is certainly is a shift starting.
Apparently Germany have only permitted type A for some time

 

[JohnW2]

I don't think anyone would code an AC RCD as C2 ...

I would hope they wouldn't, but eric seemed to imply that he might.

... but there is certainly is a shift starting.

That is my concern, unless/until we can find enough information to facilitate a proper judgement.

Apparently Germany have only permitted type A for some time

Yes, I gather that's the case, but I'm not sure what it proves.

As I have observed, recent events do not inspire much confidence in relation to Germany's 'banning' of things. The initially 'banned' the use of AZ Covid-19 vaccine in the over-65s, and then just a few weeks later restricted its use TO the over-60s (i.e. 'banned' it's use in the under-60s) - perhsp suggesting that they do 'banning' on the basis of 'knee-jerks', rather than proper thought!

Kind Regards, John

 

[ericmark]

I would hope they wouldn't, but eric seemed to imply that he might.

The only case where I would be worried about type AC is with solar panels and electric car charging where a fault could cause a smooth DC to be injected, i.e. where stored DC is over the AC voltage. And only because I simply don't know if DC could be injected or not, and only with a TT supply as with TN the RCD is secondary protection, only with TT is it primary protection.

But where a RCD exists be it wrong type or size, can anyone claim the installation is potentially dangerous? I think not, but would not say some one was wrong if they said it was.

As to RCD feeding RCD that was the case with daughters house, a RCD socket was fitted for items used in garden, then consumer unit changed so all RCD protection, and the old socket left in place, and when some thing was left out in the rain plugged in and it tripped the RCD's the main house RCD was reset without problem as the item in garden now isolated, as to if RCD socket tripped due to earth leakage or as active type and power lost is beside the point, it did allow everything else to be reactivated.

But when my dad fed a FCU from a FCU both with 13A fuses, I really had a problem as I simply did not expect two fuses, the same with two RCD's, and also we have BS EN 61008-1 and BS EN 61009-1 and most the RCD's as part of socket, FCU, or in line do not show those numbers. To my mind I don't care, if the RCD tester shows it does not trip and does trip as required, I am not really worried about any BS EN number, but on another forum when I suggested using a RCD FCU to comply, was told can't do that as the BS EN number is wrong.

So conforms to BS 7288 1990 MK Sentrysocket rcd protected active control device trips with loss of mains 10 mA tripping current on 200 kΩ scale pressing test button does not show a resistance between any terminals, although switched on I would assume it would need power as active type. BS 7288 1990 is specification for socket outlets incorporating residual current devices now withdrawn replaced with BS 7188 2016

712.411.3.2.1.2 Where an electrical installation includes a PV power supply system without at least simple separation between the a.c. side and the d.c. side, an RCD installed to provide fault protection by automatic disconnection of supply shall be type B according to IEC 60755 Amendment 2.
Where the PV convertor is, by construction, not able to feed d.c. fault currents into the electrical installation, an RCD of type B according to IEC 60755 Amendment 2 is not required.

All very well and good, but how does one know where the PV convertor is, by construction, not able to feed d.c. fault currents into the electrical installation? The same with

As of the 1st January 2019 either a Type B RCD must be used or a Type A with 6mA DC protection included in the Pod-Point (see detail on packaging to determine what protection is required),

once fitted and packaging gone, how does one know which RCD type is required.

Yes the PV or Car charging point installer may have done courses to tell them what is required, but for the EICR inspector how does he know which boiler, PV system, or car charging point needs type AC, A, F or B?

 

[JohnW2]

I would hope they wouldn't, but eric seemed to imply that he might.

The only case where I would be worried about type AC is with solar panels and electric car charging where a fault could cause a smooth DC to be injected ...

Ah - having now looked back, I think you may have confused me with a typo. When you wrote ...

..... what I am asking is could one really issue a Code C2 either because 100 mA not 30 mA or type AC not type A?
Personally I would say no, all one can do is issue a Code C2.

Having now more carefully read what preceded it, it looks as if the bit I've highlighted in red should probably actually have read "Code C3". Is that the case?

Mind you, to be frank, if it were me (which it never will be) I don't think that I have yet been able to find enough information to enable me to put my hand on my heart and even say that I "recommended" changing Type AC RCDs/RCBOs to something else in a domestic installation - so I might well not code that at all

Kind Regards, John

 

[ericmark]

Mind you, to be frank, if it were me (which it never will be) I don't think that I have yet been able to find enough information to enable me to put my hand on my heart and even say that I "recommended" changing Type AC RCDs/RCBOs to something else in a domestic installation - so I might well not code that at all

Kind Regards, John

I agree with that, I can't find enough information either, and like you I would note it in report, but likely would not give it a code.

However if I find some one else has given it a code, I could not say that's wrong either.

Lucky I don't work domestic, so where I work, I have access to all the paperwork so if solar panels were fitted I would know if there was any potentially danger which is unlikely with a TN three phase supply. Also I know the other electricians, and if we do something it is done as a team, and we are aware of why everything has been done.

The domestic inspector walks into a home very likely lacking all the paperwork, and even if there wading through it all is not fast, we are use to on line instructions, we hunt for words, if I look at a domestic boiler installation instructions I will hunt key words, like OpenTherm and ebus and bus to find out what that boiler requires, paper instructions are a lot slower, and in the main domestic EICR is either ½ day or a day, it is not practical to charge by the hour, so there is limited time on site, and if you find there is a Pod Point Solo Unit EV charging unit, how do you know if that unit needs a type A or B RCD? Same applies with Solar Panels.

As long as you have the installation certificate for the charging point or solar panel and the installer has said it complies, should you really need to question what he has said?