What is a RCD suitable for a low energy DC modulating pump according to IET wiring regulations.

[ericmark]

As title the Worcester Bosch instructions say "The correct type of RCD must be employed where additional protection is required that is suitable for a low energy DC modulating pump according to IET wiring regulations." to my mind it would have been better simply say type AC, A, F, or B, but I have not got the latest edition, so what does it say?

 

[JohnW2]

As title the Worcester Bosch instructions say "The correct type of RCD must be employed where additional protection is required that is suitable for a low energy DC modulating pump according to IET wiring regulations." to my mind it would have been better simply say type AC, A, F, or B, but I have not got the latest edition, so what does it say?

See if you can find the answer to your question here ...

531.3.3 Types of RCD
Different types of RCD exist, depending on their behaviour in the presence of DC components and frequencies. The
appropriate RCD shall be selected from the following:
(i) RCD Type AC: RCD tripping on alternating sinusoidal residual current, suddenly applied or smoothly
increasing
(ii) RCD Type A: RCD tripping on alternating sinusoidal residual current and on residual pulsating direct current,
su ddenly applied or smoothly increasing.
NOTE 1: For RCD Type A, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct
current up to 6 mA.
(iii) RCD Type F: RCD for which tripping is achieved as for Type A and in addition:
(a) for composite residual currents, whether suddenly applied or slowly rising, intended for circuit supplied
between line and neutral or line and earthed middle conductor
(b) for residual pulsating direct currents superimposed on smooth direct current.
155
NOTE 2: For RCD Type F, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct
current up to 10 mA.
(iv) RCD Type B: RCD for which tripping is achieved as for Type F and in addition:
(a) for residual sinusoidal alternating currents up to 1 kHz
(b) for residual alternating currents superimposed on a smooth direct current
(c) for residual pulsating direct currents superimposed on a smooth direct current
(d) for residual pulsating rectified direct current which results from two or more phases
(e) for residual smooth direct currents, whether suddenly applied or slowly increased, independent of
polarity.
NOTE 3: For RCD Type B, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current
up to 0.4 times the rated residual current (IΔn) or 10 mA, whichever is the highest value.
For general purposes, Type AC RCDs may be used.
NOTE 4: For guidance on the correct use of RCDs for household and similar use, see PD IEC/TR 62350.
NOTE 5: Some typical fault currents in circuits comprising semiconductors are given in Annex A53, Figure A53.1.

Kind Regards, John

 

[ericmark]

To get DC we use a rectifier and smoothing circuit and this will not in the main result in any DC being injected on the AC wave from unless there is a fault. OK years ago we may have used half wave rectifiers but even then rarely on the untransformed supply, so to get DC injection in the main looking at fault conditions.

So looking at a diode failing on the mains voltage being supplied to the capacitors before it is turned into high frequency AC and transformed down to voltage being used, or being fed directly to a motor, and much depends on other safe guards, for example if with a diode faulty the equipment will malfunction then it is unlikely it will be used for any length to time.

If my boiler was to cause DC injection it would not effect any of the main house sockets, as it takes its supply from a RCBO feeding the flat, and in real terms for any appliance to go faulty causing DC injection and also have a earth leakage fault on the same circuit at the same time is rather unlikely with 14 RCBO's.

However the installer of the boiler needs to make a risk assessment, and I question if that can be done with information given? The phrase "where additional protection is required" makes me consider the idea it is only a problem with TT supplies, but that is not what it says.

The manufacturers instructions should be taken into consideration, so we have to consider or assess that risk, so if I was to consider the risk was minimal and I get it wrong, then what would happen to me? We look at court cases and say there by the grace of god goes I, and it is tempting so watch ones back, and go OTT.

So let me put it this way, if found with an EICR that a type AC RCD was fitted with this boiler, what should go down on the report.
a) Nothing
b) Nothing only if TN supply
c) Note that the boiler should be on a dedicated circuit.
d) Note that the boiler should be on a dedicated circuit only with a TT supply.
e) Code C3 with TT supply
f) Code C3 with any supply
g) Code C2 with TT supply
h) Code C2 with any supply
I think I would do nothing, but again if any of the above was done I could not say it was wrong. If risk of death is 5 in 100,000 would that mean C2? Clearly the government thinks those odd are too high with Covid 19 so I would say also too high with electrical wiring, but I don't know the odds of anyone getting a fatal shock due to RCD failure due to DC injection, so how can I as a simple electrician assess the risk? Should the manufacturer simply say use type A RCD's?

 

[JohnW2]

If my boiler was to cause DC injection it would not effect any of the main house sockets, as it takes its supply from a RCBO feeding the flat, and in real terms for any appliance to go faulty causing DC injection and also have a earth leakage fault on the same circuit at the same time is rather unlikely with 14 RCBO's. ............. So let me put it this way, if found with an EICR that a type AC RCD was fitted with this boiler, what should go down on the report.

I imagine that opinions will vary quite a lot. My first personal first reaction is to say:

IF you really believed (and I personally don't have a clue **) that connecting your boiler would carry a significant risk of impairing the satisfactory functioning of that RCD, the:

• If a TT installation, that would leave all circuits protected by that RCD potentially without reliable fault protection (and the same would be true, for that one circuit, even if the boiler had a 'dedicated' RCBO or RCD). I would think that would be at least a C2, and some people might even say C!.
• 
  
• If a TN installation, then if any of the circuits 'protected by' the RCD were actually required to have RCD protection (e.g. sockets, lights, buried cable) then the potential absence of satisfactory/reliable protection would presumably warrant a C3 (as explicitly suggested in BS7671).

On the other hand, if you did NOT believe that connecting your boiler would carry a significant risk of impairing the satisfactory functioning of that RCD, then presumably you would not even mention the matter on an EICR.

** As I've said before, despite trying hard, I have yet to find anything which gives me much of a feel as to how real/'significant' a problem this 'RCD type' issue really is in domestic environments. However, given that many people (and the regs) seem to regard 'RCD protection' as very important (as well as being pretty crucial in TT installations), if something as 'innocent' as connecting a boiler (or countless other 'everyday' things) could undermine the reliable functioning of a Type AC RCD, then I would expect/hope that they would be outlawed (in the UK, as well s elsewhere), particularly in TT installations. Unless/until someone can prove me wrong, I'm therefore not very inclined to get very excited about this issue - for an EICR or anything else.

For what it's worth, of the people in this forum, it seems to only be yourself and one other person who are particularly concerned.

Kind Regards, John

 

[AndyPRK]

What brand rcbo or rcd do you currently have ?

 

[ericmark]

I am not really concerned, with 14 independent RCD's (RCBO's) if one was affected as same time as there being a fault it would be very unlucky, and with a TN supply not really relying on the RCD protection, it is more down to the rather vague statement by Worcester Bosch, low energy DC modulating pump tell one nothing, it does not even say if low voltage or extra low voltage, or any indication on how the DC is generated.
We have seen where electric car charging points have included circuit protection which means only a type A is required, and others where type B is required, it is all down to what is inside the charging pod, so what I read the Worcester Bosch statement as saying is.

We know the items used may effect the operation of an RCD but we can't be bothered to find out if it actually will effect the operation of the RCD so we have made the statement "The correct type of RCD must be employed where additional protection is required that is suitable for a low energy DC modulating pump according to IET wiring regulations." so if it is found the boiler has stopped an RCD working we can wash our hands of any responsibility and lay it all at the installers feet.

We know which RCD type is required depends on how much DC leaks, zero, 6 mA, 10 mA, or over means AC, A, F or B so we need the manufacturers to actually state this equipment can cause up to 6 mA or 10 mA or over leakage, or should be protected with a type A, F, or B the vague statement by Worcester Bosch is useless.

For an inspector to have to read installations instructions on equipment installed is bad enough, but to then issue such a vague statement it would seem only option would be to avoid Worcester Bosch with a barge pole.

 

[JohnW2]

.... so what I read the Worcester Bosch statement as saying is. .... "We know the items used may effect the operation of an RCD but we can't be bothered to find out if it actually will effect the operation of the RCD so we have made the statement "The correct type of RCD must be employed where additional protection is required that is suitable for a low energy DC modulating pump according to IET wiring regulations." so if it is found the boiler has stopped an RCD working we can wash our hands of any responsibility and lay it all at the installers feet.

Yes, that may well be the case, and given the lack of readily available information about this, I suppose that one can't really blame them.

As I said, I have 'bothered' quite hard, and have yet to really gain any feeling at all for how 'real' an issue this actually is. However, as I've said, given that (in relation to sockets and lighting circuits) a designer has no control over what might be plugged in or connected to the circuit, if it were felt that there were a potential significant issue, one would expect Type AC RCDs to be simply banned.

It therefore seems likely that since (unlike the situation in some other countries) they have not be banned in the UK, 'they' (in the UK) must (rightly or wrongly) be of the view that there is not a 'potential significant issue' - particularly given that, as I illustrated BS7671 says "For general purposes, Type AC RCDs may be used.", and I would have thought that 'general purposes' would include domestic sockets and lighting circuits (despite the fact that 'any' types of loads could be connected to them), wouldn't it?

Kind Regards, John

 

[ericmark]

I seem to remember a government survey, but hunt gets "registered community designs" or "Recreational Craft Directive" as the meaning of RCD. I seem to remember reading about a survey where they decided the chance of a collection of faults which resulted in some being injured due to the failure of a RCD to operate was so slim not to need worrying about, but sure that was also said about ELCB-v at some point.

Reading the instructions for an electric vehicle charging point I found " 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." I would like to know more about the 6mA DC protection, can we get a warning light to alert us when we have some DC, not to auto disconnect, but to warn.

 

[JohnW2]

....I seem to remember reading about a survey where they decided the chance of a collection of faults which resulted in some being injured due to the failure of a RCD to operate was so slim not to need worrying about.....

If you're talking about failure of an RCD to operate because it was the wring 'type' then, although, as I've said, I've yet to find any useful information or facts, I strongly suspect that taht may well be the case.

Let's face it, as I've said so many times, my personal view is that, in terms of 'impassionate cost-benefit', RCDs per se are rather questionable. If it were suggested that an investment >£1billion on some 'measures' could result in the number of road deaths reducing by a dozen or two per year, the ('impassionate') wisdom of that might be questioned. In the context of such thinking, if I think about the (I suspect very small) risk that an RCD may not operate when it should because it is the wrong 'type', then one (at least I !) has to wonder whether it is really something "worth worrying about'.

Kind Regards, John