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RCD types, an informed source of information?

alanblaby

RCD types. I'm getting more confused the more I read into it.

Ok, so we changed from Type AC to Type A, as backfed DC current from appliances could saturate the RCD coil, and stop it operating in fault conditions. Right, I can see that.

It's now been brought to our attention that some RCDs are not bi-directional, so need to be changed to bi-directional if current is likely to flow back through the RCD. Ok I can understand that more than the Type A vs AC.

Now, I'm going to fit a Heat Pump. Until recently, these were fitted on Type A RCDs, and still are by the bulk of Installers. (Manufacturers instructions are useless, one I have here in front of me says nothing about overcurrent protection, it just says a public supply of >16 and <75 amps).

Reading the Hager definitions,(linked below) it appears I need a Type A, or B, or possibly a Type F, as most HPs now use variable speed inverters. So how do I narrow it down as to what I require?

Do I go for the Type A, which Hager have listed as for  "Single phase in­vert­ers,"

The Type B, which says "In­vert­ers for speed con­trol, ups, EV char­ging where DC fault cur­rent is >6mA, PV "

Now note, they recommend the Type B for PV, so that is another I will have to change.

Or, do I go for the Type F, which says " some air con­di­tion­ing con­trol­lers us­ing vari­able fre­quency speed drives "

Clearly, there is a dilemma here. Without Manufacturers direction, I need to ensure compliance with this Countrys requirements - 7671 et al.  So the Type A could be fine, the Type B, probably, but I may need the Type F.

And, after reading the Hager explanation, I now also need a Type B bi-directional for any PV supplies.

So, how do we choose what to go for?  And If I go for a 'B', what does the Type F do differently?

And, on a similar subject, if the backfed DC current can affect the RCD on that circuit, can it also affect the other RCDs in the DB? They are all connected to a common neutral and earth, so could the backfed DC make its way through the neutral/earth to stop adjacent RCDs tripping under fault conditions if they are Type AC, or another type that can be affected in another way, say a F and A?

And, one I hinted at a few weeks ago, what happens when you fit a new DB, and find out one of the largest DB suppliers does not do a Type B or F?

Personally, I think this is getting to be a real mess, that's why people are still putting in Type A's, when, if reading into it, a B or F is required, but, actually finding out what is required is so difficult.

Hager link: hager.com/.../selection-of-rcd-types

Parents

  • I had a similar dilemma. It was like getting blood out of a stone when it came to nailing down the heat pump manufacturer's recommendation. I highlighted the various problems with VFDs and RCDs to them (Nibe - a Swiss company I think) and they came back and recommended a type B RCD which I fitted as a stand-alone exclusively for the unit itself. After that, they informed me that they were updating their installation instructions to include this requirement! Fancy that! ME having to educate THEM! God knows how many so-called uni-degree'd clever people work for that organisation, yet a lowly non-degree'd wire monkey like myself had to draw this issue to their attention!

    Chuck a stand alone type B in and forget it.

  • in reply to whjohnson

    As it happens, I called into the wholesalers today to see if they could get a type B. They couldnt get a Type B RCBO, but Hager , Fusebox and another (Lewden?) all do Type B RCDs.

    So it'll be a Fusebox or Lewden separate box, as the Hager one was £300+, with the others at around £115.

    Of course, that would be 2 of them, as PV is recommended to be on its own RCD, so it will cost more for the correct circuit protection for two circuits than it has for a 10 way Hager RCBO board.

    I do wonder why I bother, as if this was for a customer, there's no way I'd get the job, as there will be someone who would be quite happy to fit it all on Type A RCBOs, at £300+ cheaper, and be totally oblivious that they were not complying.

  • in reply to alanblaby

    Try Chint - that's what I ended up with, everyone else's brands were quoted as telephone numbers in excess of £600 +VAT ea. I think the Chint came in at just under £100.

  • in reply to whjohnson

    Fancy that! ME having to educate THEM!

    You may have far more unique knowledge than you realise, especially when one has been around the block for longer than the folk writing the advice have been alive - it may be a good idea to get used to the situation.

    Having been on the other side of the fence, and working as a consultant designing things, with teams with lots of folks with degrees and higher qualifications, I can assure you it does not make the team infallible;-) It is also very hard to write instructions for something when the conditions that the customers use it are  not quite what you expected at the white board stage, and by no means all equipment designers have ever spent any time at all installing kit,  for example having held SWA glands or wired things in conduit. (which is why so many boxes of electronics seem to have woefully inappropriate terminals and too little volume for the cables we'd like to use.)

    Where I work, as what we make is expensive, we tend to send out training/ installation chaps with it, and they come back with a lot of very useful intelligence that we can use to understand what does and does not work in the field, but that is not so easy for mass produced equipment. In an ideal world, makers would enclose a 'how would I improve my product' questionnaire, to gather the corrective feedback, and feed that into the improved "mk 2 "version, but then it would almost never get filled in usefully.

    But in this case you may very well be the first person to actually ask the RCD question, as others will have muddled through and chatted to the dealer or something, but there is a lot to be said for going to the horse's mouth.

    If no one else does either, how will they ever know what or how to improve?

    Mike

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  • in reply to whjohnson

    Fancy that! ME having to educate THEM!

    You may have far more unique knowledge than you realise, especially when one has been around the block for longer than the folk writing the advice have been alive - it may be a good idea to get used to the situation.

    Having been on the other side of the fence, and working as a consultant designing things, with teams with lots of folks with degrees and higher qualifications, I can assure you it does not make the team infallible;-) It is also very hard to write instructions for something when the conditions that the customers use it are  not quite what you expected at the white board stage, and by no means all equipment designers have ever spent any time at all installing kit,  for example having held SWA glands or wired things in conduit. (which is why so many boxes of electronics seem to have woefully inappropriate terminals and too little volume for the cables we'd like to use.)

    Where I work, as what we make is expensive, we tend to send out training/ installation chaps with it, and they come back with a lot of very useful intelligence that we can use to understand what does and does not work in the field, but that is not so easy for mass produced equipment. In an ideal world, makers would enclose a 'how would I improve my product' questionnaire, to gather the corrective feedback, and feed that into the improved "mk 2 "version, but then it would almost never get filled in usefully.

    But in this case you may very well be the first person to actually ask the RCD question, as others will have muddled through and chatted to the dealer or something, but there is a lot to be said for going to the horse's mouth.

    If no one else does either, how will they ever know what or how to improve?

    Mike

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