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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

  • It is a mess, the RCD makers have scarecly caught up with type A, and the goal posts are on the move. And as you say, makers whose instructions are universal and 'translated from the foreign' are not up to speed either.

    The only solution is to be very aware of the fault conditions that are credible in a given scenario, and those that are not.

    An EV charger for example circulates a Dc between what is in effect a pilot signal wire and the CPC, so the length of CPC between car and charger carries a DC.

    It is just about possible to imagine that damage to that section of cable may cause a fault current that returns via the supply NE bond through the neutral coil of any RCD between that link and the charge point, perhaps blinding those RCDs - and for this reason the advice is as it is. 

    In equipment where there is no such additional "earth plus DC" path - and be careful, as some innocent looking installations with interlinked mains devices may in effect have this,  there is arguably a greatly reduced risk. 

    Faults within power supplies that involve shorts between live and neutral or live and earth either via a diode or not, are likely not to persist for long,, as the diode will go bang and ADS will operate as normal. A diode between earth and neutral may do odd things, but at worst it will trigger a type A.

    It is the sort of thing where some drawings and worked examples would be helpful.

    Mike.

Reply

  • It is a mess, the RCD makers have scarecly caught up with type A, and the goal posts are on the move. And as you say, makers whose instructions are universal and 'translated from the foreign' are not up to speed either.

    The only solution is to be very aware of the fault conditions that are credible in a given scenario, and those that are not.

    An EV charger for example circulates a Dc between what is in effect a pilot signal wire and the CPC, so the length of CPC between car and charger carries a DC.

    It is just about possible to imagine that damage to that section of cable may cause a fault current that returns via the supply NE bond through the neutral coil of any RCD between that link and the charge point, perhaps blinding those RCDs - and for this reason the advice is as it is. 

    In equipment where there is no such additional "earth plus DC" path - and be careful, as some innocent looking installations with interlinked mains devices may in effect have this,  there is arguably a greatly reduced risk. 

    Faults within power supplies that involve shorts between live and neutral or live and earth either via a diode or not, are likely not to persist for long,, as the diode will go bang and ADS will operate as normal. A diode between earth and neutral may do odd things, but at worst it will trigger a type A.

    It is the sort of thing where some drawings and worked examples would be helpful.

    Mike.

Children
  • in reply to mapj1

    Faults within power supplies that involve shorts between live and neutral or live and earth either via a diode or not, are likely not to persist for long,, as the diode will go bang and ADS will operate as normal.

    On TN systems for sure. On TT though where you might have a couple of hundred Ohms in the Earth fault loop, a reliance on the RCD for ADS and many more volts of touch voltage generated per amp of leakage or earth fault current, it might not be quite so reassuring. Hopefully bonding will limit the risks, indoors at least. Given that vast tracts of western and southern Europe are TT as the norm, I can see why a lot of the advise from international manufacturers takes a pessimistic view.

       - Andy.

  • in reply to AJJewsbury

    Its going to be hard for electricians to research potential issues on a day to day basis and many companies look for excuses to always install the cheapest solution, even if it may not be safe . There are soany components going in to equipment to deal with emc etc there are are probably many potential failure points.

    Also the only way to get more complex solutions down to a sensible price is to have everyone using g the same solution, which drives volume.

    As an industry maybe we should go straight to an f type bi directional or simillar as a standard requirement. I don't know what's involved technically but suspect its much less than a afdd. Afdd prices have dropped quite a lot with modest volumes. 

    Given the number of switched mode power supplies in use plus the 6mA limit of type A's I think just taking a big step would make good sense to provide a long term solution.

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