heat pumps and dual RCD boards new install

with a minimum trigger point of 150mA above 1kHz.

I'm in the process of fitting a heat pump at the moment - and the manufacturer's instructions say something very similar. They do however note that an RCD is only require where installation conditions require one - so on a TN system with no concealed (or armoured) cables, I reckon I'll be OK without one.

will BS7671 be ammended to include what is likely to become a bigger problem as more are fitted

I'm not sure BS 7671 needs an amendment - the problem is not unique to heat pumps, and it's possible not all heat pumps will have this issue. We're already obliged to follow manufacturer's instructions - so that should cover it.

   - Andy. 

Once the people at the IET and BSI realise that these are metal boxes placed outside, expect ever more elaborate and impossible to comply with rules, like with EV chargers.

AJJewsbury said:

with a minimum trigger point of 150mA above 1kHz.

I'm in the process of fitting a heat pump at the moment - and the manufacturer's instructions say something very similar. They do however note that an RCD is only require where installation conditions require one - so on a TN system with no concealed (or armoured) cables, I reckon I'll be OK without one.

Oh yes, that would be ideal. Even getting a run of SWA through the house may be cheaper than having to buy the Hager B type RCD, which, with the enclosure and circuit breaker, would be more than £200. Daikin say, in their training courses, that their HPs are fine without RCDs,  they say to comply with 'local' wiring regs.

On the opposite spectrum, Vaillant say you have to fit a B type RCD to ensure compliance with their warranty. Now why would that be? Surely the RCD is not there to protect the HP, and if it is, I'm not sure that theory is going to work in the real world, as 40mS of 150mA stray current would surely fry the circuit board.

On the opposite spectrum, Vaillant say you have to fit a B type RCD to ensure compliance with their warranty.

That's interesting - it's a Vaillant I'm fitting! The instructions do take a bit of interpreting (e.g. they seem to like the word "partition" when I think "isolator" would be clearer and C-type "fuses")

anyhow the installation manual says: "To protect people, type B universal-current-sensitive residual-current circuit breakers must be used if these are stipulated for the installation site. Tripping must be short-time delayed and suitable for the use of inverters (tripping characteristic > 1 kHz)."

There are also some wiring diagrams that show the supply side, complete with metering and overcurrent protective devices but without any RCDs (just a mention in the text that if RCDs are used and you use the twin supply arrangement, you'll need a separate RCD in each supply).

I do recall reading similar but different wordings elsewhere in the (small mountain of) documentation that came with it (I'm sure I've read the 150mA figure somewhere too) which may not have had the "if stipulated for the site" qualification, but can't put my hand to it at the moment.

But in the round it seems to me they're trying to say "if you need an RCD, make sure it's a B-type (with certain characteristics) " rather than "you will always need a B-type RCD".

If the RCD was for the benefit of the heat pump they surely need to specify a nominal rating (which I don't recall reading anywhere, just the type and >1kHz immunity), so at the moment at least, I think I've enough evidence for my no-RCD position!

   - Andy.

AJJewsbury said:

That's interesting - it's a Vaillant I'm fitting! The instructions do take a bit of interpreting (e.g. they seem to like the word "partition" when I think "isolator" would be clearer and C-type "fuses")

Yes, clear as mud again.

The links below may help.

https://www.vitoenergy.co.uk/vaillant-rcd-update/

And direct from Vaillant UK, note the ** on page 2 beside RCD, and the description below it.

https://professional.vaillant.co.uk/downloads/aproducts/renewables-1/arotherm-plus/arotherm-plus-spec-sheet-1892564.pdf

So why is a RCD required? Covering all bases, or do they expect the RCD to protect the outside unit? Either way, it hasnt been thought through very well. Vaillant used to be one of the better HP Makers, especially as they came from a boiler background, but they have lost ground recently, and there is a bot fo controversy about some of their HPs, in not being able to supply the quoted power output in colder weather, but they tried to keep that quiet.

Hager make a type B HP rcd, should a hp share an rcd with other circuits? Or do we apply similar rules as evse and solar?

If tripping is time delayed as someone stated, buried cables won't be allowed.

Yes, clear as mud again.

Indeed - the explanation from those links seems to be that you need a B-type as A- and F-types aren't suitable... And still no specification of what rating of RCD is needed - 30mA, 100mA, 300mA, 500mA, 1A, 10A... ? It's almost as if someone has been listening to some of the urban myths and thinks that all domestic final circuits need additional protection by RCD,

   - Andy.

Yes, clear as mud again.

Indeed - the explanation from those links seems to be that you need a B-type as A- and F-types aren't suitable... And still no specification of what rating of RCD is needed - 30mA, 100mA, 300mA, 500mA, 1A, 10A... ? It's almost as if someone has been listening to some of the urban myths and thinks that all domestic final circuits need additional protection by RCD,

   - Andy.