heat pumps and dual RCD boards new install

Yes, it's entirely reasonable,  If we know that the heat pump (or any other machine) is liable to inject significant higher frequency current, or DC, into its incoming supply, then it needs its own rcd if the device in the consumer unit cannot respond adqeuately - and often it's a type AC or A & couldn't do so.

Even if there were no earth-fault in the heat pump, and thus its own rcd didn't trip, an upstream rcd in the consumer unit could easily trip, or else be 'blinded' to an unrelated earth-leakage in a different sub-circuit.  If we were determined to squeeze the heat pump circuit into an exisiting consumer unit, if single phase, it would need to be rated at not more than 11kW (50A), it would need to have a type B rcd with a switched neutral, it would probably need two DIN-module spaces & it would need a modified (split or further split) busbar to give it its own separate feed. 

A separate consumer unit, (not fed from the main one), would be the sensible option.

Not forgetting that the heating capacity is quite a lot higher than the electrical load. They are 'just' fridges in reverse ;-)

A 12kW heat pump may only be a 6kW peak electrical (guessing figures a bit based on poor CoP values). Always check the specs!

Generally, any HP install I do has a separate small DB, fed via a circuit breaker in the main DB. Of course with spilt load boards that may not be possible, so new tails to feed it via Henly style connector blocks.

I havent done one for around 8 months, but was using Fusebox Type B RCDs, around £160 for the RCD and box. Two pole, and fit all the requirements. Or so I thought. Fusebox have now said they are not suitable for HP use.

Here is an extract from an email from them:

"HPs must be installed with a Type B RCD rated to 20kHz or greater, and with a minimum trigger point of 150mA above 1kHz. Unfortunately Fusebox RTB630302M & RTB630304M do not currently meet this criteria. For any further enquiries please do not hesitate to ask."

Now, they are not offering any compensation for those that have fitted them, and dont offer any clue as to why they do not meet the (presumably?) BS/EN standards. They also dont state what the failure mode is on the ones they were selling as suitable for use.

So, how do we know if a RCD is suitable for use, as I only required a Type B RCD, it was sold as a Type B, but, apparently, it does not meet the required standard.

Is this a failure to adhere to the standards that apply to RCD production, or are they supplying inferior products?

So many questions, and very few answers.

Hager have started to do a Type B recently, so I think I'll be buying that next time, but how do I know their offering will comply with the relevant standards?

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.