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RCD Duty Category for ground source heat pump drive

whjohnson
I have been tasked with the provision of suitable electrical supply for a Nibe F1155 ground source heat pump unit and have some queries which I hope you may be able to answer.


The installation installer has requested a single phase 40A supply protected by a Type C mcb which I am fine with.


The problem arises with RCD protection. The incoming electrical supply is a TT arrangement (overhead line with no earthing facility provided by the supply company) and the means of earthing is via an earth electrode buried in the ground. BS7671 requires that all sub circuits originating from such an arrangement shall be protected by a 30ma RCD due to the high earth fault loop resistance associated with a TT electrical supplies.


So, to meet the requirement I must fit 30 ma RCD protection. I am led to understand that the unit employs an inverter arrangement to drive the pump/compressor motors.  As you are most likely aware, inverters/vfds can cause tripping problems with conventional domestic Type A RCDs. My query is what would you recommend? I am led to understand that a Type B or Type F RCD unit may be more suitable but the cost of this item is significantly more than that of a conventional double pole Type A/C RCD.


Also, I don't 'do' many TT Supplies and just want to run this past you -


Installation will go from single phase TT to 3 phase metered TT, with the existing phase feeding the rest of the house and with a different phase dedicated only to the heat pump installation.


I intend to fit a 100A Wylex switch fuse with a 60A fuse, and feed this with 16mm meter tails and earth directly from the supply company's isolator..

From the switch fuse a 3 core 10mm SWA goes to the heat pump room where it will be terminated into a 4 way metalclad cu.

This cu will feed a lighting cct via a 6A rcbo, a double socket via a 16A rcbo, and a back-up 3kw immersion heater for the heat pump via a 16A 30ma rcbo.

There will also be a 40A type C mcb which will feed the heat pump isolator in 10mm tails. I will need to incorporate the correct RCD into the tails somewhere before the heat pump but I can do that with a separate 2 way metal enclosure, most likely adjoined to the side of the cu with a conduit coupler.

Now, I need to earth this lot together. There is presently a single earth rod outside for the rest of the installation. Can I connect to this? Or Should I install a separate earth rod for the new supply from a different phase?

Also, I intend to earth the armour of the SWA at both ends and use the third core as a cpc - is this ok with a TT supply?


As I said, I am quite rusty on TT supplies because I usually get them PME'd when I find them, but this is not possible here because the supply is end-of-line and the phase -neutral loop impedance is too high (0.42 ohms) Western Power said yesterday that they would have to upgrade the cable before a PME solution could be provided and I can't see that happening any time soon.

So, having just received 2 quotes for a 2 pole 63A Type BX RCD, one came in at £270+VAT for a Doepke item and £470-odd+VAT for a ABB one.


The specs given indicate a total load of 12KW which is around 50-odd amps.

This includes 4KW for a built-in immersion heater within the pump unit.

There'll also be a back-up immersion heater & cylinder which will take another 3KW.

Since this lot will be off a different phase I don't see a supply capacity problem.

That said, Western Power downrated the cut-out fuses yesterday when they came to change the old knackered cut-outs on account of the high loop impedance value - too high to put back the old 100A ones they said.


Just had a response from Nibe themselves -


"The RCD's fitted with these units are usually type A, B & F, the important thing is that they state they are compatible with non-linear loads or inverter driven products."


Comments gents?

How much of a real-world problem is this when a conventional RCD with a Type A/C duty rating is employed on such applications? I know the regs state that you must follow the equipment manufacterer's instructions, but I have discussed this issue with other contractors and none have ever fitted a specialist RCD for such applications, even my wholesalers don't stock them and had to look up exactly what they were before they could quote.
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    BS7671 requires that all sub circuits originating from such an arrangement shall be protected by a 30ma RCD due to the high earth fault loop resistance associated with a TT electrical supplies.



    You might want to re-read BS 7671 on that point - as I read it you only need to provide ADS for each circuit - there's no specific limit on the RCD's rating - just as long as it disconnects within the required time and satisfies RA x IΔn ≤ 50V. (BS 7671 even permits overcurrent protective devices for such a purpose if Zs is low enough - very unlikely in practice but theoretically allowable). So in principle there's nothing wrong in using say a 100mA S-type for ADS if that's the most suitable choice. Normally you'd expect an earth electrode to have a resistance below 200Ω for reasons of stability/reliability - so there's usually no need to limit RCDs to 30mA.


    30mA are often called for for additional protection (equally in TN and TT systems) - typically for sockets, domestic lighting circuits, circuits feeding bathrooms and where you have insulated & sheathed cables concealed (<50mm deep) in walls. With a bit of care though, it should be possible to avoid those situations for heat pump supply.


        - Andy.
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  • 0

    BS7671 requires that all sub circuits originating from such an arrangement shall be protected by a 30ma RCD due to the high earth fault loop resistance associated with a TT electrical supplies.



    You might want to re-read BS 7671 on that point - as I read it you only need to provide ADS for each circuit - there's no specific limit on the RCD's rating - just as long as it disconnects within the required time and satisfies RA x IΔn ≤ 50V. (BS 7671 even permits overcurrent protective devices for such a purpose if Zs is low enough - very unlikely in practice but theoretically allowable). So in principle there's nothing wrong in using say a 100mA S-type for ADS if that's the most suitable choice. Normally you'd expect an earth electrode to have a resistance below 200Ω for reasons of stability/reliability - so there's usually no need to limit RCDs to 30mA.


    30mA are often called for for additional protection (equally in TN and TT systems) - typically for sockets, domestic lighting circuits, circuits feeding bathrooms and where you have insulated & sheathed cables concealed (<50mm deep) in walls. With a bit of care though, it should be possible to avoid those situations for heat pump supply.


        - Andy.
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