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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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    In the Deopke paper, the chart near the top of page 11 says that Type A RCDs are not suitable for heat pump applications,



    Isn't that just for 3-phase systems? Type A looks like it might be OK for single phase.


    Presumably a lot depends on the construction of the heat pump system. If it has the usual arrangement of a rectifier/smoothing system producing d,c. that is then switched by the inverter to drive the motors, then a pure d.c. fault could only arise from the 'middle' section - so if that part of the system was of class II construction than that kind of fault could be avoided - likewise if the inverter and motors were within a class II enclosure then faults to earth of both pure d.c. and high frequency a.c. residual currents could be avoided.

     

    to consumer unit - front-end 30ma Type A/C RCD main switch.





    My thinking is that a Type A\\C won't be affected if the feed to the pump RCD/Type C mcb is taken off the incoming side of the Type A/C - i.e. in connected in parallel.




    Ah, so the AC RCD isn't the main incomer then - more of a split load arrangement (in which case a switch disconnector as the incomer might be appropriate - rather than messing about with labels to try and explain that what looks like in incoming devices doesn't in fact isolate all the outgoing circuits).


    Might be simpler (and provide better discrimination) to use DP switching RCBOs for the other outgoing circuits and then an MCB plus suitable RCCB for the heat pump (presuming the type of RCD needed for the heat pump isn't available as an RCBO).


      - Andy.


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  • 0

    In the Deopke paper, the chart near the top of page 11 says that Type A RCDs are not suitable for heat pump applications,



    Isn't that just for 3-phase systems? Type A looks like it might be OK for single phase.


    Presumably a lot depends on the construction of the heat pump system. If it has the usual arrangement of a rectifier/smoothing system producing d,c. that is then switched by the inverter to drive the motors, then a pure d.c. fault could only arise from the 'middle' section - so if that part of the system was of class II construction than that kind of fault could be avoided - likewise if the inverter and motors were within a class II enclosure then faults to earth of both pure d.c. and high frequency a.c. residual currents could be avoided.

     

    to consumer unit - front-end 30ma Type A/C RCD main switch.





    My thinking is that a Type A\\C won't be affected if the feed to the pump RCD/Type C mcb is taken off the incoming side of the Type A/C - i.e. in connected in parallel.




    Ah, so the AC RCD isn't the main incomer then - more of a split load arrangement (in which case a switch disconnector as the incomer might be appropriate - rather than messing about with labels to try and explain that what looks like in incoming devices doesn't in fact isolate all the outgoing circuits).


    Might be simpler (and provide better discrimination) to use DP switching RCBOs for the other outgoing circuits and then an MCB plus suitable RCCB for the heat pump (presuming the type of RCD needed for the heat pump isn't available as an RCBO).


      - Andy.


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