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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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  • 0

    Inverter Air conditioning units require a certain RCD mcb Breaker type and rating electrical supply


     
    Since the advent of Inverter heat pump air conditioning  there is the occasional problem  with electrical RCD breakers going out due to rating issues. Because inverter air conditioning units use a DC driven compressor there can be issues with the transfer from AC to DC which creates a lot of EMF thus causing the RCD mcb breaker to trip. The inverter board uses the a.c. supply going to the compressor to reduce or speed it up and for some reason low rated i.e 30mA RCD's have a rating problem.
     

    thumbnail10.JPG air_con_rcd.JPG


     
    The biggest problem with a underrated RCD breaker is that it can be a periodic issue, i.e the air conditioning units work for 2 months then all of a sudden a issue arises where the supply blows. The obvious first thought is that the inverter air conditioning unit itself has a major problem such as a PCB or compressor fault. This can waste valuable site time and customer down time while a fault is attempted to be rectified and on occasion parts being replaced.
    The now obsolete but easier to electrically supply, fixed speed air conditioning systems used a direct on line motor compressor, either the compressor was Off or On according to the thermostat requirements. This made them a easier to rate electrically as they were a inductive rated load which usually meant using a C type breaker with no consequence to a RCD. The fixed speed models produced no high output  EMF because there were no inverter boards inside the outdoor unit with heavy electronic board based triacs and coils producing heavy electrical noise.

     

     

    Which RCD Breaker Is Best To Use With Inverter Air Conditioning


     
    It is best to use a (greater than) 100mA RCD breaker as this will give the electrical supply sufficient support to hold in while the air conditioning inverter system ramps up and slows down. Many electricians are unaware that air conditioning systems require a rating high than 30mA and it has to be said most of the time a 30Ma RCD will work. But if you are a electrician of air conditioning engineer with a issue with a RCD which keeps tripping it just might not be rated for the project.


    C.

     

Reply
  • 0

    Inverter Air conditioning units require a certain RCD mcb Breaker type and rating electrical supply


     
    Since the advent of Inverter heat pump air conditioning  there is the occasional problem  with electrical RCD breakers going out due to rating issues. Because inverter air conditioning units use a DC driven compressor there can be issues with the transfer from AC to DC which creates a lot of EMF thus causing the RCD mcb breaker to trip. The inverter board uses the a.c. supply going to the compressor to reduce or speed it up and for some reason low rated i.e 30mA RCD's have a rating problem.
     

    thumbnail10.JPG air_con_rcd.JPG


     
    The biggest problem with a underrated RCD breaker is that it can be a periodic issue, i.e the air conditioning units work for 2 months then all of a sudden a issue arises where the supply blows. The obvious first thought is that the inverter air conditioning unit itself has a major problem such as a PCB or compressor fault. This can waste valuable site time and customer down time while a fault is attempted to be rectified and on occasion parts being replaced.
    The now obsolete but easier to electrically supply, fixed speed air conditioning systems used a direct on line motor compressor, either the compressor was Off or On according to the thermostat requirements. This made them a easier to rate electrically as they were a inductive rated load which usually meant using a C type breaker with no consequence to a RCD. The fixed speed models produced no high output  EMF because there were no inverter boards inside the outdoor unit with heavy electronic board based triacs and coils producing heavy electrical noise.

     

     

    Which RCD Breaker Is Best To Use With Inverter Air Conditioning


     
    It is best to use a (greater than) 100mA RCD breaker as this will give the electrical supply sufficient support to hold in while the air conditioning inverter system ramps up and slows down. Many electricians are unaware that air conditioning systems require a rating high than 30mA and it has to be said most of the time a 30Ma RCD will work. But if you are a electrician of air conditioning engineer with a issue with a RCD which keeps tripping it just might not be rated for the project.


    C.

     

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