Connecting a 100ma RCD for Ground Source Heat Pump

Discussion in 'Electricians' Talk' started by dave longster, Sep 13, 2017.

  1. dave longster

    dave longster New Member

    I would really appreciate some advice please, so that I can order the necessary parts and instruct my Electrician accordingly.

    I am installing a Ground Source Heat Pump which requires a supply using a 100ma RCD and 40 amp C Rated MCB. The instructions say this is to avoid nuisance tripping and the 100ma RCD is to provide protection against fire.

    This is part of a new build barn conversion, so the starting point is a BG metal twin 30ma RCD CU, which also allows for unprotected ways or RCBOs. The CU is already fitted and supplies the circuits to the conversion.

    There is not much room to split the incoming meter tails and I wondered whether we could use a spare fuseway for the 40a C Rated MCB then into a separate enclosure and through the 100ma RCD to the heat pump, or is there an easier way ?

    With many thanks.
  2. nigel willson

    nigel willson Well-Known Member

    It's up to your sparks how he does the job at the end of the day. You should discuss it with him.
    sparky Si-Fi likes this.
  3. MGW

    MGW Active Member

    I would use one side of the consumer unit with a 100 mA RCD and C40 MCB and other side an isolator and RCBO's, however to use a 100 mA RCD would also present other problems in the supply cable used.

    Before the requirements for 30 mA @ 40 mS RCD's for nearly all circuits, it was common to have freezers, fridges, cookers and other items either where loss of supply would cause losses or likely to course loss of supply mainly mineral insulated heating elements to not have RCD protection. However today we have seemed to find ways around using larger RCD's or no RCD's and in domestic premises I would be asking should one be using a different product?
  4. leesparkykent

    leesparkykent Well-Known Member

    Yest you could install a 40A C type mcb in between the main switch and 1st RCD as by the sounds of it you have a high integrity consumer unit. From the 40A mcb you could then take it through the 100mA RCD then out to the GSHP isolator.

    You will have to use an installation method that doesn't require additional protection by means of a 30mA RCD.

    The above assumes you are using the distributors means of earthing and the installation isn't TT.
  5. dave longster

    dave longster New Member

  6. dave longster

    dave longster New Member

    Thanks for the replies so far. The earthing is via earthing rods and not via the distributors cabling. An alternative solution would be most welcomed.
  7. Lectrician

    Lectrician Screwfix Select

    Good job BG do a type C. Not all domestic consumer unit brands have that choice.
  8. leesparkykent

    leesparkykent Well-Known Member

    Scrap that...just remembered it's a 40A mcb. Being over 32A and TT the maximum disconnection time is 1 second you could simply replace the main switch for a 100mA S type RCD and install the MCB in the space that was previously an unprotected way.

    Here's an S type/time delayed 100mA RCD...The only thing is they are rated to 80A so your electrician will have to decide if the design of the whole installation after diversity will not exceed 80A..Obviously being 100mA and time delayed it can't be used as additional protection so you will still have to opt for a installation method that doesn't require RCD protection like SWA, earthed metallic conduit, clipped direct to the finished surface, trunking etc. If you want to bury the cable in the fabric of the building which I assume you do then the easiest option where the cables run down walls at a depth <50mm would be to use SWA for the circuit. If using SWA isn't particle then you could use twin and earth for the internal part of the circuit and use sufficient mechanical protection to stop penetration by screws, nails and the likes of where the cable runs in a wall at a depth <50mm. Like this stuff note it's not standard capping so will be special order from a wholesalers...when I buy it I get it through electric centre/edmundsons.

    If the circuit was 32A or under you couldn't use the S type RCD for the same purpose as the maximum disconnection time would be 0.2 seconds which the S type/time delayed may/probably won't achieve.

    Being time delayed it will provide discrimination between itself and the 30mA RCD's.
    Last edited: Sep 13, 2017
  9. retiredsparks

    retiredsparks Well-Known Member

    I would...never...allow unqualified DIY customers to "instruct" me on how to do my job.
    I give them MY professional opinion and solutions with the related costs and guarantees and registrations.
    If they don't like it they can go elsewhere...
    sparky Si-Fi and nigel willson like this.
  10. dave longster

    dave longster New Member

  11. dave longster

    dave longster New Member

    Many thanks for all the replies which have been gratefully received. Special thanks to Leesparky for going into such detail. Regards.
    leesparkykent likes this.
  12. sparky Si-Fi

    sparky Si-Fi Well-Known Member

    Let the sparks do his job as stated

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