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Discuss Ok to connect AC cable to MCB in Consumer Unit??? in the Solar PV Forum area at

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  1. danmarc

    danmarc EF Member

    Hi all, searched the forum for any info on this but to no avail...

    I have been installing PV systems for ages now, and have been connecting the AC cable to an MCB on the RCD side of the consumer unit whenever there is a spare way..
    I spoke to another installer and he told me that this cant be done, the cable must be connected to an MCB an RCD protected garage unit which is fed via tails taken from the main switch or link boxes, because under fault conditions if the RCD trips the inverter will still be producing power for up to 5 minutes, and the busbar (and circuits) will still be live.

    A way to get round this is to install an RCBO in the consumer unit, but there is not always a spare way on the non-RCD side...

    Dont the inverters have a shut-off built in in the absence of an AC input?

    Can anyone shed any light on this?

  2. yellowvanman

    yellowvanman Electrician's Arms

    Inverter will stay on for a max of 5 seconds (not 5 minutes),.

    Whichever way you look at it you need to put the MCB on a circuit that does not share an RCD with other circuits that require a 0.4 second disconnection time. Some choices are:

    Connect a seperate garage CU unit and use only one MCB from that CU (as you've been told)

    If you've got a 17th Edition CU - have you got one that has 2 RCD busbars and another seperate busbar (can't remember how these circuits are referred to) - typical 17th Edition CU would be 2-4-4 for instance.

    If you've got a 16th Edition board put the PV on the non RCD side.
    • Like Like x 1
  3. Markc

    Markc CCTV / Alarms

    Big question this one....

    The inverter is only required to reduce touch voltage within 5 seconds and not 0.4 seconds. Undoubtably when testing the RCD serving your inverter you did not allow it to go back into generation mode before continuing with the RCD test. (1/2 x both side of sin wave, 1 x both sides and 5 x both sides) if in generation the 30mA RCD Probably will not trip within 0.4 seconds. If this RCD is also serving sockets outlets, shower or any other circuit requiring supplementary protection then by connecting the inverter it has created a potentially serious situation. There is nothing stopping you fitting a RCBO but again not on a shared RCD.

    Now the question is over cables embedded in walls at less than 50mm. According to the regs a cable installed in this manner with no earthed mechanical protection requires 30mA protection. But again the inverter will not stop the RCD from tripping but the touch voltage could still be present for 5 seconds. There has been discussions on wether to fit a RCBO to both ends of the inverter AC supply (at inverter and at consumer unit) a reasonable but complicated and may be costly approach. We fit either SWA or not embed in walls.

    Where a none RCD way is available we use it, where a small board is protected by an overall RCD we might replace this with a main switch and fit RCBO's to the required outgoing circuits and MCB to the PV. Where there is no option we will fit a two way board with main switch and a MCB solely for the PV.

    There are no additional requirements under BS7671 that require PV to be on a 30mA RCD other than the standard requirements but even then it wouldn't work as it is designed to!

    And so the argument will continue...............
  4. danmarc

    danmarc EF Member

    Thanks for the input chaps. Makes a lot more sense now.
  5. rich.250

    rich.250 Electrician's Arms

    Business Name:
    Rich Williams Electrical
    If only more companies did this, I have come across pv on single
    Rcd boards four times in 2 months on condition reports, and yes it takes about 5secs to loose potential, from what I've seen.
    Could be rather Dangerous!!
    Thanks for the post.
    Hope People can see the Error
  6. moggy1968

    moggy1968 Electrician's Arms

    there is also a suggestion that if you do need an RCD for reasons under 1771 then it should be a type B RCD (as against a type b MCB which is something entirely different), which are £2-300
    • Like Like x 1
  7. danmarc

    danmarc EF Member

    What about for a TT system?
  8. moggy1968

    moggy1968 Electrician's Arms

    If you need an RCD for other reasons (i.e. under 7671, such as on a TT supply) and the inverter doesn't have glvanic isolation then you need a type B RCD (712.411.
    most inverter manufacturers specify 100ma RCD minimum
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