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Domestic consumer unit rating with PV and battery storage.

Mspark

Having read the COP on  Electrical Energy Storage Systems and  completed the IET course on the same subject I had a query regarding the rating of domestic consumer units and switch gear which I addressed by email to NICEIC technical. I also came across an older discussion on this forum but am still no closer to a definitive answer. I've included my findings and would welcome constructive input. 

Post by GKenyon in previous thread

Because an EESS charges the battery as well as as discharging it, you will need to check the rating of the CU is not exceeded. For example, if the CU is rated for 100 A, and there's a 100 A service fuse, and a 16 A output battery storage system - by feeding 16 A in at one end through an OCPD, because that OCPD gets hot it contributes to the total heat load in the CU - therefore the CU should be rated for 116 A.

My question to NICEIC.

Hello
Please can you help with the following.
Domestic installations with PV and/or battery storage.
551.7.2 Where the generating set is connected to either the main consumer unit or via a separate consumer unit via Henley blocks the rating of the consumer units shall be protected by a OCPD InA≥In+Ig(s).
Where In = 100A DNO fuse and Ig(s) = 16A MCB or 2 x 16A MCB's which would be 116A or 132A, what inspection code should be given on an EICR where a standard domestic consumer unit is fitted which has a rating of 100A. Can any allowance be given on connected load being less than 100A or as the regulation relates to the rated current of the assembly and is a "shall" requirement does the load have no influence on the code assigned. 
Answer from Certsure

The Certsure Technical Helpline provides general information and guidance for compliance with the British Standard BS 7671, the Requirements for Electrical Installations, and matters concerning electrical safety within electrical installations designed, constructed, inspected, and tested to BS 7671. Without detailed knowledge of your installation, we cannot offer advice specific to your installation and can only generically provide comments based on the information you have provided.

The intent of the regulation is to ensure that the assembly is not overloaded with the additional generating set, as the main fuse may not protect the assembly if for example the internal busbar is pulling 116A.

Regulation 536.4.202 states: see regulation

From the viewpoint of an EICR, we would be looking for evidence that the assembly is being overloaded, such as burning, distorting and the likes.

The above regulation allows for diversity to be taken into account, so we can exercise our engineering judgement in declaring whether or not the assembly is suitably protected.

We trust that we have answered your current question; however if you require any further information or clarification, then please do not hesitate to contact us either by e-mail to helpline@certsure.com or by telephone on 0333 015 6628

I've read 536.4.202 and am interested on your views on the last paragraph with the shall requirement and how this ties in with the answer given by certsure. 536.4.3.2 is also relevant but has not been mentioned in the reply.

Thanks for your time.

Parents
  • 0

    So if I connect my 16 A output battery system (that can charge from the mains) through a 20 A breaker, I need a cable with current carrying capacity in the installation conditions to be at least 36 A.

    But why? I can see no reason for such a requirement. The cable will either be carrying 20A in one direction or 16A in the other - never 36A. I feel sure there must have been some mis-interpretation going on somewhere. There seems to be an assumption that an energy storage system is a load AND a source - whereas if actually (to my mind) either a load OR a source - and treating the two different situations distinctly (as we usually do where differing permutations are possible) doesn't lead us to thinking that Iz ≥ In + Ig is a requirement when there are no loads on that circuit while generating.

      - Andy.

  • 0 in reply to AJJewsbury
    AJJewsbury said:
    But why? I can see no reason for such a requirement.

    Imagine a plug-in inverter that connects into a socket-outlet final circuit (and, when not connected to the grid, they stop outputting power, so assumed to be "safe" on a plug in some countries).

    Not common in this country, but apparently they do exist in Europe, particularly on the balconies of flats in Germany.

    Some manufacturers abroad have also proposed similar for battery storage (offering "backup" to the relevant socket-outlet circuit by tripping an RCD or similar device in the CU if the mains goes off).

    So, that kind of thing is why 551.7.2 is as it is. And the requirements need to remain in place because of that. However, there's no saying an exception couldn't be made for battery storage systems (as it sort of is with Solar PV by virtue of Regulation 712.551.7.2).

    AJJewsbury said:
    I feel sure there must have been some mis-interpretation going on somewhere. There seems to be an assumption that an energy storage system is a load AND a source - whereas if actually (to my mind) either a load OR a source - and treating the two different situations distinctly

    Agreed in part. Not mis-interpretation, but requirements for one thing that have an unintended consequence on something else, as I said a couple of days back, and there is no reason why a dedicated circuit for a battery storage system (alone) should have to comply with those requirements, but at present there is no exception.

    However, despite the IET raising this in the CoP since 2017, and despite all those who reviewed the First and Second Editions of that CoP through public consultation as well as committee reviews, it is only now battery storage is becoming more in demand (and some installations have not been done in accordance with the recommendations of the CoP and requirements in BS 7671) that people are starting to see the issue.

    Certainly one on the list for further discussion.

Reply
  • 0 in reply to AJJewsbury
    AJJewsbury said:
    But why? I can see no reason for such a requirement.

    Imagine a plug-in inverter that connects into a socket-outlet final circuit (and, when not connected to the grid, they stop outputting power, so assumed to be "safe" on a plug in some countries).

    Not common in this country, but apparently they do exist in Europe, particularly on the balconies of flats in Germany.

    Some manufacturers abroad have also proposed similar for battery storage (offering "backup" to the relevant socket-outlet circuit by tripping an RCD or similar device in the CU if the mains goes off).

    So, that kind of thing is why 551.7.2 is as it is. And the requirements need to remain in place because of that. However, there's no saying an exception couldn't be made for battery storage systems (as it sort of is with Solar PV by virtue of Regulation 712.551.7.2).

    AJJewsbury said:
    I feel sure there must have been some mis-interpretation going on somewhere. There seems to be an assumption that an energy storage system is a load AND a source - whereas if actually (to my mind) either a load OR a source - and treating the two different situations distinctly

    Agreed in part. Not mis-interpretation, but requirements for one thing that have an unintended consequence on something else, as I said a couple of days back, and there is no reason why a dedicated circuit for a battery storage system (alone) should have to comply with those requirements, but at present there is no exception.

    However, despite the IET raising this in the CoP since 2017, and despite all those who reviewed the First and Second Editions of that CoP through public consultation as well as committee reviews, it is only now battery storage is becoming more in demand (and some installations have not been done in accordance with the recommendations of the CoP and requirements in BS 7671) that people are starting to see the issue.

    Certainly one on the list for further discussion.

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