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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.

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  • 0

    551.7.2 also means that, as a battery storage system is a load as well as a generator, the cable connecting it has to be "double rated"

    Eh? Are you suggesting a storage system that can draw say 16A to charge the battery or supply 16A from the battery needs to be connected using a 32A cable even if it's on it own dedicated circuit?

    That sounds rather silly to me. The storage system can't be a load and a source simultaneously - it's either one or the other - at least in nett effect - as you can't have current flowing in two directions in the same conductor at the same time - Messers Ohm and Kirchhoff would be spinning in their graves,

    Either the current flow is to the charger, in which case it's in effect just a simple load, or the current is away from the generator and then as far as BS 7671 is concerned it's just a simple generator with no loads on the circuit - in that situation it's no longer a final circuit (one supplying loads), the generator is connected on the supply side of all protective devices for final circuits and so the Iz ≥ In + Ig requirement doesn't apply.

    Any 'self consumption' by the storage system (e.g. running control circuitry of the storage system when generating) is an internal matter for the storage equipment I'd argue - same as a conventional generator re-charging its stater battery or running AVC circuitry - and we'd never consider a conventional generator to be a load simultaneously.

    The only situation where Iz ≥ In + Ig might make sense is where you'd have the storage system and other loads (e.g. socket outlets) on the same circuit as the storage system  - so parts of that circut's wiring could potentially draw In (from the grid) plus Ig (from the generator) without triggering any overload protection.

      -  Andy.

Reply
  • 0

    551.7.2 also means that, as a battery storage system is a load as well as a generator, the cable connecting it has to be "double rated"

    Eh? Are you suggesting a storage system that can draw say 16A to charge the battery or supply 16A from the battery needs to be connected using a 32A cable even if it's on it own dedicated circuit?

    That sounds rather silly to me. The storage system can't be a load and a source simultaneously - it's either one or the other - at least in nett effect - as you can't have current flowing in two directions in the same conductor at the same time - Messers Ohm and Kirchhoff would be spinning in their graves,

    Either the current flow is to the charger, in which case it's in effect just a simple load, or the current is away from the generator and then as far as BS 7671 is concerned it's just a simple generator with no loads on the circuit - in that situation it's no longer a final circuit (one supplying loads), the generator is connected on the supply side of all protective devices for final circuits and so the Iz ≥ In + Ig requirement doesn't apply.

    Any 'self consumption' by the storage system (e.g. running control circuitry of the storage system when generating) is an internal matter for the storage equipment I'd argue - same as a conventional generator re-charging its stater battery or running AVC circuitry - and we'd never consider a conventional generator to be a load simultaneously.

    The only situation where Iz ≥ In + Ig might make sense is where you'd have the storage system and other loads (e.g. socket outlets) on the same circuit as the storage system  - so parts of that circut's wiring could potentially draw In (from the grid) plus Ig (from the generator) without triggering any overload protection.

      -  Andy.

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  • 0 in reply to AJJewsbury
    AJJewsbury said:

    Eh? Are you suggesting a storage system that can draw say 16A to charge the battery or supply 16A from the battery needs to be connected using a 32A cable even if it's on it own dedicated circuit?

    That sounds rather silly to me. The storage system can't be a load and a source simultaneously - it's either one or the other - at least in nett effect - as you can't have current flowing in two directions in the same conductor at the same time - Messers Ohm and Kirchhoff would be spinning in their graves,

    No, I'm not suggesting that at all.

    It's just a consequence of Regulation 551.7.2, which in this particular respect has been in BS 7671 for quite some time.

    Now we've invented Battery Storage as part of prosumer's electrical installations, perhaps it's time the Regulation was reconsidered?

    However, this has been highlighted in the IET Code of Practice for Electrical Energy Storage Systems since the 1st Edition (2017), so nothing new to see here.

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