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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.

  • 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.

  • 0

    My error is making an assumption that the inverter, on-site generation and storage will all be installed with a single connection and the batteries will only charge from the on-site generation, but it could of course be a load and charge the batteries from the DNO network, so it can be a load as well.

    But that means the total rating of the protective devices in the consumer unit cannot add up to more than the the rating of the consumer unit and the upfront DNO and/or installation upfront fuse.

    If the battery storage is a load then only 84 amps is available for the rest of an installation with a 100 amp fuse.

    I am a guy who has back into the habit of turning the televisions off at the wall rather than leaving them on standby, it would be very worrying for me to get anywhere near using the 60 amp capacity of my supply never mind twice as much as that.

    When I bought my data logger I tried it out at hand, it didn’t record much as it doesn’t record usage below 10 amps, over a week or so it did very briefly record just over 30 amps being drawn, but not for long.

    Obviously we don’t have electric heating and electric vehicles, yesterday I spent over a hundred pounds on diesel, but that’s a different issue as it’s on expenses and someone else is paying.

  • 0

    Maybe double pole MCB's as the incoming device on each consumer unit is the answer, 3 pole MCB's could also be used on three phase boards. Domestic consumer units are going to be running with 7kW EV chargers, 3kW-9kW battery chargers, 5kW-12kW ASHP all on low tariffs for prolonged periods during the night.

    The heat dissipated through the double pole MCB could be substantial. Add to this the derating factor for higher temperature inside the enclosure and a grouping factor as shown on the Hager guidance. I can see either lots of smaller consumer units or larger MCB's connected to larger final circuit cables.

    When BS 7671:2018+A2:2022 is fully implemented in October how am I going to address reg 132.16 where an existing solar system is connected to an installation that I need to alter or add to. If the 100A assembly does not have correct overcurrent protection I cannot alter or add to it.

  • 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.

  • 0 in reply to Sparkingchip
    Sparkingchip said:
    My error is making an assumption that the inverter, on-site generation and storage will all be installed with a single connection and the batteries will only charge from the on-site generation, but it could of course be a load and charge the batteries from the DNO network, so it can be a load as well.

    Definitely ... battery storage might be intended to charge from the mains "charge when cheap" type approach. It's certainly been put forward as an option.

  • 0 in reply to Mspark
    Mspark said:
    When BS 7671:2018+A2:2022 is fully implemented in October how am I going to address reg 132.16 where an existing solar system is connected to an installation that I need to alter or add to. If the 100A assembly does not have correct overcurrent protection I cannot alter or add to it.

    Agreed ... unless you find out how big that service cutout really is!

    Just to add, though, this is not new in March 2022.


    The IET Code of Practice for Electrical Energy Storage Systems has had the same advice since the 2nd Edition (published late 2020), as the change relates to a change in IEC 61439-1, not directly BS 7671 ...

  • 0 in reply to Sparkingchip
    Sparkingchip said:
    If the battery storage is a load then only 84 amps is available for the rest of an installation with a 100 amp fuse.

    That's not how maximum demand and diversity works.

  • 0

    There is going to have to be load management and curtailment in place in many installations.

  • 0 in reply to Sparkingchip

    Yup

  • 0

    So, if an electric vehicle is going to be used as a big storage battery will the EVSE circuit need to be double rated as it’s fed from both ends?

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