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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

    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?

    Sorry Graham, I'm not following. If we had a storage system that say could supply 16A or draw 16A from the grid - connected to dedicated circuit - what rated cable are you suggesting we need?

       - Andy.

Reply
  • 0

    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?

    Sorry Graham, I'm not following. If we had a storage system that say could supply 16A or draw 16A from the grid - connected to dedicated circuit - what rated cable are you suggesting we need?

       - Andy.

Children
  • 0 in reply to AJJewsbury
    AJJewsbury said:
    Sorry Graham, I'm not following. If we had a storage system that say could supply 16A or draw 16A from the grid - connected to dedicated circuit - what rated cable are you suggesting we need

    Because a storage system that can charge from the grid is connected (by definition) to a load-side of an OCPD (it is a load as well as a source), and the circuit meets the definition of a final circuit at least when the battery is charging ("Final circuit. A circuit connected directly to current-using equipment,", and current-using equipment being equipment that converts electrical energy into another form of energy, in this case chemical energy) then the requirements if (i) through (iv) inclusive of 551.7.2 applies, and the first is:

    Iz ≥ In + Ig

    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.

    Table 4D5 for T&E, Ref Method C is 4 sq mm, but Method 103# is 16 sq mm !!!

    Yes, I know this doesn't make sense for a circuit whose only load is a grid-connected battery storage system, but it's what there is at the moment I'm afraid. And as I said earlier, this has been highlighted in the IET CoP for EESS since the 1st Ed in 2017.

  • 0 in reply to gkenyon

    Except the sign of Ig wrt  In should be reversed, as the currents should be subtracted, (in some more complex system where charging and generation could be simultaneous present) so that the total is 4A (in the load direction).

    The signs only add when there is a common load is fed from two parallel sources of generation.

    Let us ignore the scope for using generation to also correct the power factor of the system, putting current out of phase with the voltage and  requiring complex maths.

    It would still be poorly worded. But over estimating  the cable size is nothing compared to the aggro of removing, reversing and refitting the protective device when the net current crosses zero.

    Mike.

  • 0 in reply to mapj1
    mapj1 said:
    The signs only add when there is a common load is fed from two parallel sources of generation.

    Or a fault in the circuit.

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