Domestic consumer unit rating with PV and battery storage.

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
  • and there's a 100 A service fuse

    From the perspective of EICR, do you know the service fuse is actually 100 A?

    So, from a coding perspective, you'd need to be careful. EICRs ought not to record assumptions as facts.

    the rating of the consumer units shall be protected by a OCPD InA≥In+Ig(s)

    "Which consumer units?" is a good question.

    Regulation 551.7.2 addresses only the assembly through which the generator (inverter or storage system) connects. It takes no account of the diversity of the loads connected to that consumer unit, only the protective device at its input, or upstream of it, along with the output rating(s) of the generator(s) connecting through it.

    Regards the other CU (if you have two and split the tails via suitable connection blocks) this needs to be protected according to instructions from the manufacturer (which may or may not take into account upstream protective devices, and therefore you may or may not need to take into account the contribution of the generator(s) upstream of this other (loads only) CU.

    I think Regulation 536.4.202 is the correct reference for the "other" regulation covering this, statement regarding overload protection being the last para of the Regulation:

    However, overload protection shall not solely be based on the use of diversity factors of the downstream circuits. To achieve overload protection of RCCBs or switches, the rated current of the OCPD shall be selected according to the manufacturer’s instructions.

    So, clear as mud really ... a lot hinges on the manufacturer's instructions (apologies to   for adding that) and their applied assembly ratings, according to BS 7671.

    Sadly, I know that doesn't make it an easy job to assess for EICR (it's not a design job; however, things are always difficult when BS 7671 changes design premises like this) and therefore the only thing you could go off is evidence of overload or clear disparity between the distributor's cutout and the rating of an assembly.

  • Perhaps there will be a need in the future for more than one domestic consumer unit, rather than assume higher rated units will be available. The current ranges are quite economical to purchase, so why increase cost by going outside the norm? We used to have at least two consumer units years ago, one for the 24 hour supply and the second for off peak storage heaters and water heating. Could we do that again?

    Z.

  • Perhaps there will be a need in the future for more than one domestic consumer unit, rather than assume higher rated units will be available.

    That would work, but only if you install another overcurrent protective device (switch with integral fuse probably - see note) before new CU, so the sum of the ratings of the generators plus the nominal rating of the overcurrent protective device, is less than the current rating of the CU connecting the generation.

    Note: The overcurrent protective device prior to the CU connecting the generators would have to be stand-alone, in its own enclosure, and not in a CU either!

Reply
  • Perhaps there will be a need in the future for more than one domestic consumer unit, rather than assume higher rated units will be available.

    That would work, but only if you install another overcurrent protective device (switch with integral fuse probably - see note) before new CU, so the sum of the ratings of the generators plus the nominal rating of the overcurrent protective device, is less than the current rating of the CU connecting the generation.

    Note: The overcurrent protective device prior to the CU connecting the generators would have to be stand-alone, in its own enclosure, and not in a CU either!

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