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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

    If the Solar PV inverter and battery inverter are mounted 10 m away from the consumer unit and the total load of the installation is being supplied by Solar PV and battery, how is the cable to the consumer unit protected for high impedance fault current. Eg where buried<50mm PVC/PVC cable would require 30mA RCD.

    An RCD at the grid-end of the circuit will still detect & trip on earth faults even if all the current is provided by the on-site generator - why? because when running in parallel with the grid the on-site generator must not have its own N-PE link - the only path for any earth fault current is via the grid supply's N-PE link - and so the fault current will pass through the grid-side RCD (if only in it's N pole to return to the on-site generator's N). That won't disconnect the generator from the fault of course, but provided the RCD opens N as well as L(s) (as required 551.7.1(ii)) then the on-site generator becomes isolated from Earth (again because it doesn't have its own N-PE link) in effect becoming a separated or IT circuit, so the shock path is broken - so the danger is removed. No need for a 2nd RCD at the generator end (it wouldn't work anyway for faults (other than on the generator itself) as there's no N-PE link on the generator.

       - Andy.

  • 0 in reply to AJJewsbury

    Thank you.

    Having now appreciated that when the inverters are generating in parallel they are operating in separated or IT mode it makes more sense. Your explanation is a great help.

  • 0 in reply to Mspark
    Mspark said:
    Having now appreciated that when the inverters are generating in parallel they are operating in separated or IT mode

    No they are not, operating IT mode, they are either connected to the grid (in which case the grid earthing arrangement applies) or they ought to be connected in TN-S arrangement. IT is not recommended for island mode, and Chapter 82 in AMD2 requires a neutral to be formed by earthing after disconnecting the live conductors to the grid.

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  • 0 in reply to Mspark
    Mspark said:
    Having now appreciated that when the inverters are generating in parallel they are operating in separated or IT mode

    No they are not, operating IT mode, they are either connected to the grid (in which case the grid earthing arrangement applies) or they ought to be connected in TN-S arrangement. IT is not recommended for island mode, and Chapter 82 in AMD2 requires a neutral to be formed by earthing after disconnecting the live conductors to the grid.

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