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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

    At the start the discussion was about 16 amp battery storage, however the V2G vehicle to grid inverters may have an output of 6 kilowatts, around 26 amps, so with that in mind what’s the short circuit current likely to be from a V2G inverter?

    https://www.indra.co.uk/v2g

  • 0 in reply to Sparkingchip
    Sparkingchip said:
    so with that in mind what’s the short circuit current likely to be from a V2G inverter?

    The "rule of thumb" is 120 % of (1.2 times) the nominal maximum output of the inverter, so 19.2 A for a 16 A output inverter, or 31.2 A for a 26 A output inverter.

    Hence, you might just get away with B6 meeting the disconnection times, but a B10 would not be considered to comply with BS 7671 for disconnection times (where you have to consider 5 times nominal rating for ADS).

    This is why the IET CoP for EESS tends towards recommending RCDs at the inverter output and RCD / RCBOs protecting final circuits in island mode.

  • 0 in reply to gkenyon

    Of course at short circuit, the voltage has fallen to zero, so there is no need for any rapid  ADS at all, so the MCB is irrelevant. The inverter output stages either folding back to current limit or going bang is an adequate and far faster safety mechanism than any MCB fuse or RCBO, happening in a fraction of a half cycle.

    Mike

Reply
  • 0 in reply to gkenyon

    Of course at short circuit, the voltage has fallen to zero, so there is no need for any rapid  ADS at all, so the MCB is irrelevant. The inverter output stages either folding back to current limit or going bang is an adequate and far faster safety mechanism than any MCB fuse or RCBO, happening in a fraction of a half cycle.

    Mike

Children
  • 0 in reply to mapj1
    mapj1 said:
    Of course at short circuit, the voltage has fallen to zero, so there is no need for any rapid  ADS at all, so the MCB is irrelevant.

    The time in which the voltage drops is definitely important. Have a look at 1st para of Regulation 419.2.

    419.2 For installations with power electronic convertors with nominal voltage U0 greater than 50 V AC or 120 V DC and where automatic disconnection is not feasible, the output voltage of the source shall be reduced to 50 V AC or 120 V DC or less in the event of a fault between a live conductor and the protective conductor or Earth in a time as given in Regulation 411.3.2.2, 411.3.2.3 or 411.3.2.4, as appropriate (see BS EN 62477-1).

    mapj1 said:
    The inverter output stages either folding back to current limit or going bang is an adequate and far faster safety mechanism than any MCB fuse or RCBO, happening in a fraction of a half cycle.

    So, that would put the time factor to bed, but if the manufacturer does not provide the relevant information on the operating times, and also the further information necessary, namely adequate methods for initial verification and periodic inspection and testing, it can't be relied upon (2nd para of Regulation 419.2):

    The power electronic convertor shall be one for which the manufacturer gives adequate methods for the initial verification and periodic inspection and testing of the installation.

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