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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

    This discussion is interesting but seems to be a little shy of the actual characteristics of the equipment. Battery inverters cannot supply significant excess current, and have protection built in to prevent this situation which otherwise would rapidly cause electronic failure. Therefore the short circuit condition is not as described, as the inverter MCB can probably never trip, the inverter will simply supply the rated current an very low voltage, or more likely shut down altogether. Therefore the mains MCB will trip too due to the excess current and the supply will not dissipate significant power in the cable, simply rated for the inverter output current.

    In a recent discussion, the problem of connections as shown in Andy's second drawing, was discussed but is probably unsatisfactory due to a different problem. In order to provide power back to the mains, although OK in Island mode, a significantly higher voltage is required to overcome any voltage drop in the connecting cables. The solution is that local generation should connect DIRECTLY to the consumer side of the meter via a dedicated circuit, not as drawing 2 on a local final circuit.

    The lack of RCD protection at this point is then not a problem either, assuming the cable is installed or chosen to make this unnecessary, such as SWA if buried or perhaps in steel conduit. Note that this arrangement also prevents any kind of overload situation of the connecting cable, so the "difficulties" with 536 etc are avoided. Over-current protection at the inverter is not required and "adding" of the protective device current is not required.

  • 0 in reply to David Stone
    David Stone said:
    Battery inverters cannot supply significant excess current, and have protection built in to prevent this situation which otherwise would rapidly cause electronic failure.

    Whilst this is technically correct, in practice if the inverter manufacturer fails to provide sufficient information to permit compliance with Regulation 419.2 an RCD is often necessary for ADS. This is required at the inverter itself, but it need not be a 30 mA RCD (and perhaps may be time delay depending in disconnection times) unless additional protection is required for other reasons (e.g. cables concealed in building fabric < 50 mm without suitable protection).

Reply
  • 0 in reply to David Stone
    David Stone said:
    Battery inverters cannot supply significant excess current, and have protection built in to prevent this situation which otherwise would rapidly cause electronic failure.

    Whilst this is technically correct, in practice if the inverter manufacturer fails to provide sufficient information to permit compliance with Regulation 419.2 an RCD is often necessary for ADS. This is required at the inverter itself, but it need not be a 30 mA RCD (and perhaps may be time delay depending in disconnection times) unless additional protection is required for other reasons (e.g. cables concealed in building fabric < 50 mm without suitable protection).

Children
  • 0 in reply to gkenyon

    I think that is wrong Graham, unless one wants additional protection. As I pointed out the inverter cannot provide excess current above its rating, and unless the fault is of a very high resistance cannot provide significant output voltage either above its rating. The problem is with the wording of 419.2, which is you suggest is requiring ADS when it is not necessary because the output voltage is completely limited by the output power rating and possible fault impedances. You are translating this as "we don't know" when this should be included in the specification, and in fact is by the maximum power output rating and time for which this may be provided. 419.2 clearly has thought of this in the part about reduced output voltage, and for that reason should be readily available from a manufacturer. Adding an inverter RCD is as Andy says unnecessary as the connection I suggest to the supply leaves all the normal RCDs to isolate faults elsewhere in the installation, and an Earth fault on the inverter cable, either SWA or steel conduit is exceedingly unlikely and not dangerous due to the limited output voltage. Due to the nature of the inverter, many people would probably suggest a type F RCD which is very expensive, and in my view unnecessary.

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