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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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    Slowly getting more information and have asked the manufacturer for confirmation on some points of protection built in to their inverters.

    Copy of Residual Current Device Protection Manufacturer's Declaration  https://kb.givenergy.cloud/article.php?id=63 this built in device is a RCM not RCD so where protection is required in island mode by the use of an RCD this would have to comply with 531.3.4.1 or 531.3.4.2. The declaration does recommend a type A RCD where one is required.

    During my research I have also found one technical report which has helped me gain a better understanding of inverter operation so thought I would share, unfortunately it is a USA based paper but has still added to my knowledge.

    https://www.nrel.gov/docs/fy10osti/46698.pdf

    Extract from paper - Both inverters test results suggest that inverters designed to meet IEEE 1547 and UL 1741 produce fault currents anywhere between 2 to 5 times the rated current for 1 to 4.25ms

    I shall wait to hear back from the manufacturer but my initial understanding is short circuit protection built in to the inverter operates much quicker than a fuse or circuit breaker, when operating in island mode no other short circuit protection will be required where the suggested minimum cable size of 2.5mm is used for connection to the EPS output on a 3.6kW rated inverter.

Reply
  • 0

    Slowly getting more information and have asked the manufacturer for confirmation on some points of protection built in to their inverters.

    Copy of Residual Current Device Protection Manufacturer's Declaration  https://kb.givenergy.cloud/article.php?id=63 this built in device is a RCM not RCD so where protection is required in island mode by the use of an RCD this would have to comply with 531.3.4.1 or 531.3.4.2. The declaration does recommend a type A RCD where one is required.

    During my research I have also found one technical report which has helped me gain a better understanding of inverter operation so thought I would share, unfortunately it is a USA based paper but has still added to my knowledge.

    https://www.nrel.gov/docs/fy10osti/46698.pdf

    Extract from paper - Both inverters test results suggest that inverters designed to meet IEEE 1547 and UL 1741 produce fault currents anywhere between 2 to 5 times the rated current for 1 to 4.25ms

    I shall wait to hear back from the manufacturer but my initial understanding is short circuit protection built in to the inverter operates much quicker than a fuse or circuit breaker, when operating in island mode no other short circuit protection will be required where the suggested minimum cable size of 2.5mm is used for connection to the EPS output on a 3.6kW rated inverter.

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  • 0 in reply to Mspark

    Agreed regards BS 7671 and fault protection, you are limited to a either Reg 419.2 for ADS (comprehensive manufacturer information), and where an RCD is either selected, or otherwise required for additional protection, it should be selected from the lists of standards in Reg Group 531.4.

    Mspark said:

    Extract from paper - Both inverters test results suggest that inverters designed to meet IEEE 1547 and UL 1741 produce fault currents anywhere between 2 to 5 times the rated current for 1 to 4.25ms

    I shall wait to hear back from the manufacturer but my initial understanding is short circuit protection built in to the inverter operates much quicker than a fuse or circuit breaker, when operating in island mode no other short circuit protection will be required where the suggested minimum cable size of 2.5mm is used for connection to the EPS output on a 3.6kW rated inverter.

    Unlike RCDs, when selecting overcurrent protection, other devices providing a level of protection not less than that given by the list of devices in 533.1.1 are permitted. So, if you could read-across from the inverter product standard (BS or BS EN preferably), or the manufacturer's confirmed data, there would be no problem relying on that for overcurrent protection. Back-up protection could also be provided.

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