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
  • You are perpetuating a myth Broadgage. A 100A fuse will never blow with 116 amps passing, and the protection current is considerably more than 100A. I regularly come across "electricians" who do not understand circuit ratings at all, and assume that fuses fail at their nominal current rating. They also assume that minor cable overloads will immediately cause melting or a fire, and even that ring finals are inherently "unsafe". I do not understand why this should be, but taking fig 3A1 in BS7671 as an example, a 5A BS88-3 fuse will carry 8A for an indeterminate period, as you see the graph does not continue because there are simply too many unknowns to the conditions. Given a 20C ambient temperature I would expect it to never blow, as the power dissipation would not raise the fusebox temperature significantly. You will notice that a 100A fuse would carry at least 150A for a similarly long period, and in a cool environment for a significant period of time, probably at least 10,000 seconds or 3 hours. The 100A rated CU could therefore be subjected to 150A, but this is unlikely to cause a failure except possibly if the main switch is expected to break this current. Given long enough the CU temperature could rise significantly, and most domestic CUs are probably not really rated for the high ambient temperatures of some switch-rooms with inadequate ventilation.

    To the OP, I agree with Andy and Mike, this is not a real or particular problem, and given normal load diversity it will not occur except possibly for insignificant periods. Domestic CUs with ratings over 100A are not available, and the switch to commercial or industrial switchgear with higher ratings is significantly expensive. Essentially you are looking for a problem where none exists, unless there is some very particular and strange domestic installation. Electric heating along with a car charger could change this, but such in one CU is probably unknown, as electric heating is usually off peak storage on cost grounds.

  • Domestic CUs with ratings over 100A are not available

    They might need to be soon, if we move to all electric heating and cooking as well as having to address EV loads.

    and given normal load diversity it will not occur except possibly for insignificant periods.

    Again, what is "normal load diversity"? Many of the assumptions we have traditionally made regarding what a "domestic load" looks like are not valid with all loads now being installed.

    For example, it has become commonplace for EV charging points to be set up for load curtailment at the rating of the distributor's cutout (or, an assumption made that this is 60 A). It could well be, that, with heat pumps, there are now long periods when the demand is way above what we were used to, for a number of hours in a row, and with other appliances on, well, who knows ?

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  • Domestic CUs with ratings over 100A are not available

    They might need to be soon, if we move to all electric heating and cooking as well as having to address EV loads.

    and given normal load diversity it will not occur except possibly for insignificant periods.

    Again, what is "normal load diversity"? Many of the assumptions we have traditionally made regarding what a "domestic load" looks like are not valid with all loads now being installed.

    For example, it has become commonplace for EV charging points to be set up for load curtailment at the rating of the distributor's cutout (or, an assumption made that this is 60 A). It could well be, that, with heat pumps, there are now long periods when the demand is way above what we were used to, for a number of hours in a row, and with other appliances on, well, who knows ?

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