Help with Reg 551.7.2 Ina >= In + Ig(s) on 3 Phase Supply

[woodster92]

Hello, I'm struggling to understand how to apply Reg 551.7.2 [in particular the formula Ina >= In + Ig(s) ] to the following situation.

I hope the attached image is clear enough, it shows an existing 3 Phase Domestic supply with:
A) CU1 is a single phase consumer unit serving the main house with circuits as shown (uses Line 3 from the 3 Phase supply)
B) CU2 is a 3 Phase consumer unit with circuits as shown which are: (i) a 3 Phase Electric Vehicle charger, (ii) the circuit labelled "spare" takes an SWA feed from Line 3 to a second new property in the back garden. I don't have a photo of the CU (lets call it CU3) in the second smaller property yet, I imagine its another 100A Mains CU.
Note that the 2 Lucy Blocks have a "100 Amp" annotation on them in red text.
I also do not know the spec of the existing 3 Phase EV Charger.

This is the existing setup to which the following will be added: installation of a single phase Solar PV system with battery storage which will be installed in the garage in close proximity to the image shown. A new single phase 63A RCD (bidirectional) Consumer Unit will be added which will serve a a 5kW Hybrid Inverter which will manage both the solar generation and the battery storage.

I think the issue in this case is that in theory according to this Reg, there could be a risk of >100Amps passing through the Lucy blocks.

In order to mitigate this risk I think a solution would be to:

1) Replace the 100A blocks with a higher rating. I have not managed to find a 150 Amp connector block but there are single pole 200A blocks available such as this: https://www.cef.co.uk/catalogue/products/1149265-200a-single-pole-connector-block

2) Install a 63A Switch Fuse and redirect the tails for CU1 to come from this to derate the CU1 Mains from 100A down to 63A using for example one of these: https://www.cef.co.uk/catalogue/products/1538601-100a-fused-switch-metal-ip40

Does anyone have any views or alternative suggestions?

Now to the nitty gritty of the Reg and how to apply it in this case, I'm unsure where to take a value of In from in this scenario, there are 2 clauses:

551.7.2 (i) The Conductors of the Final Circuit shall meet the following requirement: Iz >= In + Ig

Where:
Iz current-carrying capacity of the final circuit conductors
In rated current or current setting of the protective device of the final circuit
Ig rated output current of the generating set

Variable	Location / Where is this value taken from ?	Value
Iz	Is this the current carrying capacity of each 25mm supply cable?	97A
In	Where is this taken from?	?
Ig	This value is taken from the datasheet of the Inverter. 32A MCB which supplies power to/from the Inverter	21.7A

Next Clause:

551.7.2 (v) Where the generating set is connected via a low voltage switchgear and control gear assembly then: Ina >= In + Ig(s)

Where:
Ina is the rated current of the assembly
In is the rated current or current setting of the incoming circuit overcurrent protective device either incorporated within the low voltage switchgear and controlgear assembly or upstream of it
Ig(s) rated output current of the generating set or sets

Variable	Location / Where is this value taken from ?	Value
Ina	Presume this is taken from the 100A Lucy blocks	100A
In	Where is this taken from?	?
Ig(s)	This value is taken from the datasheet of the Inverter. CU3.Circuit Number 1 is a 32A RCBO which supplies power to/from the Inverter. There is only 1 generating set	21.7A

 

[Upton Sparks]

Just stick the 63amp fused connector before the Henley block , or Lucy block as you call it, and your all good,

as for your 2nd point, ,

the first part of the regulation is talking about the current carry capacity of the cable, in this case the meter tails, I'm pretty sure 25mm tails are good for 116amps.

the 2nd part is talking about your assembly, ie the fuse board, the switches, the busbars, the Henley block, so from your example above, (In) is the service fuse again, and (Ig) is your generated load, so if you don't do anything then the fuse board in theory could have 120amps flowing through it, when its rated at 100amps,

but you know that already, that why you're putting the 63amp fused Isolator in, you are just putting it in the wrong place, it need to protect the Henley block as well,

then you need to label the fuse board to indicate why you've done that to warn other before they mess around with it, and change the 63 amp fuse to a 100 amp fuse,,

page 78 of the CoP for EESS, has a good example for you,

hope that helps,