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Domestic Battery Storage - G83/G98 - Earthing Arrangements + More

vantech
Hi,


I am looking to add a battery storage system to my own home (I am an NICEIC registered contractor, just to make it clear this is a project for myself). This will be a battery only solution, and will not feature any solar arrangements. 


If there are any experts here that can advise on a few questions, that would be superb. 

 
  • GN98 (formally GN83) allows systems up to 16A per phase to be connected without the need for any sort of site inspection before the install (so the solution is notified after). I plan on doing this myself, and I am not part of a microgeneration scheme, but assume self certification is ok, following Part P requirements and issuing the relevant certs to the NIC. 


 

  • The battery I plan on using is the Alpha ESS (combined battery and inverter), which is grid tied with separate backup output(s) for things like lighting circuits and small power circuits. I have a smart meter, however I do not have a feed-in tariff, therefore by means of a CT connected to the battery I plan to set the feed-in to zero using the inverters user interface, so that the battery just delivers power to loads as and when its required. My question here however leads me onto earthing - when running in backup mode, the inverter automatically disconnects the grid and provides power on its dedicated backup connection - what do I do with the Earthing? I am on a TN-C-S supply from the DNO - I would assume an earth rod is required, connected into the MET (as the DNO earthing cannot be relied upon during an outage) which would be combined the DNO earthing (so a rod in the ground connected to the MET and bonded onto the battery inverter). Assuming this is correct, does the inverter need to tie its backup output neutral and earth conductors together, forming its own TN system (it could even be doing this internally I'm not sure and need to check). 


 

  • Anything else I may be missing?



Thanks.

  • 0
    Agree - you cannot wire it in a way where is a DP switch that might remove the NE link from the circuit but leave the house wiring powered by the inverter - depending if it is one or two CUs, if there are henley blocks in the tails etc that may or may not apply.

    I'm not keen on contactors N-E that are set by something that has to 'know the state' because it is another possible failure mode, but it may be unavoidable here

    My G-code comment was to say that I am not going to be drawn as to if G98 assumes things that may or may not apply in this case and if it is the right one to be looking at. (Not that I  have any reason to doubt your assumption is reasonable , I'm just warning you that I am not considering it for now, as I am not upto date with these battery power things and what standards they do or do not meet) Nor am I going to be sidetracked by the 'is this worth it?' question. You are not selling this to someone else under the pretence they will make a fortune, if  you were I might be more concerned, but it is a private experiment, and in that sense, a rather  interesting one. Let us know how it goes will you.

    M.
  • 0
    vantech:

    GN98 (formally GN83) allows systems up to 16A per phase to be connected without the need for any sort of site inspection before the install (so the solution is notified after). I plan on doing this myself, and I am not part of a microgeneration scheme, but assume self certification is ok, following Part P requirements and issuing the relevant certs to the NIC. 


    I am not quite sure how that works in the UK these days. The certificates for G.98 need to end up with your Meter Operator & DNO. It does tell you what you need to do in G.98. The standard also tells you what certificates / documentation you need. Obviously being Grid Tied, you will need a copy of the type tests on the inverter, which includes all its settings. G.98 also requires a block diagram showing how it is all connected together. Again, the standard tells you what you need.


    Depending on the DNO, they may require to witness what happens with a power failure, and it’s subsequent restoration. With a solar inverter, this is seeing the inverter shut down on loss of mains, then restart when the mains reappears. If your system is able to operate islanded -that is it continues to supply your house when the supply goes off, they may expect to see both phase and neutral to the metering dropped out by a suitable contactor or MCCB, as well as how it resynchronises when the mains returns. Obviously if it doesn’t synchronise before coming back and doesn’t briefly shut down altogether, you are likely to lose some fuses as a minimum, but may also destroy the inverter - the grid supply will always win when it comes to fault current! I would also expect to see the customer neutral connected to earth by a suitable contactor immediately after the supply disconnects, and immediately before it reconnects. 


    Regards,


    Alan. 


  • 0
    I would assume an earth rod is required, connected into the MET (as the DNO earthing cannot be relied upon during an outage) which would be combined the DNO earthing (so a rod in the ground connected to the MET and bonded onto the battery inverter). Assuming this is correct, does the inverter need to tie its backup output neutral and earth conductors together, forming its own TN system (it could even be doing this internally I'm not sure and need to check).

    Correct. If it can run in two different modes - grid tied and islanded - then in grid mode you must not have N-PE link and in island mode (assuming it's intended to be TN) you must have a N-PE link. Also in islanded mode all your live conductors (including N) need to be isolated from the grid.


    Using a contactor/relay  for the N-PE link (but not for the earthing connections) is feasible - but would often be built into the storage system itself - hopefully with some logic that attempted to do things in a sensible order - disconnect from grid, close local N-PE link, apply islanded power - with checks at each stage that the appropriate contacts have indeed opened/closed as required before allowing the next stage.


    The IET do have a publication that covers just this sort of thing in quite some detail: https://shop.theiet.org/code-of-practice-for-electrical-energy-storage-systems-2nd-edition


        - Andy.

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