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Battery backup earthing - PME supply

vantech

Hi, 

I am installing a battery (Alpha ESS) to a PME system. 

The battery charges during off-peak and discharges during the day during peak hours. There is no solar. 

This relatively straight forward and will be notified to the DNO. 

The system features a back-up supply, where if the grid supply fails, a set of loads can be powered from a dedicated “backup” output on the inverter. This output is already separate from the grid input. 

The question I have is around earthing. When in normal operation the inverter is connected to the PME supply via the MET in the distribution board and the “backup” output is earthed through this connection. When the grid fails (outage, for example) my thinking is that the PME “earth” cannot be relied upon for the backup circuit (lost neutral for example). 

What would be the best course of action to resolve this situation? 

  1. Earth rod at the customer premises connected to the MET? 
  2. Rely on the manual and it’s wiring diagram? 
  3. Other? 
     

The inverter manual gives no indication. 

Thanks.

Battery Inverter Manual: 

https://www.alpha-ess.com/Upload/Images/20190814093353_165226.pdf

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  • 0

    AJJewsbury: 
     

    Sorry Andy, whilst this would at first appear correct, you also need to disconnect the supplier's neutral from the installation for a number of reasons. Failure to do so can render RCD protection inoperative (or could cause nuisance tripping) … and that may well be important because of the reduced EFLI in island mode.

    I disagree that the entire installation needs to be disconnected from the supplier's N - rather just the portion that's supplied by the embedded generation needs to be galvanically isolated. Just as if we had a UPS that was serving just a few critical items - the remainder of the installation remains connected to the (dead) grid supply during power cuts.

       - Andy.

    Yes, that is true, the Neutral must be disconnected from any part of the installation supplied by the backup.

    Not all UPS work like that, as we discussed in another thread recently. If you take your supply at LV, you can't rely on the distributor's N-E link, and of course you can't just “earth the neutral” upstream of the UPS to fix that without disconnecting the Neutral.

    So in that respect, UPS vs backup are identical.

    The IET CoP covers both options - “backup” for part of an installation, and “backup” for the whole installation. There still has to be a means of switching L & N to the distributor's supply from the backup system, and forming N-E connection.


    Anyway, the point I was making, that others make to me, is that you're not “isolated from the distributor's Neutral” if you keep the distributor's means of earthing connected - it's the Neutral conductor that needs separating, NOT the whole system from the distributor's means of earthing (and neutral).

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  • 0

    AJJewsbury: 
     

    Sorry Andy, whilst this would at first appear correct, you also need to disconnect the supplier's neutral from the installation for a number of reasons. Failure to do so can render RCD protection inoperative (or could cause nuisance tripping) … and that may well be important because of the reduced EFLI in island mode.

    I disagree that the entire installation needs to be disconnected from the supplier's N - rather just the portion that's supplied by the embedded generation needs to be galvanically isolated. Just as if we had a UPS that was serving just a few critical items - the remainder of the installation remains connected to the (dead) grid supply during power cuts.

       - Andy.

    Yes, that is true, the Neutral must be disconnected from any part of the installation supplied by the backup.

    Not all UPS work like that, as we discussed in another thread recently. If you take your supply at LV, you can't rely on the distributor's N-E link, and of course you can't just “earth the neutral” upstream of the UPS to fix that without disconnecting the Neutral.

    So in that respect, UPS vs backup are identical.

    The IET CoP covers both options - “backup” for part of an installation, and “backup” for the whole installation. There still has to be a means of switching L & N to the distributor's supply from the backup system, and forming N-E connection.


    Anyway, the point I was making, that others make to me, is that you're not “isolated from the distributor's Neutral” if you keep the distributor's means of earthing connected - it's the Neutral conductor that needs separating, NOT the whole system from the distributor's means of earthing (and neutral).

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