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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: 
     

    This would then form an island TN-S system for the inverters output?

    Correct. Typically all the "Earths" are (DNO and local electrode) are permanently connected together - the DNO's can't be relied upon during a grid failure and the local rod becomes just another bonded extraneous-conductive-part when the grid supply is working. But in island mode all the live conductors (N as well as L) of the inverter are kept completely separate from the DNO's.

       - Andy.

    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.

    Further, the DNO won't like it if they isolate the PME main for maintenance, and potentially find voltage on it they weren't expecting.

    Regardless, it's effectively a requirement of ESQCR.

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  • 0

    AJJewsbury: 
     

    This would then form an island TN-S system for the inverters output?

    Correct. Typically all the "Earths" are (DNO and local electrode) are permanently connected together - the DNO's can't be relied upon during a grid failure and the local rod becomes just another bonded extraneous-conductive-part when the grid supply is working. But in island mode all the live conductors (N as well as L) of the inverter are kept completely separate from the DNO's.

       - Andy.

    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.

    Further, the DNO won't like it if they isolate the PME main for maintenance, and potentially find voltage on it they weren't expecting.

    Regardless, it's effectively a requirement of ESQCR.

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