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Battery backup earthing - PME supply

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

Parents
  • gkenyon: 
     

    This is currently considered to be a specialist area, and the earthing can get complicated depending on the type of system and whether there is sufficient information available from the manufacturer. The IET has published a specific Code of Practice for this: IET Code of Practice for Electrical Energy Storage Systems.

    The independent output is not permitted to rely on the distributor's means of earthing in the UK - this is covered in BS 7671 (Regulation 551.4.3.2.1). You will need a consumer earth electrode, but this can be connected to the MET.

    The IET Code of Practice for Electrical Energy Storage Systems recommends such a “backup supply" operates in TN-S in island mode. Again, requirements for transfer of supply and switching means of earthing must comply with a number of specific Regulations in BS 7671.

    Will you be connecting it as a “switched alternative” to the mains for? If so, you need to comply with the requirements of G98 / G99. The detail of the required switching mechanisms are contained in the IET Code of Practice. Some “inverters” have the management system and switching (backup isolator and N-E bonding relay) in place to achieve this in a compliant manner. 

    After having a quick look at the manual you posted a link for, I don't think there's sufficient information in there to tell me whether it has an appropriate switching to achieve backup earthing as required in the UK, and you'll need more information or a more detailed schematic (unless you have that already).

    If the inverter “backup” output is “floating” ("earth free" or “IT”), it should only be used for a single piece of equipment that is not connected to anything else via communications cables etc. The IET Code of Practice does not recommend that a “floating” or IT arrangement “backup” output is connected to part of a fixed installation.

    There's also the consideration of a suitable location for the batteries and inverter, and specific recommendations in BS 5839-6 relating to linked fire detection and alarm if inverter and/or battery are in a location that is not frequently visited in the premises. 

    Thanks for this. 

    I will ask the manufacturer for specifics. 

    If the inverter has an automatic relay creating the E-N link on mains failure, i presume a supplementary earth rod will then make the system complaint (at least from an earth perspective). 
     

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

    Thanks. 

Reply
  • gkenyon: 
     

    This is currently considered to be a specialist area, and the earthing can get complicated depending on the type of system and whether there is sufficient information available from the manufacturer. The IET has published a specific Code of Practice for this: IET Code of Practice for Electrical Energy Storage Systems.

    The independent output is not permitted to rely on the distributor's means of earthing in the UK - this is covered in BS 7671 (Regulation 551.4.3.2.1). You will need a consumer earth electrode, but this can be connected to the MET.

    The IET Code of Practice for Electrical Energy Storage Systems recommends such a “backup supply" operates in TN-S in island mode. Again, requirements for transfer of supply and switching means of earthing must comply with a number of specific Regulations in BS 7671.

    Will you be connecting it as a “switched alternative” to the mains for? If so, you need to comply with the requirements of G98 / G99. The detail of the required switching mechanisms are contained in the IET Code of Practice. Some “inverters” have the management system and switching (backup isolator and N-E bonding relay) in place to achieve this in a compliant manner. 

    After having a quick look at the manual you posted a link for, I don't think there's sufficient information in there to tell me whether it has an appropriate switching to achieve backup earthing as required in the UK, and you'll need more information or a more detailed schematic (unless you have that already).

    If the inverter “backup” output is “floating” ("earth free" or “IT”), it should only be used for a single piece of equipment that is not connected to anything else via communications cables etc. The IET Code of Practice does not recommend that a “floating” or IT arrangement “backup” output is connected to part of a fixed installation.

    There's also the consideration of a suitable location for the batteries and inverter, and specific recommendations in BS 5839-6 relating to linked fire detection and alarm if inverter and/or battery are in a location that is not frequently visited in the premises. 

    Thanks for this. 

    I will ask the manufacturer for specifics. 

    If the inverter has an automatic relay creating the E-N link on mains failure, i presume a supplementary earth rod will then make the system complaint (at least from an earth perspective). 
     

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

    Thanks. 

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