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. 

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. 

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.

vantech: 
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. 

No!, you can't quite assume that. Whilst it may form “TN-S”, it has to also include a [compliant] switching arrangement to appropriately disconnect the DNO's neutral.

 

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.

Indeed, as Andy suggests, earths solidly strapped, and all pole change-over (Ls and N ) from DNO to inversion, so load N is lifted off the DNO N during island.

M.

gkenyon: 
 

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.

Hi,

If the inverter makes a direct connection between earth and its “backup“ neutral output, the disconnection of live and neutral “backup” outputs would already be isolated to prevent feedback into the inverters “grid“ input (otherwise the grid input of the inverter would become live powering non-essential loads). 

So the inverter itself (at least the type I have posted) will disconnect incoming L+N from its battery back up output. 

If we installed a change over switch that disconnects L+N into the inverter, or indeed the whole installation, there would still be a permanent connection through the earth conductor (as this has been switched in automatically) on the intertevers backup output and the N-E link on the PME system (for example on the cut-out). 

Albeit the L+N can be isolated firmly, there will also be a ”link” of sorts between the PME connection, the ground rod and the N connection of the inverters output.

The inverter is designed to provide an instantaneous change over to prevent loss of power to essential loads, so the use of a manual change over is not required. 

What impact (if any) will the PME earth (inside the cut-out or the street, wherever the DNO may have done this) to neutral link have, with regards to isolation? 

Thanks. 

This area seems to be a minefield of potential complications.

Z.

There is no problem having the DNO's earthing terminal connected to MET at the same time as your own earth electrode (just like extraneous-conductive-parts). In fact, a consumer's supplementary earth electrode is recognized by BS 7671. See Fig 3.9 and the descriptions below it.

What you MUST NOT do is connect N to PE within your installation (downstream of the DNO's service head), without disconnecting the distributor's neutral first - regardless of whether the supply is TN-C-S (PME), TN-S, or TT. This is not only an issue for protective devices, but also potentially contravenes ESQCR.

It's absolutely fine for the appropriate switching devices (island mode isolator to disconnect the distributor's live conductors, all lines and Neutral and the N-E bond relay to form TN-S when the grid is fully disconnected) to be part of the inverter, battery management system, etc., provided they meet these requirements.

You will, of course, need to provide the additional consumer earth electrode if one is not already present. The value of electrode resistance required will depend on the size of RCD used for the island-mode circuits ("essential loads") and whether there are any other complications such as TT-d outbuildings or EV charging points.

You must ensure that protection against electric shock and overcurrent are available in all modes of operation.

Because an EESS charges the battery as well as as discharging it, you will need to check the rating of the CU is not exceeded. For example, if the CU is rated for 100 A, and there's a 100 A service fuse, and a 16 A output battery storage system - by feeding 16 A in at one end through an OCPD, because that OCPD gets hot it contributes to the total heat load in the CU - therefore the CU should be rated for 116 A.

All of this is covered in the IET Code of Practice for Electrical Energy Storage Systems.

gkenyon: 
 

There is no problem having the DNO's earthing terminal connected to MET at the same time as your own earth electrode (just like extraneous-conductive-parts). In fact, a consumer's supplementary earth electrode is recognized by BS 7671. See Fig 3.9 and the descriptions below it.

What you MUST NOT do is connect N to PE within your installation (downstream of the DNO's service head), without disconnecting the distributor's neutral first - regardless of whether the supply is TN-C-S (PME), TN-S, or TT. This is not only an issue for protective devices, but also potentially contravenes ESQCR.

Ah I see, so you’re saying the order of change over needs to ensure line conductors are broken before the N-E link is made by the inverter, and then broken first before the DNO supply is reinstated?

I would hope (wrongly or rightly) the inverter has been designed to take care of this, where it’s “backup” output shuts down, breaks the N-E link and then reinstates what is DNO supplied/synchronised power onto its backup output. 

I can see some inverters on the market have separate automatic change over devices using sets of contactors, however they don’t seem to address the issue of the N-E bond. 

This is something from Solax - install guide at the bottom of the page.

https://www.wattuneed.com/en/mounting-and-accessories/21133-x1-eps-box-solax-0768563817090.html

My thinking here is that actually this device breaks the grid input before it allows connection of the inverters backup supply. I think a simple relay could be added here to make a E-N link on switch over.