Alan B said:

I am installing an EV charger. At the moment my plan would be to a bit more investigation but ultimately will just connect the TNCS and TT earth together and to the EV db.

As Andy says, difficult to tell without seeing the installation. I would be very wary of simply connecting the two without checking the island mode design through fully, because depending on how the inverter is arranged, the you might just "re-earth" a neutral in the installation, which is potentially a breach of ESQCR, and might cause unwanted operation of RCDs in the installation. It would effectively be an 'N-E fault'.

But ... leaving them separated could potentially cause problems, because I'm sure you're aware that BS 7671 requires simultaneously accessible exposed-conductive-parts to be connected to the same earthing system where ADS is used (Reg 411.3.1.1).

There are apparently a number of inverters out there at the moment that offer so-called "island mode" or "power backup" which cannot be installed according to the UK's installation standards/requirements and UK industry guidance for installations, in respect of that functionality.

Just one example of differences in approach internationally, is that a PME type system is used in other countries in Europe, such as Germany.

• In Germany, there is no problem relying on the N-PE connection at the origin of the installation, and most installations already have an earth electrode (foundation earth electrode), and I believe that island mode is simply achieved in Germany by disconnecting the line conductors from the grid.
• Not so in the UK, where we have Regulation 551.4.3.2.1 (can't rely on distributor's means of earthing for switched alternative supplies, i.e. island mode) simply because the N-PE connection and the earthing of the PEN is wholly outside the installation and the consumer's control. So our island mode approach (supported by G98/G99, which by virtue of Regulations 21 and 22 of ESQCR are effectively mandatory where they apply as it provides the "distributor's requirements" mentioned in the legislation) is to disconnect all live conductors (including Neutral) and then re-reference the Neutral by closing a 'N-E Bond Relay' or 'System Referencing Relay' (reverse procedure when returning to connected mode).

Alan B said:

I am installing an EV charger. At the moment my plan would be to a bit more investigation but ultimately will just connect the TNCS and TT earth together and to the EV db.

As Andy says, difficult to tell without seeing the installation. I would be very wary of simply connecting the two without checking the island mode design through fully, because depending on how the inverter is arranged, the you might just "re-earth" a neutral in the installation, which is potentially a breach of ESQCR, and might cause unwanted operation of RCDs in the installation. It would effectively be an 'N-E fault'.

But ... leaving them separated could potentially cause problems, because I'm sure you're aware that BS 7671 requires simultaneously accessible exposed-conductive-parts to be connected to the same earthing system where ADS is used (Reg 411.3.1.1).

There are apparently a number of inverters out there at the moment that offer so-called "island mode" or "power backup" which cannot be installed according to the UK's installation standards/requirements and UK industry guidance for installations, in respect of that functionality.

Just one example of differences in approach internationally, is that a PME type system is used in other countries in Europe, such as Germany.

• In Germany, there is no problem relying on the N-PE connection at the origin of the installation, and most installations already have an earth electrode (foundation earth electrode), and I believe that island mode is simply achieved in Germany by disconnecting the line conductors from the grid.
• Not so in the UK, where we have Regulation 551.4.3.2.1 (can't rely on distributor's means of earthing for switched alternative supplies, i.e. island mode) simply because the N-PE connection and the earthing of the PEN is wholly outside the installation and the consumer's control. So our island mode approach (supported by G98/G99, which by virtue of Regulations 21 and 22 of ESQCR are effectively mandatory where they apply as it provides the "distributor's requirements" mentioned in the legislation) is to disconnect all live conductors (including Neutral) and then re-reference the Neutral by closing a 'N-E Bond Relay' or 'System Referencing Relay' (reverse procedure when returning to connected mode).