justinneedham:

This is all worthwhile. Thank you.. 

Skipping over for the moment the practicalities of the system.


Graham writes "I would think that the system will go TN-S in island mode? At least that's what the IET Code of practice for electrical energy storage systems recommends. The earth electrode is required to comply with Regulation 551.4.3.2.1, and not necessarily to form a "TT". 

- I will buy the COP, but before I receive that. You imply TN-S? This and all the adjacent properties being TNCS right now. Even if it were connected TN-S at the supply head, it would more-or-less be TNCS via the neighbours bonding to the gas.

Since consumers are not generally permitted to combine N & E functions in a single conductor in their installations according to ESQCR, the arrangement in island mode can only be described as TN-S.


However, as you correctly point out, the arrangement will remain connected to the PME earthing system (but this may well be the case anyway because of extraneous-conductive-parts). Therefore, certain provisions regarding the use of the PME earthing terminal continue to apply to the installation in island mode as well as connected mode.

Mike writes "having a system where you wish to load shed when on inverter really does affect the CU layout.., "

Indeed yes.  I didn't want to complicate my original post on this, but that's something else on the list for my customer who wants this facility but doesn't necessarily understand the considerable potential for extra wiring complexity.

This is also discussed in the IET CoP.Ultimately the easiest approach may well be two CUs, one with "connected mode only" loads, and the other which is energized in both connected mode and island mode.

I will add a final twist.. The Customer has an early-adopter EV charger also. Installed when external use on TNCS still had the "reasonably practicable"  get-out in 17th edition. The unit is fitted just inside up-over garage door, alongside the consumer units, and is exclusively used outdoors, cable under the swing-door. Customer has suggested pulling the EV charger into the new CU.. all very well, but I've turned this down since I can't fulfill the need for a fully isolated TT. Meanwhile I'm told that Mr Tesla has suggested "fixing" the problem by connecting it to "his" new rod, - This not quite worked out comment is where my original puzzle came from.

If Tesla is serous, then this is either:

A) Implying that the new rod is separate from the incoming earth in his installation, yet to use it and connect the EV charger would be ignoring inevitable proximity to the alternative supplier earthing system. Non starter then on two counts, because we've discounted the former (rod is connected to the PME) and the latter (proximity of two earthing systems).

..or B) The new rod will miraculously meet Annexe A733.3 for 70V touch voltage. - Not a chance in hell.

Agreed here - the earth electrode resistance will be tricky.


An alternative approach may be to use an open-PEN detection device. HOWEVER ... there are some tricky nuances here also:

• A device as described in 722.411.4.1 (iii) could be used, but for a single-phase installation a measurement earth electrode will be necessary. The measurement earth electrode would have to be separated by at least 2 m below ground from any buried uninsulated conductive parts connected to the PME earthing system. I know of at least one product of this type.

• A device as described in 722.411.4.1 (iv) would be ineffective in island mode. However, unless the car is being used as part of the island mode power supply, would you want the car to remain connected? You certainly couldn't charge it fully unless you had very large storage batteries in your EESS. So therefore the vehicle could be "electrically disconnected" (L, N and PE) in island mode. There may also be some voltage issues in connected mode in installations with solar PV inverters (and perhaps connected mode EESS inverters) causing nuisance tripping.

• If you are using a device the manufacturer claims complies with 722.411.4.1 (v), you will be reliant on that manufacturer's guidance for using it in an installation with EESS.

I am going to conclude that Tesla might be a little unclear about things in that respect, - or this is Chinese whispers playing havoc.

Plan:

Don't do anything about the EV charger- leave it alone on it's existing CU, because I can't change it at all without replacement with a fangled one having monitoring device 722.411.4.1(iii).

You could argue it complied with an earlier version of BS 7671, and you've not made things worse.

Issue relevant text about the above.

Find out if customer really wants backup circuits and if so to spell that out to me ASAP.

Backup of all circuits isn't really practicable - probably not recommended by Tesla either?

Fit new CU (with or without second backup circuits CU), and let Tesla get on with their bit.

 

 Agreed

justinneedham:

This is all worthwhile. Thank you.. 

Skipping over for the moment the practicalities of the system.


Graham writes "I would think that the system will go TN-S in island mode? At least that's what the IET Code of practice for electrical energy storage systems recommends. The earth electrode is required to comply with Regulation 551.4.3.2.1, and not necessarily to form a "TT". 

- I will buy the COP, but before I receive that. You imply TN-S? This and all the adjacent properties being TNCS right now. Even if it were connected TN-S at the supply head, it would more-or-less be TNCS via the neighbours bonding to the gas.

Since consumers are not generally permitted to combine N & E functions in a single conductor in their installations according to ESQCR, the arrangement in island mode can only be described as TN-S.


However, as you correctly point out, the arrangement will remain connected to the PME earthing system (but this may well be the case anyway because of extraneous-conductive-parts). Therefore, certain provisions regarding the use of the PME earthing terminal continue to apply to the installation in island mode as well as connected mode.

Mike writes "having a system where you wish to load shed when on inverter really does affect the CU layout.., "

Indeed yes.  I didn't want to complicate my original post on this, but that's something else on the list for my customer who wants this facility but doesn't necessarily understand the considerable potential for extra wiring complexity.

This is also discussed in the IET CoP.Ultimately the easiest approach may well be two CUs, one with "connected mode only" loads, and the other which is energized in both connected mode and island mode.

I will add a final twist.. The Customer has an early-adopter EV charger also. Installed when external use on TNCS still had the "reasonably practicable"  get-out in 17th edition. The unit is fitted just inside up-over garage door, alongside the consumer units, and is exclusively used outdoors, cable under the swing-door. Customer has suggested pulling the EV charger into the new CU.. all very well, but I've turned this down since I can't fulfill the need for a fully isolated TT. Meanwhile I'm told that Mr Tesla has suggested "fixing" the problem by connecting it to "his" new rod, - This not quite worked out comment is where my original puzzle came from.

If Tesla is serous, then this is either:

A) Implying that the new rod is separate from the incoming earth in his installation, yet to use it and connect the EV charger would be ignoring inevitable proximity to the alternative supplier earthing system. Non starter then on two counts, because we've discounted the former (rod is connected to the PME) and the latter (proximity of two earthing systems).

..or B) The new rod will miraculously meet Annexe A733.3 for 70V touch voltage. - Not a chance in hell.

Agreed here - the earth electrode resistance will be tricky.


An alternative approach may be to use an open-PEN detection device. HOWEVER ... there are some tricky nuances here also:

• A device as described in 722.411.4.1 (iii) could be used, but for a single-phase installation a measurement earth electrode will be necessary. The measurement earth electrode would have to be separated by at least 2 m below ground from any buried uninsulated conductive parts connected to the PME earthing system. I know of at least one product of this type.

• A device as described in 722.411.4.1 (iv) would be ineffective in island mode. However, unless the car is being used as part of the island mode power supply, would you want the car to remain connected? You certainly couldn't charge it fully unless you had very large storage batteries in your EESS. So therefore the vehicle could be "electrically disconnected" (L, N and PE) in island mode. There may also be some voltage issues in connected mode in installations with solar PV inverters (and perhaps connected mode EESS inverters) causing nuisance tripping.

• If you are using a device the manufacturer claims complies with 722.411.4.1 (v), you will be reliant on that manufacturer's guidance for using it in an installation with EESS.

I am going to conclude that Tesla might be a little unclear about things in that respect, - or this is Chinese whispers playing havoc.

Plan:

Don't do anything about the EV charger- leave it alone on it's existing CU, because I can't change it at all without replacement with a fangled one having monitoring device 722.411.4.1(iii).

You could argue it complied with an earlier version of BS 7671, and you've not made things worse.

Issue relevant text about the above.

Find out if customer really wants backup circuits and if so to spell that out to me ASAP.

Backup of all circuits isn't really practicable - probably not recommended by Tesla either?

Fit new CU (with or without second backup circuits CU), and let Tesla get on with their bit.

 

 Agreed