justinneedham:

Who here has first-hand of Tesla Powerwall? On cursory inspection, on-line technical info is sparse.


One of my regular domestic customers has had a quote for a Tesla Powerwall, including the Tesla isolating gateway to allow some property consumption in selected backup circuit(s) during network outage. Meanwhile I am quoting for CU upgrade to go in ahead of that work so that it can take place. The latter is basic work, no problems, were it not for the former which is providing me with a bit of a conceptual challenge. Now the first bit below is second-hand from the Tesla installer via my customer. Hence parts of it might be lost in translation, but as far as I have received from Tesla installer:


"Go ahead and to the CU, - it won't affect the Tesla."

The Tesla gateway contains SPD so I "don't need" to fit SPD in my CU. (But Tesla only up to 1KV, so not fully meeting AFAICT, 442.2.2)

Now the biggie: Tesla say they will be fitting a separate earth rod such that (as far as I understand it..), when the gateway entirely disconnects the network including earthing conductor (which after the supply head is no longer PEN 461.2).. then the gateway  "islands" the whole property on the TT via it's switch 537.1.5. 551.6 .

Would it be "TT"? 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".


Island mode (see BS HD 60364-8-82) is NOT associated with "TT island" as often used by the EV charging community ... which is why I really disagree with the use of the term "TT island" which does NOT appear in any of the standards, or indeed the IET guidance.

I'm not so familiar with switched alternative to public supplies, and in the absence of anything from Tesla, those who are able to might add to my reasoning below (or otherwise abuse it):


1) The property bonding and MET would still be connected to incoming PEN via gas/water pipe and next-door house(s).

This is the reason the system will go TN-S when in island mode. It's also the reason you can't easily generate a TT system in island mode.

2) What happens to the Tesla earth rod in normal (grid connected) operation? I'm guessing it's wired by the installer such it's effectively just a bonded extraneous conductive part. 

It is nothing other than an extraneous-conductive-part in connected mode, although if the resistance is low enough it may help keep touch voltages down, on exposed-conductive-parts, or extraneous-conductive-parts, that are accessible outdoors, particularly away from the main building(s), in the event of a failure of the PME neutral conductor.

3) Under loss of network power, Tesla gateway disconnects incoming supply and TNCS earthing conductor and now floats on the parallel impedance of it's Ra and the bonding. During this time,the disconnected property would (due to high-ish Ra) likely get bootstrapped close to whatever voltage is on the incoming gas/water/bonding.

Is this the case? The recommendation of the IET CoP are to leave the distributor's means of earthing connected, but to isolate the Live Conductors of the supply.

4) Although there are two earthing systems, there aren't two simultaneously accessible earthing systems..  Because in normal (network) use the TT electrode would be just a bonded extraneous part, and in "island" mode the TNCS earthing is not connected, - But to make this claim, we'd then need to also state that the TNCS "not-MET" (with installation earth no longer connected) was a bonded extraneous conductive part. 

To separate them would require a suitable distance below ground of all parts connected to the PME, and everything connected to the TT system. This is in practice in most domestic premises at least, not achievable, not least because of extraneous-conductive-parts, hence the recommendation to leave the distributors means of earthing connected, and to isolate the live conductors (each L, and N). N isolated to prevent two N-E bonds, which causes all sorts of havoc, although I fully appreciate that technically it remains connected via the means of earthing.

5) Loss of PEN externally: The Tesla I'm guessing would detect any possible lift in earth potential of the TNCS MET and could then switch into a safe condition (house disconnected entirely, other than bonding to PEN), and un-powered, or even maybe continue to generate for the house backup circuits, while floating on whatever fault condition PEN/bonding voltage was imposed on it. 

Not sure of that ... unless it contains a means of detection according to 722.411.4.1 (iii) of either BS 7671:2018 or BS 7671:2018+A1:2020.

Have I reasoned this out right?

justinneedham:

Who here has first-hand of Tesla Powerwall? On cursory inspection, on-line technical info is sparse.


One of my regular domestic customers has had a quote for a Tesla Powerwall, including the Tesla isolating gateway to allow some property consumption in selected backup circuit(s) during network outage. Meanwhile I am quoting for CU upgrade to go in ahead of that work so that it can take place. The latter is basic work, no problems, were it not for the former which is providing me with a bit of a conceptual challenge. Now the first bit below is second-hand from the Tesla installer via my customer. Hence parts of it might be lost in translation, but as far as I have received from Tesla installer:


"Go ahead and to the CU, - it won't affect the Tesla."

The Tesla gateway contains SPD so I "don't need" to fit SPD in my CU. (But Tesla only up to 1KV, so not fully meeting AFAICT, 442.2.2)

Now the biggie: Tesla say they will be fitting a separate earth rod such that (as far as I understand it..), when the gateway entirely disconnects the network including earthing conductor (which after the supply head is no longer PEN 461.2).. then the gateway  "islands" the whole property on the TT via it's switch 537.1.5. 551.6 .

Would it be "TT"? 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".


Island mode (see BS HD 60364-8-82) is NOT associated with "TT island" as often used by the EV charging community ... which is why I really disagree with the use of the term "TT island" which does NOT appear in any of the standards, or indeed the IET guidance.

I'm not so familiar with switched alternative to public supplies, and in the absence of anything from Tesla, those who are able to might add to my reasoning below (or otherwise abuse it):


1) The property bonding and MET would still be connected to incoming PEN via gas/water pipe and next-door house(s).

This is the reason the system will go TN-S when in island mode. It's also the reason you can't easily generate a TT system in island mode.

2) What happens to the Tesla earth rod in normal (grid connected) operation? I'm guessing it's wired by the installer such it's effectively just a bonded extraneous conductive part. 

It is nothing other than an extraneous-conductive-part in connected mode, although if the resistance is low enough it may help keep touch voltages down, on exposed-conductive-parts, or extraneous-conductive-parts, that are accessible outdoors, particularly away from the main building(s), in the event of a failure of the PME neutral conductor.

3) Under loss of network power, Tesla gateway disconnects incoming supply and TNCS earthing conductor and now floats on the parallel impedance of it's Ra and the bonding. During this time,the disconnected property would (due to high-ish Ra) likely get bootstrapped close to whatever voltage is on the incoming gas/water/bonding.

Is this the case? The recommendation of the IET CoP are to leave the distributor's means of earthing connected, but to isolate the Live Conductors of the supply.

4) Although there are two earthing systems, there aren't two simultaneously accessible earthing systems..  Because in normal (network) use the TT electrode would be just a bonded extraneous part, and in "island" mode the TNCS earthing is not connected, - But to make this claim, we'd then need to also state that the TNCS "not-MET" (with installation earth no longer connected) was a bonded extraneous conductive part. 

To separate them would require a suitable distance below ground of all parts connected to the PME, and everything connected to the TT system. This is in practice in most domestic premises at least, not achievable, not least because of extraneous-conductive-parts, hence the recommendation to leave the distributors means of earthing connected, and to isolate the live conductors (each L, and N). N isolated to prevent two N-E bonds, which causes all sorts of havoc, although I fully appreciate that technically it remains connected via the means of earthing.

5) Loss of PEN externally: The Tesla I'm guessing would detect any possible lift in earth potential of the TNCS MET and could then switch into a safe condition (house disconnected entirely, other than bonding to PEN), and un-powered, or even maybe continue to generate for the house backup circuits, while floating on whatever fault condition PEN/bonding voltage was imposed on it. 

Not sure of that ... unless it contains a means of detection according to 722.411.4.1 (iii) of either BS 7671:2018 or BS 7671:2018+A1:2020.

Have I reasoned this out right?