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Switched alternative to public supply. Tesla Powerwall and gateway. Have I got this 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 .


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).

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. 

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.

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. 

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. 

Have I reasoned this out right? 












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  • 0
    Ignoring the uncertain financial arguments for buying the thing,  which may well benefit from a cool headed re-visit, a large battery, a charger for the same and an inverter in a box has similar  requirements electrically   to  any back up generator of the kind that can also be paralelled with the mains supply.

    An earth electrode does not a TT installation make, if the genset is connected to  a system with a CPC - it is just TN-S, only when you are the generator, not the local substation you have to have the earth electrodes and NE links that a substation and or the DNO wiring would normally provide. This is because it is quite possible that a loss of supply may mean loss of CPC function and loss of NE bond as well, depending which bit of the DNO wiring has been hit by an enthusiastic digger bucket.


    I am quite sure that when islanded  the powerwall does have a metallic connection to the house wiring CPC, and if you'd like MCBs and RCDs to work, there had better be an NE link switched in as well.


    Now ignoring the 2nd hand advice, I'd say having a system where you wish to load shed when on inverter really does affect the CU layout, unless there are to be lots of downstream contactors on individual loads, as it becomes more like an economy 7 rig - either 2 CUs or a contactor and a split one.

    If your new CU feeds a mix of sockets to be kept on, and ones to be dropped, from the same final circuit, that will make it very difficult in practice to achieve the separation required.
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  • 0
    Ignoring the uncertain financial arguments for buying the thing,  which may well benefit from a cool headed re-visit, a large battery, a charger for the same and an inverter in a box has similar  requirements electrically   to  any back up generator of the kind that can also be paralelled with the mains supply.

    An earth electrode does not a TT installation make, if the genset is connected to  a system with a CPC - it is just TN-S, only when you are the generator, not the local substation you have to have the earth electrodes and NE links that a substation and or the DNO wiring would normally provide. This is because it is quite possible that a loss of supply may mean loss of CPC function and loss of NE bond as well, depending which bit of the DNO wiring has been hit by an enthusiastic digger bucket.


    I am quite sure that when islanded  the powerwall does have a metallic connection to the house wiring CPC, and if you'd like MCBs and RCDs to work, there had better be an NE link switched in as well.


    Now ignoring the 2nd hand advice, I'd say having a system where you wish to load shed when on inverter really does affect the CU layout, unless there are to be lots of downstream contactors on individual loads, as it becomes more like an economy 7 rig - either 2 CUs or a contactor and a split one.

    If your new CU feeds a mix of sockets to be kept on, and ones to be dropped, from the same final circuit, that will make it very difficult in practice to achieve the separation required.
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