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Double wound safety transformer for EV supply.

Hi everyone, I have only posted once before so thanks to anyone who replies!

I am following on from the earlier "70 volt PEN conductor not allowed to exceed post", and looking into supplying a client with an electric vehicle power supply from a three phase isolating transformer BS 7671 722.413 (1.2): " The circuit shall be supplied through a fixed isolating transformer.."

The general consensus seems to be that an external IP box with an RCD (Type B) and a tethered lead is the standard to follow, and this may be the only option with a 230 volt domestic supply, but why not use a 3 phase 400 volt step down or tapped, safety double wound isolation transformer in a standard 100 -200 ampere or above industrial units/sheds?

( Subject to load and diversity).

The answer often stated when I have asked sparks/engineers is that in-rush current are too high but a type D CB BS 60898 will 'let through' the in-rush ( the transformer manufacturer agrees), and will still give at 5 seconds- (final circuit exceeding 32 A) 0.44 ohm EL ( 10oC) , so is achievable in many situations local to Birmingham.

I was then going to run a fused cable out to an external isolated IP 65 box with a Type 2 socket to IP44 or above ( 722.55.101).

Isn't it better to engineer a solution to the upcoming electric charger deluge, rather than buying (insert well known manufacturer name here), and lots of single phase loads usually dumped onto L1?

I would be interested in any thoughts or problems you may consider....

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0 gkenyon over 5 years ago

RichardCS2:

The problem with that approach (unless I have missed something) is it only offers protection from broken CNE conductors that supply only the installation in question and carry no load currents from other installations. Given that faults most often occur at joints and that a majority of the joints are usually shared it all seems a bit dubious. Yes, in that situation there will be some benefit from the PME electrodes and potentially some from load balance, but it seems rather optimistic to think that it will always, or even usually, hold the voltage down to an acceptable level.

Agreed, but this needs to be balanced against the fact that the "ME" in "PME" means "Multiple Earthing". If you follow the requirements of G12/4 for street lighting installations (and bus shelters) in this respect, the effect of other loads is lessened.

EV charging does, in line with the point you've made, present a set of different situations - namely loads that are too large to apply similar "multiple earthing" rules, and bring anticipated touch voltages down to an acceptable level.

Similarly, the face of modern supplies to premises has changed. We no longer have swathes of burled metal pipework supplying premises, and connected to the MET ... and in existing premises, any external metal pipework is becoming limited to the last few metres as the metal pipes int he street are being replaced with plastic. Hence, simply "progress" is changing the face of PME.
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0 gkenyon over 5 years ago

RichardCS2:

The problem with that approach (unless I have missed something) is it only offers protection from broken CNE conductors that supply only the installation in question and carry no load currents from other installations. Given that faults most often occur at joints and that a majority of the joints are usually shared it all seems a bit dubious. Yes, in that situation there will be some benefit from the PME electrodes and potentially some from load balance, but it seems rather optimistic to think that it will always, or even usually, hold the voltage down to an acceptable level.

Agreed, but this needs to be balanced against the fact that the "ME" in "PME" means "Multiple Earthing". If you follow the requirements of G12/4 for street lighting installations (and bus shelters) in this respect, the effect of other loads is lessened.

EV charging does, in line with the point you've made, present a set of different situations - namely loads that are too large to apply similar "multiple earthing" rules, and bring anticipated touch voltages down to an acceptable level.

Similarly, the face of modern supplies to premises has changed. We no longer have swathes of burled metal pipework supplying premises, and connected to the MET ... and in existing premises, any external metal pipework is becoming limited to the last few metres as the metal pipes int he street are being replaced with plastic. Hence, simply "progress" is changing the face of PME.
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Double wound safety transformer for EV supply.

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