Double wound safety transformer for EV supply.

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.."

I'd take a step back and think carefully about that approach first. Isolating transformers can be used in a couple of different ways - either to provide a separated supply (i.e. no deliberate connection to earth as per section 413) or with one pole of the secondary deliberately earthed (to form a local TN system as per 411).


The trouble with the first approach is that many, if not most, electric cars check for a sound connection to earth (presumably a L-PE loop in practice) - and flatly refuse to charge if it's absent or has a high resistance. So a section 413 approach, although all very well and good and very safe in theory, probably isn't going to actually work in practice.


The other approach of turning the secondary into a TN system (normally TN-S) means you need to obtain an earth connection to connect the secondary to - in theory that could be the a local electrode or even the c.p.c. of the primary circuit. If it's a PME supply then using the primary circuit's c.p.c. is obviously out, which leaves a local electrode (and the need to keep the EVSE system out of reach of anything connected to the PME earth)  - but if you're going for that you might as well just TT the EVSE and not bother with the expensive transformer at all.


There have been suggestions along the lines of and isolating transformer with the EVSE's PE conductor connected to the secondary "neutral" but without any kind of Earth connection - but that approach is completely outside of BS 7671 methods and has all kinds of potential risks that would need to be addresses (e.g. the secondary PE being capacitively coupled to the primary and so floating up to a hazardous voltage) and you wouldn't be able to describe the installation as complying with BS 7671.


    - Andy.

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.."

I'd take a step back and think carefully about that approach first. Isolating transformers can be used in a couple of different ways - either to provide a separated supply (i.e. no deliberate connection to earth as per section 413) or with one pole of the secondary deliberately earthed (to form a local TN system as per 411).


The trouble with the first approach is that many, if not most, electric cars check for a sound connection to earth (presumably a L-PE loop in practice) - and flatly refuse to charge if it's absent or has a high resistance. So a section 413 approach, although all very well and good and very safe in theory, probably isn't going to actually work in practice.


The other approach of turning the secondary into a TN system (normally TN-S) means you need to obtain an earth connection to connect the secondary to - in theory that could be the a local electrode or even the c.p.c. of the primary circuit. If it's a PME supply then using the primary circuit's c.p.c. is obviously out, which leaves a local electrode (and the need to keep the EVSE system out of reach of anything connected to the PME earth)  - but if you're going for that you might as well just TT the EVSE and not bother with the expensive transformer at all.


There have been suggestions along the lines of and isolating transformer with the EVSE's PE conductor connected to the secondary "neutral" but without any kind of Earth connection - but that approach is completely outside of BS 7671 methods and has all kinds of potential risks that would need to be addresses (e.g. the secondary PE being capacitively coupled to the primary and so floating up to a hazardous voltage) and you wouldn't be able to describe the installation as complying with BS 7671.


    - Andy.