EVs, Street furniture, PME and TT configurations

Andy, you're quite right, and this is what's described in section 5.3.4 of the IET Code of Practice for EV Charging Equipment Installation (3rd Edition, but it's been in there since the 1st Edition with no-one worrying about it). Provided that ALL of the following are met, it should be OK for Electrical Separation:

• the protective conductor on the secondary is not connected to any conductor (including protective conductor) of the primary

• Isolating transformer meets requirements for electrical separation

• There is an RCD (30 mA for EV supply equipment) immediately after the secondary.

• Only one vehicle is supplied by each transformer (one transformer per charge point).

(Side note: for those that are still a little queasy about the situation, unearthed systems with protective conductors are discussed in BS 7430 ... as Andy says it's not really covered in sufficient detail in BS 7671 at the moment ... but I really don't think this particular situation needs to go down that rabbit hole as it's one separated system feeding one vehicle as per Section 722.)

Just so I'm clear - what's proposed is a separated system but with c.p.c.s (and any exposed-conductive-parts) bonded to one of the live conductors - is that right? So the closest BS 7671 equivalent would be a separated system supplying more than one item of current-using equipment (418.3) but with a first fault already present. (Even though it's only supplying one item, the presence of c.p.c.s and their connection to exposed-conductive-parts makes it unlike 413 arrangements.)


Has such an arrangement been tried & tested? I've got some worries about it. I'm familiar with SELV-like circuits from class II equipment suffering from capacitive coupling from the main supply (typically several interconnected items of domestic AV equipment) where disconnecting/reconnecting ELV interconnecting leads with the equipment live (contrary to the manufacturer's instructions) can sometimes produce a very obvious crackle and tingle and I'm sure someone here (Alan?) has described similar problems with DNO transformers where the LV system can float at around half the HV voltage if the LV earth is lost. So my worry would be with an isolating transformer supplying EVSE that there would be a risk of capacitive coupling with the earthed primary circuit coupling the secondary circuit to say something like half mains voltage above true earth potential (so each pole of the separated circuit might be at that +/- 115V say) - and so exposed-conductive-parts could be held at a significant voltage above true earth. Of course without galvanic connection to earth or the primary circuit any shock current would be limited by the capacitance between the transformer windings - almost certainly well below levels that would be fatal - but possibly high enough to be felt. But even a perceived shock could still be a significant problem - not just from the point of view of being disconcerting and causing members of the public to question the safety of the installation, but from in a highway environment any involuntary 'jump back' could put someone into the path of an approaching vehicle.


Maybe I'm worrying unnecessarily about this - do isolation transformers necessarily have an earthed screen between windings to eliminate this effect? Or would things necessarily be wired in such a way (e.g. c.p.c. connected end of the secondary winding matches the N end of the primary winding) so things are cancelled out to an acceptable level?


   - Andy.

Andy, you're quite right, and this is what's described in section 5.3.4 of the IET Code of Practice for EV Charging Equipment Installation (3rd Edition, but it's been in there since the 1st Edition with no-one worrying about it). Provided that ALL of the following are met, it should be OK for Electrical Separation:

• the protective conductor on the secondary is not connected to any conductor (including protective conductor) of the primary

• Isolating transformer meets requirements for electrical separation

• There is an RCD (30 mA for EV supply equipment) immediately after the secondary.

• Only one vehicle is supplied by each transformer (one transformer per charge point).

(Side note: for those that are still a little queasy about the situation, unearthed systems with protective conductors are discussed in BS 7430 ... as Andy says it's not really covered in sufficient detail in BS 7671 at the moment ... but I really don't think this particular situation needs to go down that rabbit hole as it's one separated system feeding one vehicle as per Section 722.)

Just so I'm clear - what's proposed is a separated system but with c.p.c.s (and any exposed-conductive-parts) bonded to one of the live conductors - is that right? So the closest BS 7671 equivalent would be a separated system supplying more than one item of current-using equipment (418.3) but with a first fault already present. (Even though it's only supplying one item, the presence of c.p.c.s and their connection to exposed-conductive-parts makes it unlike 413 arrangements.)


Has such an arrangement been tried & tested? I've got some worries about it. I'm familiar with SELV-like circuits from class II equipment suffering from capacitive coupling from the main supply (typically several interconnected items of domestic AV equipment) where disconnecting/reconnecting ELV interconnecting leads with the equipment live (contrary to the manufacturer's instructions) can sometimes produce a very obvious crackle and tingle and I'm sure someone here (Alan?) has described similar problems with DNO transformers where the LV system can float at around half the HV voltage if the LV earth is lost. So my worry would be with an isolating transformer supplying EVSE that there would be a risk of capacitive coupling with the earthed primary circuit coupling the secondary circuit to say something like half mains voltage above true earth potential (so each pole of the separated circuit might be at that +/- 115V say) - and so exposed-conductive-parts could be held at a significant voltage above true earth. Of course without galvanic connection to earth or the primary circuit any shock current would be limited by the capacitance between the transformer windings - almost certainly well below levels that would be fatal - but possibly high enough to be felt. But even a perceived shock could still be a significant problem - not just from the point of view of being disconcerting and causing members of the public to question the safety of the installation, but from in a highway environment any involuntary 'jump back' could put someone into the path of an approaching vehicle.


Maybe I'm worrying unnecessarily about this - do isolation transformers necessarily have an earthed screen between windings to eliminate this effect? Or would things necessarily be wired in such a way (e.g. c.p.c. connected end of the secondary winding matches the N end of the primary winding) so things are cancelled out to an acceptable level?


   - Andy.