Double wound safety transformer for EV supply.

Thoughts after about 10 seconds of consideration.

Drawing the load current from phase to phase would elegantly sidesteps the question of PEN and rise of earth /neutral

On the secondary side, connect one side of the  230 to a local electrode and call it neutral of a TN-S 230V supply, keep away from the suppliers PEN derived earth.

Do not connect the secondary winding to the suppliers earth, but do earth the core, in case of failure of primary insulation.


I think there are things that can be done for inrush - though we need to know how much primary current are we talking - I have used negative temperature slope  inrush limiters with great success on transformers up to about 5kVA, and on much larger (circa 50kVA), we have designed in a staggered start, where we connect to the mains via a (bosky ) resistance of a few ohms on one contactor and a few mains cycles later a second contactor shorts out the resistor Given the transformer price, the extra cost of the slow start components is negligible. Resistor does need containment to catch the explosion if the second contactor fails to come in on time. And in the second one, it did have. That was  an "oh dear" ? moment if ever there was.

Thanks for the reply Mike.



Very interesting to note your comments on connecting on side one side of the secondary to earth via an electrode. Would this make the secondary side no longer isolated though? Could stray voltages from the PEN network come back up the electrode and CPC spoil the double insulated advantages?


Earthing the core is sound advice.


Your comments on staggered start are interesting because our clients is talking about numerous chargers being installed, possibly all coming on-line when people return late afternoon with their cars. Spreading and managing loads with electric vehicles surely will be a major issue coming down the line. I will have a look at 'negative temperature slop inrush limiters', and also the contactors and ( bosky) resistor for the slow start components. Your point about' catching' the resistor if there is a timing issue is a good one!?.


Thanks Simon

examples of limiters

to be mounted in places where the fact they get quite hot is not an issue. - basically these are resistors designed to self heat, and the resistance falls when hot to nearly a short circuit. Need to be in metal box on ceramic choc terminal block or similar.

Thanks for the reply and information Mike.


The resistor getting hot won't be an issue as it could be mounted in the transformer enclosure, I would just need to ensure good ventilation for both.


I can mention this if we order with the manufacturer and see if they can size up allowing for this and some ceramic connectors as well.


Re: Contactors and staging/staggering the start up's. we could just put delay timers onto the contactors and perhaps even incorporate a stop circuit as well as isolation.


Thanks  for your comments


Simon.

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.

Thanks Andy for the reply.


Re: Earthing and double insulated:

I have been trying to find different charging options for different cars. The Porsche charger ( not mine!) I installed ( early 2017), was a plug in unit which came with the vehicle and plugged straight into the mains, but we did it through an RCD and isolator, circuit, breaker, IP rated etc,.

Admittedly it was a type A RCD, and not the DC sensitive Type B which is quoted in the 18th. I'm confident that the Porsche lead was two wire heavy duty cable ( no earth?), but I could be wrong.

But your point that manufacturers have an intelligent charger that senses the earth is important, do they send a current down the earth and monitor it back, and is this the reason for the Type B RCD's, as they give back a small DC monitoring fault ?


I wouldn't fancy being a cable jointer and finding during street mains connection alterations that several houses have EV's, and have just plugged them into wall sockets. The sum of the accumulation of potential  DC currents ( individually under 6 mA so the RCD Type B won't trip) in the PEN, when terminating, could be a major hazard. 


I take your point about earthing and using the electrode or converting to TNS, but then you are back to using ADS with the Type B RCD, and all the load being put onto one phase often L1.

Compliance with BS7671is a must, as you state, so losing the double insulated Electrical Separation cannot happen, but I am wondering why 722.413 has been left in ( Electrical separation). Perhaps just to allow future innovation or a separate rod just for the socket connection. The transformer and housing are yet to be priced but I would think they will be cheaper than the ( insert manufacturer) cost quoted at £ 375.00 for a wall pod with Type B RCD, plus the electrode.


Many thanks for your comments.

Simon 

The transformer and housing are yet to be priced but I would think they will be cheaper than the ( insert manufacturer) cost quoted at £ 375.00 for a wall pod with Type B RCD, plus the electrode.

Since most cars seem to expect a mode 3 connection there's probably no getting away from a "wall pod" in some form or other - if you installed a simple BS EN 60309 (or BS 1363) socket then someone would have to buy a "granny lead" (i.e. a lead with all the wall pod gubbins built into it) in order to connect to most cars.  It's that gubbins that handles the communication with the vehicle - including telling it the maximum power available. It all depends on whether the charge point is for the exclusive use of one particular vehicle, or is intended for more general use.

 

But your point that manufacturers have an intelligent charger that senses the earth is important, do they send a current down the earth and monitor it back, and is this the reason for the Type B RCD's, as they give back a small DC monitoring fault ?

I don't know for sure but I would hope that they'd use a (tiny) a.c. signal to test the earth connection - just because prolonged d.c. currents are known to encourage corrosion in buried metalwork.

I wouldn't fancy being a cable jointer and finding during street mains connection alterations that several houses have EV's, and have just plugged them into wall sockets. The sum of the accumulation of potential  DC currents ( individually under 6 mA so the RCD Type B won't trip) in the PEN, when terminating, could be a major hazard. 

I doubt they would be that bothered (but wait to see if UKPN posts a reply). DNO's jointers are used to working live with tens if not hundreds of amps flowing in the PEN (they have various techniques of working that keeps the PEN continuous at all times) - a few hundred mA extra (d.c. or otherwise) wouldn't make much difference I suspect. If the PEN has to be cut then the lines are cut first, so any EVSE would loose power & disconnect so shouldn't be leaking anything (d.c. or otherwise) back to the mains. As I understand it high d.c. leakage currents are only really a problem where there are RCDs that could be disabled by it - and our DNOs don't do RCDs on the LV system.

and all the load being put onto one phase often L1.

I guess your option there, if you have more than one (single phase) charge point, is to share them around the available phases - e.g. connect the first to L1, the second to L2 and so on. If you have 3-phase charge points then rotate the phases (e.g.  connect L2,L3,L1 to the 2nd charge point and L3,L1,L2 to the 3rd and so on) so even if the vehicles only use what they see as L1, the load is better balanced from the installation's point of view.

 

but I am wondering why 722.413 has been left in ( Electrical separation). Perhaps just to allow future innovation or a separate rod just for the socket connection.

I suspect, from a regulation point of view, it's not so much that 722.413 is describing an extra option - that option is there anyway from the general body of the regulations (section 413 itself) - it's just regulating how section 413 should be applied within the context of EVSE. It might even be useful in some peculiar circumstances (e.g. a dedicated supply for a particular vehicle that didn't require an earth connection). An separate rod just for the socket (well usually the EVSE wall pod) would make it a TT system (still under 411) - actually a very commonly adopted solution.


   -  Andy.

Easy peasey, just use this method using the support pole as an earth electrode. You are all overthinking the matter. Run your three phase supply cable in a nice colourful buried plastic tube and all's well.

https://www.youtube.com/watch?v=zSO9vrjmGZo


Z.

More E.V. charging safety concerns.

https://www.dailymail.co.uk/sciencetech/article-7086061/Electric-vehicle-drivers-UK-risking-death-electrocution-charging-car-home.html


Z.

The IEEE has looked into this matter before.

https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8303690


Z.