Double wound safety transformer for EV supply.

SL1:

gkenyon:




The situation that does arise, however, is high inrush current of the rather sizeable transformers. Yes, this can be overcome, but of course costs are now increasing, and we're competing against "forget it, I'll just plug it in to a socket-outlet", which of course is far less practical, and far less safe, but doesn't have the price tag attached.



 

Hi Mr G Kenyon,


The original idea of my questioning the possibility of using a Double Wound Safety Transformer was to look at an alternative, ( in an industrial and offices based environment), to installing the 'box on the wall with the Type 3 sockets/tethered lead and RCD'- insert manufacturer. Your last line of your quote (above) talks about using the transformer against a socket outlet and cost ( Mode 1). Cost is big driver of bad practises, and I have witnessed at office premises a coiled extension lead plugged into a socket to charge an EV.


My original point was that 722.413.1.2 allows for the supply of one EV from one unearthed source, using a fixed isolating transformer complying with BSEN 61558-2-4.

So Using mode 2 ( BEAMA Guide to electrical vehicle infrastructure), with a dedicated 3 phase circuit (regular charging), a circuit breaker rated for the transformer's in-rush current type D, could be rated at 7.4 KW for industrial use using a BS 60309-02 socket, source: BEAMA Guide, subject to the vehicle protocols.

The secondary side earth cable PE would come from the secondary side of the transformer start point.

The Mode 2 cable providing in cable and protection device (IC-CPD) and downstream RCD protection. 


I have asked a transformer manufacturer for costs on this and in an industrial situation, using the safety transformer, and BS 60309-02, and not duplicating an RCD in a dedicated box, could prove cheaper to purchase and install than the units currently being supplied as a standard, and spreading load across three phases would assist energy efficient electrical installations compared to single phase load balancing.

I would appreciate any comments on this idea.


Regards


Simon

 

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.

 

gkenyon:

SL1:

gkenyon:




The situation that does arise, however, is high inrush current of the rather sizeable transformers. Yes, this can be overcome, but of course costs are now increasing, and we're competing against "forget it, I'll just plug it in to a socket-outlet", which of course is far less practical, and far less safe, but doesn't have the price tag attached.



 

 

Just a thought on this ... Regulation 722.411.4.1 provides other options for three-phase installations. However, I guess such an approach would not be precluded, provided the requisite RCDs are used as indicated in the IET Code of Practice and to meet the requirements of section 722 of BS 7671.

 

Thanks for you comments.

However, isn't it the case that if you follow Section 722.413.1.2,  and use a Safety double insulated transformer Section 413 Electrical separation is in use.

Therefore would the additional RCD's be required and isn't the RCD built into the manufacturers lead sufficient anyway? Surely a risk assessment could easily acknowledge that the separation system used, along with the manufacturers vehicle building-in RCD with purpose built lead and Type of socket.


The point of the exercise was to use the transformer  protective measure rather than have the RCD so adding in the RCD on top would then defeat the idea of needing the protective separation measure, and introduce the CEN PME 70 volt rms etc problems.

 Isn't it an either/ or situation offered by Section 722?



Regards


Simon




 

Interestingly no one has suggested a reason why a charging RCD should be DC sensitive.

An interesting reply Andy, but the earth connection to the car is not a protective conductor in the sense of a conventional circuit, it is simply there to enable the electronics to power the car charger up. If you think about it, it doesn't provide any protection to anything, as the supply is IT, and a double fault etc. is required to get any dangerous situation.

It would ensure that the RCD tripped if the live faulted to the car bodywork, but even so this would not be a danger to any person or livestock. If there was a neutral fault the RCD would probably trip due to current diverted via the fault, but again could in no way be considered dangerous. The "even so" means that the RCD is not the primary protection of the circuit, it is additional protection against multiple faults, single ones not being dangerous.

It has been extremely difficult to design a completely satisfactory and safe car charging installation with a PME supply, and particularly with the possibility of a broken CNE. This scheme is probably as safe as is possible when cars are class 1, and I would certainly recommend that all new car designs are class 2, because that change is fairly easy and cheap, and removes the charging dangers unless damaged flexible cables are involved.

CliveS:

Could one of you please draw a circuit diagram showing the electric car and battery and voltage, the DC leads; the main transformer/supply, diodes and protection fuses or circuit beakers with earthing lead arrangement so that we can all understand what is required to construct a vehicle charging station safely.  Please avoid using and initials like PME, YY or CPC  unless their are defined in words. Thank you.

AJJewsbury:

Interestingly no one has suggested a reason why a charging RCD should be DC sensitive.

From what I've been able to gather, it's not safe (at present) to assume that there's any galvanic isolation between the AC mains supply and the vehicle's propulsion battery & associated circuitry when on change - indeed it seems that at least one current model re-uses the charger power electronics in the power train (I'm guessing during regenerative breaking) so it's all quite tightly coupled together. It seems they don't use the traditional chassis return for the d.c. side (at least not for the propulsion system) but as usual with vehicle electrics there's only single insulation between the internal wiring and chassis. While on charge the vehicle's chassis/bodywork is likely to be connected to the supply's protective conductor. So, as one example, a single fault from say the battery circuit (at perhaps a few hundred volts d.c.) to the chassis, while on charge from a conventional TN or TT supply, would potentially mean a d.c. fault current flow from the battery, to bodywork, via the protective conductor to the EVSE and hence via the means of earthing back to the supply N point and then along the N back through the vehicle's charging circuit to the battery -ve. That d.c. fault current would be flowing through the N side of any RCDs within that loop - both on the EVSE circuit and upstream which may also protect other circuits - potentially blinding them (D-loc style) to any imbalance. Similarly faults from within the charging circuit or elsewhere in the vehicles' wiring could mean pulsed rather than smooth d.c. or at a whole variety of driving voltages.

CliveS:

Could one of you please draw a circuit diagram showing the electric car and battery and voltage, the DC leads; the main transformer/supply, diodes and protection fuses or circuit beakers with earthing lead arrangement so that we can all understand what is required to construct a vehicle charging station safely.  Please avoid using and initials like PME, YY or CPC  unless their are defined in words. Thank you.