EV Charging RCD

Gyaneshwar Kumar said:

For a 30 kW DC EV charger on a TN-S system, does the RCD need to disconnect the neutral as well?

What is the RCD doing and why has it been specified?

Thanks for your response. The RCD has been included based on the charger supplier’s recommendation. The charger has a built-in Type B RCD, but they have advised providing a Type A RCD in the DB as well. This seems to be a requirement in the wiring regs as well. The RCD is not required for ADS. 

In case of any insulation faults etc causing leakage current on the DC side, I understand the DC RCD within the charger should trip. 

In the past, I have designed larger DC chargers with separate power units that had galvanic isolation between the AC and DC sides. In those cases, the manufacturer advised that a RCD is not required on the AC side and in most cases not recommended as  the standing leakage current was high enough to potentially cause nuisance tripping on a 30mA RCD.

There's something not quite adding up here. In general you can't put an A-type RCD upstream of a B-type (on the assumption that the B-type is needed for d.c. residual currents) since the threshold of a B-type tripping (e.g. 60mA d.c. for a 30mA nominal a.c. device) is far above what might interfere with the correct operation (e.g. "blind" ) an A-type (typically anything in excess of 6mA).

I had presumed that the "B type" was on the d.c. side to protect the "socket outlet" to the vehicle (or more likely a chunky tethered flex with a vehicle connector on the end for d.c. systems) and there was galvanic separation between the d.c. and a.c. sides so d.c. leakage couldn't escape on to the a.c. side live conductors. But you now seem to be suggesting that's not the case...

I had pondered with the idea that your RCD wasn't part of the EV installation (covered by 722) - but merely an circuit that supplied the EV installation - and thus would only need to comply with general rules (and so didn't need to switch N). The Scope of section 722 is "...circuits intended to supply electric vehicles..." (my emphasis) - so if the d.c. side could be considered a separate circuit, the I can see an argument that the a.c. circuit that supplies the mode 4 charger isn't a circuit that directly supplies the vehicle.

I suspect a bit more information is needed...

   - Andy.

yes, the charger supplier got me confused as well. I had a chat with them again, and I understand what they have is a DC leakage protection on the Dc side, galvanic isolation between AC and DC and another type B RCBO on the AC side where the supply cable connects. They are still recommending type A RCD in the upstream Distribution board and I don't understand why. The cable doesn't need any RCD protection for ADS. I am trying to get more information now. Thanks for your help. 

yes, the charger supplier got me confused as well. I had a chat with them again, and I understand what they have is a DC leakage protection on the Dc side, galvanic isolation between AC and DC and another type B RCBO on the AC side where the supply cable connects. They are still recommending type A RCD in the upstream Distribution board and I don't understand why. The cable doesn't need any RCD protection for ADS. I am trying to get more information now. Thanks for your help.