DC Fault Current Monitoring for EV Charging Station

Fault currents can put the usual earth-fault protection out of play, and create security risks. So how do we solve this problem? First let us learn about some basics about RCD(residual current device).

RCD Types

In the standards for the types of RCD used in Europe, there are three categories:

Type AC - This only responds to AC leakage currents, which are likely to arise in appliances and wiring that have no electronics. Most installed RCDs are of this type.

Type A - This response to AC leakage currents, and also to pulsed DC currents. There is a slow move to using this type for most applications.

Type B - This response to AC and pulsed DC like the Type A, but also to pure DC leakage. Since pure DC doesn't work with transformers, a different mechanism is needed to detect the DC and so Type B units are fundamentally more expensive to make (some designs are also physically bigger).

(Grasen uses Type B RCD in our OCPP Commerical AC EV Charger)

For EV charging, since specific regulations were introduced there has been a requirement to use at least Type-A; Type-B is obviously better, but the regulation didn't require it except if it was known that the installation was specifically subject to DC leakage greater than 6mA.  

Type-B RCDs are hard to find and very expensive, and won't easily fit into some existing enclosures. One common solution is to implement DC detection in the EV charger - this can be simple and lightweight. Then there is a safety unit in the charging station that makes sure to switch off the charging if DC fault currents occur.