How to use residual current monitors to ensure electrical safety when charging electric vehicles
thresholds in the EV charging circuit are 6 mA DC and 30 mA AC. System designers can now easily implement specific personal protection requirements in the charging circuit by selecting an RCD type of the appropriate standard. Table 2 lists residual current forms and trip tolerance of the different RCD or ground fault circuit interrupter (GFCI) types.
The IEC 62196 standard, therefore, defines two residual current protection options: the use of an all-current sensitive RCD of Type B (or Type B+), or an RCD of Type A in conjunction with a residual DC monitoring system according to IEC 62955 with IΔn DC ≥ 6 mA. The DC fault current monitoring can be arranged in the wall box, in the building electrical installation, or at both locations. Since an AC-sensitive Type-A or Type-F RCD is usually present in building electrical systems, designers can cost-effectively add 6 mA residual DC monitoring to Mode 3 wall boxes or charging stations, as well as to the in-cable control boxes (ICCB) of Mode 2 charging cables (Figure 1, cases 2 and 3). Charging modes for EVs The EV battery can be charged via different charging modes, depending on the available on-site power connection, connection plug, charging cable, and the charging technology installed in the vehicle, as well as in the charging station. In Europe, the electrical energy can be fed into the vehicle via single- phase AC (230 volt/3.6 kilowatt (kW), three-phase AC (400 volts/22 kW), or via high-voltage DC charging stations (up to 1000 volts DC/500 kW). Figure 2 illustrates the four charging modes defined in the IEC 61851 standard.
Installing RCDs in the EV charging circuit Type-A or Type-F RCDs only detect AC residual current and DC pulsating current, which is insufficient for protecting an EV charging circuit. A wide range of straight DC residual currents that can occur in the onboard charger or battery management system must also be considered.
Tripping characteristic of RCD types In general, personal protection against electric shock on electrical installations is regulated by IEC 60479 and UL 943. Both standards define significant AC and DC residual currents in the range of 6, 30, 100, 300, 500, and 1000 mA, at tripping times ranging from 20 to 500 ms. Common tripping
Figure 1: EVSE devices must add a DC RCM downstream of the AC-sensitive Type-A RCD (case 2), or have one connected to AC mains directly via a Type-B RCD (case 4). (Image source: goingelectric.de)
Table 2: Tripping characteristics of different GFCI or RCD types. (Table source: abb.com)
Figure 2: Illustration of the four charging modes defined in the IEC 61851 standard. (Image source: bestchargers.eu)
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