How to use residual current monitors to ensure electrical safety when charging electric vehicles
Mode 1 (single-phase AC charging up to 3.6 kW; default mode of charging) In this case, the electric or hybrid vehicle is connected to a standard 230-volt household socket using a simple passive cable and will be charged using low power at a maximum of 3.6 kW via the onboard charger. This charging scenario does not provide sufficient protection against residual DC for the user. Usually, only an AC- sensitive Type-A RCD is installed in the building's electrical system.
Mode 2 (single/three-phase AC charging up to 22 kW via an ICCB charging cable) A Mode 2 charging cable equipped with a Type 2 vehicle plug contains an ICCB that performs safety and communication functions when charging EVs using domestic and three-phase sockets to prevent overloading them. The following protection functions must be integrated with the ICCB: ■ Determination of polarity and protective conductor (PC) monitoring; only a few ohms of loop impedance are permitted between neutral and the PC.
■ Testing of the electrical connection between the PC and the metal body. ■ An AC and DC residual current circuit breaker prevents current accidents. ■ Monitoring/shutdown of the charging process in the event of anomalies (for example, current fluctuations due to corroded plug contacts or cable breakage). ■ Monitoring of the temperature inside the ICCB and both plugs and perform shutdown if necessary.
■ Control of the charging power: Pull-down resistors on the control pilot (CP) wire to signal the cable current load rating to both the wall box and the EV; the pulse width modulation (PWM) signal on the charging control (CC) wire signals the wall box charging power capability to the EV. Mode 3 (single-phase/three- phase AC charging up to 22 kW via wall box) For EV charging, a passive Mode 3 cable is connected to a wall box in private households or a public AC charging station in parking lots. Both have integrated the same protection functions as the ICCB above. Mode 4 (direct battery DC fast charging up to 500 kW) DC high-power charger (DC/HPC) stations for EVs deliver significantly higher charging currents compared to Mode 2 and Mode 3. Shock protection from residual AC and DC is implemented in this supercharger; the different charging cables are always firmly attached. Measure AC and DC fault currents in the EVSE circuit RCMs, from the RCM14 series from Littelfuse Inc., detect DC and/or AC residual currents in AC or DC systems and deliver an
Figure 3: If a fault current (I g ) flows into the ground potential via the human body, the GFCI total current differs from zero, and the circuit breaker trips. (Image source: Littelfuse)
output signal to control an external disconnect (cutoff relay). In contrast, RCDs and residual current circuit breakers (RCCBs) have an integrated cutoff relay. AC residual currents are detected using an inductive current transformer (CT). For this purpose, the current forward conductor (IL) and current return conductor (IN) are fed through a soft magnetic toroidal core, causing both current vectors to normally compensate for each other and add up to zero. If a fault current (I g ) flows into the ground potential via the human body in the circuit behind the detector, the RCM or GFCI total current differs from zero, and the circuit breaker trips (Figure 3).
By integrating a fluxgate magnetometer probe into a slot of the toroidal core and compensating the magnetic flux to zero by means of a compensation coil, the CT can also detect differential DC. More accurate than Hall effect sensors or shunt resistors, this method detects tiny DC fault currents from 6 mA at heavy DC load currents up to 500 amperes (A). RCMs featuring control output for disconnector Littelfuse’s RCM14 series is ideal for use in ICCB charging cables for EVs (Mode 2) and EV charging stations (Mode 3). They are available in three residual current detection options in accordance
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