How to use residual current monitors to ensure electrical safety when charging electric vehicles By Jens Wallmann Contributed By DigiKey's European Editors
Power electronic circuits such as rectifiers, switching converters, frequency inverters, along with inverter and phase-angle control systems, have a wide variety of load current characteristics. The resulting potential residual currents are categorized as sinusoidal AC, pulsed DC, and straight DC. These residual current forms are dangerous to humans. Table 1 shows typical load current signals of various circuit topologies and the resulting residual current waveforms. Columns 1 to 3 assign RCD types suitable for detection.
Frequent charging of the high- voltage battery of electric vehicles (EVs) translates to high mechanical stress requirements for charging cables and connectors. If the insulation breaks and live metal parts are exposed, or shunts appear in the onboard electronics, life-threatening residual currents can occur in the EV user’s body. Particularly problematic are various DC residual current forms that cannot be detected by the AC- sensitive Type-A residual current devices (RCDs). To prevent electric shock accidents, manufacturers of EV supply equipment (EVSE) must incorporate RCDs into their power electronics products that trip within a few milliseconds for both AC and DC residual currents of a few milliamperes (mA). This article explains forms of residual currents, how to measure them, and where to install the RCD in the charging circuit. It then introduces residual current monitors (RCMs) from Littelfuse that system designers can use to add protection against DC electric shock in their EVSE devices in a cost and time-efficient manner. The article also demonstrates which EV charging modes these current sensors are suitable for and how they are used.
Residual currents in the EV charging circuit Charging EVs at voltages up to 400 volts AC and 1000 volts DC requires extensive protective measures for the EV user when handling charging equipment. Due to the harmonic-rich and asymmetric switching pulses of charging stations and onboard chargers, as well as several hundred volts of DC link voltage, various types of AC and DC residual currents can occur via shunts, coupling effects, insulation faults, and leakage faults.
Table 1: Fault current forms and their detection according to the type of RCD that is most suitable (columns 1 to 3). (Image source: Wikipedia)
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