Abstract: An electrical power supply system comprising: an electrical connector; a signal transmitter configured to send an electrical signal having a first voltage waveform to the electrical connector; a voltage measurement module configured to measure a second voltage waveform at the electrical connector; and controlling means comprising one or more processors configured to compare the shape of the second voltage waveform measured by the voltage measurement module with the shape of the first voltage waveform of the electrical signal; and control a transfer of electrical power to and/or from the electrical connector based on the comparison. The electrical signal may be an alternating current signal comprising two pulses, which may have equal duration and equal and opposite amplitude.