Abstract: An overload relay protects a motor or other device from an overload condition and a power supply unit supplies power to the overload relay. The overload relay has: shunt resistors, each being locatable in the current path of a respective supply line of a power supply for the motor or other device; and control mechanism connected to each shunt resistor and arranged to receive signals representative of current through each shunt resistor. The control mechanism produces a trip signal to interrupt the continuity of the power supply if it determines that an overload condition exists based on one or more of the signals. The power supply unit has: a rectifier producing a direct current from one or more supply lines of an AC power supply connected to the motor or other device; and at least one transformer coupling the power supply unit to the overload relay.