Patent ID: 8861163

Claim:
A protection relay comprising: an input circuit that detects a state of an external device according to whether or not an external input voltage is larger than a preset threshold voltage; and an operation unit that fetches a detection signal detected by the input circuit and performs a protection relay operation, wherein the input circuit includes: a switching unit that is made conductive by a divided voltage obtained by voltage-dividing resistors that divide the external input voltage when the external input voltage is higher than or equal to the threshold voltage, and a photocoupler that is operated by a constant current of a constant current output circuit and outputs an operation signal to the operation unit when the switching unit is made conductive, wherein the protection relay further comprises a threshold voltage adjusting unit that adjusts a voltage division ratio of the voltage-dividing resistors to adjust the threshold voltage for the external input voltage while maintaining the divided voltage obtained by the voltage-dividing resistors at a voltage that makes the switching unit conductive, by changing a value of a register included in the voltage-dividing resistors.