SPDs’ operating characteristics are defined by several technical characteristics. The following are some technical characteristics that should be considered: maximum continuous operating voltage, ac or dc application, nominal discharge current (defined by a magnitude and waveform), voltage-protection level (the terminal voltage that is present when the SPDs are discharging a specific current) and temporary overvoltage (a continuous overvoltage that can be applied for a specific time without damaging the SPDs).

SPDs must be able to change states quickly for the brief time and discharge the transient current without failing. The device must also minimize the voltage drop across the SPDs’ circuit to protect the equipment it is connected to. Finally, SPDs’ function should not interfere with the normal function of the electrical power system.

The SPDs have an integral self-protecting device that disconnects from the circuit when the device fail. To make the disconnection apparent, many SPDs display a flag that indicates their disconnect status. Indicating the SPDs’ status via an integral auxiliary set of contacts is an enhanced feature that can provide a signal to a remote location. Another important product characteristic is the SPDs utilize a finger-safe, removable module that allows a failed module to be easily replaced without tools or the need to de-energize the circuit.