How Does a Surge Protective Device Work?

Surge Protective Device (SPD) is a device used to protect electronic equipment from power surges or transient voltages. They are connected in parallel with the load power circuit that needs protection and can also be used in power supply networks at all levels. This article will delve into the working principle of surge protective devices and their important role in electrical systems.

The Role of Surge Protective Devices

Surge protective devices control transient voltages by diverting or limiting surge currents, protecting sensitive electronic equipment connected to them, such as computers, televisions, washing machines, and safety circuits (such as fire detection systems and emergency lighting). These devices contain sensitive electronic circuits that are prone to damage from transient overvoltages; thus, surge protective devices play a crucial role in protecting electrical installation systems.

The primary function of the surge protective device (SPD) is to protect electrical systems and equipment from surge events by limiting transient voltages and diverting surge currents. Surges can come from external sources, the most intense usually being lightning; they can also come from internal electrical load switching. These internal surges (accounting for 65% of all transients) include sources such as load turning on and off, relay and circuit breaker operations, heating systems, motors, and office equipment.

Without proper SPD, transient events can damage electronic equipment and lead to costly downtime. Therefore, the importance of surge protective devices in protecting electrical equipment cannot be overstated.

How Does an SPD Work?

In the most basic sense, when a transient voltage appears on the protected circuit, SPD will limit the transient voltage and divert the current back to its source or ground. To achieve this, the SPD must contain at least one nonlinear component that switches between high impedance and low impedance states under different conditions.

Under normal operating voltages, the SPD is in a high impedance state and does not affect the system. When a transient voltage appears on the circuit, the SPD switches to a conduction state (low impedance), diverting the surge current back to its source or ground, thus limiting or clamping the voltage to a safer level. After the transient transfers, the SPD automatically resets back to its high impedance state.

Working Principle of SPD

Surge protective devices contain at least one nonlinear component (such as a varistor or spark gap) whose resistance varies with the applied voltage. Their function is to divert discharge or pulse currents and limit overvoltages to downstream devices.

The working principle of the surge protector is as follows

Normal Operation

In the absence of a surge, the surge protection device has no effect on the system it is installed in. It acts as an open circuit, maintaining isolation between live conductors and ground.

During a Surge

When a voltage surge occurs, the surge protection device will reduce its impedance in nanoseconds and divert the surge current. At this point, the SPD behaves like a closed circuit, short-circuiting the overvoltage and limiting it to values acceptable to the electrically connected downstream equipment.

Post-Surge

Once the pulse surge ceases, the surge protection device will restore its original impedance and return to the open circuit state, continuing to monitor the voltage conditions in the electrical system.