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What is a Surge Protective Device (SPD)?

The Surge Protective Device (SPD) is a component of the electrical installation protection system. This device connects in parallel to the power supply circuit of the loads that it has to protect. The surge protective device redirects electrical currents like nominal discharge current from a short circuit. It does that using either a solid-state contact or an air-gap switch. In addition, the surge protective device serves as a load-safe shutoff device for overcurrent conditions and a recloser that controls the voltage level above the rated voltage or low voltage in the event of a fault condition. We can also use the surge protective device at all levels of the power supply network. This approach is often the foremost commonly used and most effective sort of overvoltage protection.

Surge protective device connected in parallel features a high impedance. In other words, the sum of the series impedance is equal to the impedance of one surge protective device. Once the transient overvoltage appears within the system, the device's impedance decreases, so surge current is driven through the surge protective device, bypassing the sensitive equipment. That is to protect equipment against overvoltage transients and disturbances, such as voltage spikes and electrical surges, frequency variations, and over-voltages caused by switching operations or lightning. When a user installs a surge strip or a surge protective device into a power line coming from a power utility that includes smoothing capacitors, surge suppressors are not necessary because these capacitors already protect from sudden changes in voltage level.