A Surge Protective Device (SPD) is designed to protect electrical systems and equipment from voltage spikes or surges. These surges can occur due to lightning strikes, power outages, or switching events within the power grid. SPDs act as a barrier to divert or absorb excessive voltage, preventing damage to sensitive equipment.

SPD stands for Surge Protective Device, which is a device that is designed to protect electrical and electronic equipment from voltage surges and transients. Here is a brief explanation of how SPDs work:

- SPDs are installed in parallel with the equipment they are designed to protect. This means that they are connected between the power supply and the equipment, and they divert excess voltage to ground.

- SPDs typically use one or more metal oxide varistors (MOVs) to absorb excess voltage. MOVs are semiconductors that have a variable resistance depending on the voltage applied to them. When voltage spikes occur, the MOVs become conductive and divert the excess voltage to ground, thus protecting the equipment.

- In addition to MOVs, SPDs may also include other components such as gas discharge tubes, capacitors, and inductors to provide additional protection against different types of surges and transients. SPD breakers come equipped with these various components to offer optimal safety and protection, catering to the specific needs of various electrical systems and appliances.

Here's how they work:

1. Detection of Surges: When there’s a sudden increase in voltage (a surge), the SPD detects it almost instantly. This surge could come from external sources like a lightning strike or internal sources like power switching.

2. Clamping or Diverting the Surge: SPDs use components like metal oxide varistors (MOVs), gas discharge tubes (GDTs), or transient voltage suppression diodes (TVS). These components are designed to conduct current only when the voltage exceeds a certain threshold. When a surge occurs, they "clamp" the voltage by conducting the excess voltage away from the protected equipment.

3. Dissipation: Once the surge is diverted, the SPD safely dissipates the excess energy, typically in the form of heat, or redirects it into the ground (depending on the type of SPD).

4. Returning to Normal Operation: After the surge has passed, the SPD returns to its normal, non-conducting state, allowing regular voltage to flow through the system again.

How Do I Select The Correct SPD?

When selecting an SPD you must make sure that the available fault current is less than or equal to the SCCR of the SPD. The nominal discharge current should be as high as possible because an SPD with a higher In will be able to handle more surges (at lower currents) then one with a lower In.

How Is an SPD Installed?

A Surge-Trap SPD is always installed in parallel with the load. When the circuit has the normal operating voltage the SPD will not be
conducting current. Once the system experiences an overvoltage the SPD will “turn on” and begin to conduct the extra voltage to ground,
allowing the load to continue running at the correct voltage. This operation is similar to a pressure relief valve in a steam system.

Overall, SPDs provide an important layer of protection against voltage surges and transients that can cause damage to electrical and electronic equipment. By diverting excess voltage to ground, SPDs can help extend the lifespan of equipment and reduce the risk of downtime and costly repairs.