What is a Surge Protective Device (SPD)?

SPD is a device designed to limit surge energy to electrical equipment. It does this by diverting or limiting surge current. An SPD is wired in parallel to the equipment it is intended to protect. Once the surge voltage exceeds its designed rating it “begins to clamp” and starts to conduct energy directly to the electrical grounding system. An SPD has a very low resistance during this time and “shorts” the energy to ground. Once the surge is over it “opens” up, so it does not trigger upstream circuit breakers.