Development of a fuse-type SPD disconnector for power supply system and the application guide

A surge protective device (SPD) is used in low-voltage power systems to protect electrical equipment from surges induced by lightning. An SPD has a drawback that its impedance becomes low in case of SPD failure. This failure can cause not only the SPD, or other devices near it, to catch fire or make an upstream over current protection device (OCPD) operate unnecessary, and also can cause serious damage to power-supply facilities and lead to loss of reliability concerning customers. Using a conventional overcurrent protector such as a low-voltage fuse or molded-case circuit breaker (MCCB) cannot reduce this risk. Since an existing overcurrent protector cannot be applied to an SPD disconnector, a special-purpose fuse, called a “fuse-type SPD disconnector” (SFD), was developed. The specification of the developed SFD is described, and guidelines for selecting the appropriate SFD in combination with an OCPD to reduce serious risk in case of SPD failure are proposed.