The overheat protection technology of varistors mainly has 5 types which are introduced in the following:
(1)Hot-melt Fuse Technology.
The technology is to install a low-melting-point metal protected by wax on a varistor through a certain Techhbs process. When the leakage current of the varistor is too large, and the temperature rises to a certain extent, the metal will be melted, thereby cutting the varistor from the circuit, effectively preventing the varistor from burning. However, hot-melt fuses have problems in reliability, and have a service life of only 5 years in an environment of enhancing thermal cycling where the hot-melt fuse need to be replaced periodically to maintain normal operation.
(2) Low-melting-point Soldering technology
Currently, this technology is used by the SPD of voltage-limiting type in most lightning arrester manufacturers. At first, add a low-melting-point soldering point to the pin of the varistor, and then pull the soldering point with a spring. When there is a large leakage current, and the temperature rises to a certain extent, the solder will be melted, and the solder joint will be quickly separated under the force of the spring, thereby cutting off the varistor from the circuit, simultaneously linking the alarm contact and sending an alarm signal. Because the low-melting -point metal will flow and crack at the force bearing point, and the solder at the solder joint under the force of the spring will also flow and crack, so the solder will age, resulting in the disconnection of device without cause or reason.
(3) Thermal Fuse Technology.
In this technology, the varistor and the hot-melt fuse are packaged in series, and to conduct the heat generated by the leakage current is transmitted to the thermal fuse with heat conduction. When the temperature rises to the set temperature of the thermal fuse, the fuse will be melted, cutting the varistor from the circuit. In addition to the same problems in service life and reliability as temperature rises as the soldering technology, thermal fuse technology also has the problems of long heat-conduction path and slow response speed. As the heat should pass through a certain heat transfer medium (filling material), the shell of thermal fuse, the internal filling material of the thermal fuse, and then be transmitted to the melt, thus determining the slow response speed of the thermal fuse.
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