The technology is to install a low-melting-point metal protected by wax on a varistor through a certain 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.
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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.
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.
This technology houses the varistors in a closed box isolated from other circuits to prevent the spread of smoke and flame of of the varistors. In the case of the malfunction of various backup protection, isolation technology is a simple and effective method. However, it will occupy large device space, and high attention should be paid to prevent smoke and flame coming out from the hole of the leads in the box.
In order to prevent the varistor from smoking, firing and exploding in the event of a failure, some manufacturers use this technology to seal the varistor. However, since the varistor will cause internal arcing when it malfuntioned, the sealing material will fail and produce carbon that will maintain the arc, which will often lead to internal short circuits and blackening of the equipment, even causing the entire equipment room in soot.
Experiments show that after the the varistor is package in the heat-shrinkable sleeve, the heat dissipation of the varistor will be affected, reducing its maximum power dissipation, which will influence the power-frequency voltage endurance capability of the varistor. From another point of view, if the heat dissipation is affected, it will also accelerate the aging of the varistor , thus reducing its service life.