The solution of avoid ZOV varistor burnout
The ZOV varistor overheat protection technology mainly has the following types:
(1) Thermal fuse technology. The technology is to install a low melting point metal protected by wax on a ZOV varistor through a certain process. When the ZOV varistor leakage current is too large, and the temperature rises to a certain extent, the low melting point metal is melted, thereby ZOV varistor from The cutting in the circuit can effectively prevent the ZOV varistor from burning. However, thermal fuses have reliability problems and have a reliable life of only 5 years in an environment that enhances thermal cycling. In a hot cycle environment, the hot melt fuses need to be replaced periodically to maintain normal operation.
(2) Using a spring to pull the low melting point soldering technology. This technology is the technology used by the current limited-voltage SPD of most lightning arrester manufacturers. A low-melting-point solder joint is added to the ZOV varistor pin, and then the solder joint is pulled by a spring. The ZOV varistor leakage current is too large, when the temperature rises to a certain extent, the solder of the solder joint is blown, and the solder joint is quickly separated under the tension of the spring, thereby cutting off the ZOV varistor from the circuit, and simultaneously linking the alarm contact and issuing Alarm signal. Because the low melting point metal will flow and ***** at the point of stress, the solder of the low melting point solder joint in the spring tension will also flow and *****, so the biggest problem of this device is that the solder will age, resulting in the device will be broken.
(3) Thermal fuse technology. The technology encapsulates the ZOV varistor and the thermal fuse in series, and uses heat conduction to conduct the heat generated by the leakage current on the ZOV varistor to the thermal fuse. When the temperature rises to the set temperature of the thermal fuse, the thermal fuse is blown. The ZOV varistor is removed from the circuit. In addition to the same life and reliability problems of temperature fuses, the use of thermal fuses to protect the ZOV varistor from overheating also has the following problems: long heat conduction path, slow response speed, heat passing through a certain heat transfer medium (filling material), The thermal fuse housing, the internal filling material of the thermal fuse, is then passed to the temperature-safe melt, thus determining the slow response speed of the thermal fuse.
(4) Isolation technology. This technology houses the ZOV varistor in a closed box that is isolated from other circuits to prevent the spread of ZOV varistor smoke and flame. In the case of various backup protection failures, isolation technology is also a simple and effective method, but it needs to occupy the equipment space of the teaching, but also to prevent smoke and flame from taking the hole in the box lead. come out.
(5) Potting technology. In order to prevent the ZOV varistor from smouldering, igniting and exploding when it fails, some manufacturers use this technology to seal the ZOV varistor, but because the ZOV varistor will cause internal arcing when it fails, the sealing material will fail and produce The production of carbon and carbon will maintain the arc, which will often lead to internal short circuit and blackening of the equipment, and even cause serious blackening of the entire equipment room.
Experiments show that after the heat-shrinkable sleeve of the ZOV varistor sleeve, the heat dissipation of the ZOV varistor is affected, and the maximum power dissipation is reduced, which affects the power-frequency voltage withstand capability of the ZOV varistor. From another point of view, Affected heat dissipation will also accelerate the aging of the ZOV varistor and affect the service life of the ZOV varistor.