Today, the small series of varistor manufacturers will tell you about 5 kinds of overheating protection technology:
(1) Hot fuse technology. This technology is to install the low melting point metal protected by wax on the varistor through a certain process. When the leakage current of the varistor is large and the temperature rises to a certain extent, the low melting point metal is fused, so that the varistor is removed from the circuit, which can effectively prevent the varistor from burning. But the hot fuse has reliability problems and only has a reliable life of about five years in an environment of enhanced thermal cycling. In the thermal cycle environment, the hot fuse needs to be replaced periodically to maintain normal operation.
(2) the use of spring pull low melting point solder technology. This technology is currently the vast majority of the lightning protection device type pressure limiting and the adopted technology of the SPD in the varistor pins add a low melting point solder welding points, and then use a spring will pull the welding points, the varistor leakage flow is big, the temperature rise to a certain extent, welding solder fusing point, under the action of tension of the spring weld rapid separation, In this way, the varistor is removed from the circuit, and alarm contacts are linked to send alarm signals. Because the low melting point metal will flow and crack at the stress point, the solder at the low melting point solder contact in the spring tension will also flow and crack, so the biggest problem of this device is that the solder will age, resulting in the device will disconnect without reason.
(3) Temperature fuse technology. In this technology, the varistor and the temperature fuse are packaged together in series, and the heat conduction temperature fuse generated by the leakage current on the varistor is connected by heat conduction. When the temperature rises to the set temperature of the temperature fuse, the temperature fuse is blown off, and the varistor is removed from the circuit. In addition to the same life and reliability of temperature fuse, there are also the following problems using temperature fuse to overheat protection of varistor: The heat conduction path is long and the response speed is too slow. The heat is passed through a certain heat conduction medium (filling material), the temperature fuse shell, the internal filling material of the temperature fuse, and then transferred to the melt of the temperature insurance, so the response speed of the temperature fuse is slow.
(4) Isolation technology. The varistor is enclosed in a sealed box that is isolated from other circuits to prevent the spread of smoke and flame. Isolation is also a simple and effective method in cases where backup protection fails, but requires large equipment space and prevents smoke and flame from escaping from the opening of the box lead.
(5) Potting technology. To prevent varistor in failure when smoke, fire and explosion, some vendors will adopt the technology varistor potting up, but as a result of varistor will appear within fails to pull arc, lead to seal material failure, and generate carbon, carbon produce and can make the arc is maintained, so often lead to internal short circuit and blackened equipment, serious and even cause the entire equipment room blackened. The experimental results show that the heat dissipation of the varistor is affected by the heat shrinkable casing, and its maximum dissipation power is reduced, thus affecting the power frequency voltage tolerance of the varistor. On the other hand, the influence of heat dissipation will also accelerate the aging of the varistor and affect its service life.
