According to the working principle of surge protection device and the characteristics of the components themselves, the internal lightning protection chip of surge protector will be affected by many times of lightning current impact, operating overvoltage, high temperature and high humidity under normal operation, causes the lightning protection chip aging, the deterioration.
In general, the initial leakage current of over-current limiting type (varistor) surge protection devices is less than 40 ΜA, and the initial leakage current of some mall surge protectors is less than 5 ΜA, but the leakage current begins to increase gradually after the rated current discharge, with the increase of discharge times, the leakage current increases continuously. When the leakage current increases to a certain value (generally, the single valve should not span 1 Ma) , the surge protection device begins to heat up, and the deterioration speed becomes faster, which is easy to cause fire. In addition, when the high-energy surge or line frequency defect, surge protector short-circuit failure, if no line protection, can not disconnect the defective line in time, will also lead to the distribution line fire, and the surge protector explodes.
To sum up, blocking the leakage current of the internal chip of the line surge protection device and the short-circuit current of the Line Power frequency are the main reasons for setting the back-up protection of the Surge Protector.
The leakage current in the surge protector is the key to the internal protection of the surge protection device.
Some of the market products initial leakage current is very small, but there will be a large increase after use, the change rate is very high. In contrast, the leakage current of some other surge protection devices is relatively large (5 ~ 30μa) , but the increase of leakage current is very small after repeated rated current discharge, which is a very important policy. The higher the change rate of leakage current, the lower the safety, reliability and service life of the surge protection device. The lower the change rate of leakage current, the higher the safety, reliability and service life of the surge protection device. When the leakage current inside the surge protector increases, the internal temperature of the surge protector rises to the limit, and the internal device is released through low temperature soldering or mechanical metal shrapnel, sensitive disconnect from power supply to ensure the safety of surge protector. Therefore, we should not look for small leakage current, but should pay more attention to the change rate of leakage current during the operation of surge protection device, generally should be less than 200% .
When there is an overload energy surge or a line power frequency defect (Tov) , the tripping point can not be guaranteed to be the highest melting point because of the presence of the afterflow or the tremendous pressure caused by the expansion of the gas, the Surge Protector is short to the ground, and the continuous short-circuit current causes the surge protector to heat up and catch fire. Therefore, when the back-up protective equipment is installed before thesurge protection devicer, the back-up protective equipment disconnects and the line is protected when the short-circuit failure of the surge protector occurs.
When the Surge Protector is in operation, the surge current not only flows through the surge protector, but also flows through all other equipment on the line, including the Surge Protector backup protection equipment. In order to prevent the back-up protective equipment from misoperation when the normal current of surge current expires, the lightning protection of surge protective equipment should be selected reasonably. In this paper, the fuse is taken as an example to analyze (the results show that the same rated current of the circuit breaker is better than the fuse surge withstand function) .
When the short-circuit tolerance or short-circuit breaking performance of the surge protector itself is greater than the expected short-circuit current at the device, the surge protector is considered to have the protection function, and the external backup protector can not be installed at this time; But the general surge protector generally can not meet the current supply system expected short-circuit current. Therefore, when the short-circuit failure of the surge protector can not effectively break the short-circuit current, the surge protector should set back-up protector and be able to break the corresponding expected short-circuit current.
The back-up protection should be selected by time delay release with C release curve, and its rated current is selected according to the maximum current of surge protector IMAX. Or select the fuse, should be the fuse with the upper end of the selective cooperation (cooperation ratio of 1/1.6) . When the rated value of the upper overcurrent protector is less than the setting value of the overcurrent protector in the lead circuit of the Surge Protector, the backup protection of the surge protector is not effective, and the lower setting value can be omitted or selected.
The current market surge protector manufacturers choose many types of back-up protective device components, complex. Common types of surge protector special backup protection equipment (SCB) , integrated fuses (Fu) , MCCB, micro-break (MCB) and so on. How to select the type and main parameters of the backup protector has not been clearly defined and accurate data, and even different product technical staff have many inconsistent ideas and propaganda, to the designers a lot of confusion.