When the overvoltage generated in the line exceeds the working voltage of SPD, SPD will release the strong overvoltage to the earth. But the transient overvoltage caused by transmission line fault will last a long time, and it will not disappear until the fault is eliminated manually. So, once the transient overvoltage exceeds the working voltage of SPD, it will keep it on until SPD fails or even fires. The backup protection of SPD is selected?
Therefore, it is necessary to select backup protection for SPD to ensure that SPD can not be broken by mistake under the impact of lightning current, and can quickly break off the current exceeding its thermal release capacity, and prevent the surge protector from starting fire. So why not choose the breaker as a backup protection device for it?
Because if the circuit breaker with low rated current is used as the over-current protection of surge protector, it may break the circuit breaker directly under lightning impulse current, which will cause damage to the electrical equipment which should be protected; while the breaker with high rated current is selected. Under the transient overvoltage, the surge protective device will continue to work and the breaker will open continuously, which will cause the surge protector to rise and rise continuously Fire. Therefore, the traditional circuit breaker is not suitable for the backup protection device of surge protective device.
After years of joint efforts in the lightning protection industry, the products specially used for SPD over current protection of surge protective devices were born in 2012.
The surge backup protector uses parallel circuit inside. When lightning strikes, the lightning current will be discharged through the discharge tube on the right without causing the contact to move by mistake; when the power frequency short-circuit current occurs in the line, the short circuit current will be connected from the left side of the electromagnetic coil to make the contact act, so as to achieve the correct action under lightning shock, and at the same time, it will be broken at a speed when the power frequency short circuit current is generated , protected SPD from fire.
In conclusion, it is important to select backup protection for SPD, which is related to the safe operation of electrical circuit. It is gratifying that on August 1, 2020, gb51348 was issued. Page 163 of the standard, 11.9.11 stipulates that the conductor connected with surge protector shall be short and straight, and the total length of lead wire shall not exceed 0.5m. The surge protector shall be equipped with over current protection device on the installation line, which shall have the following capabilities:
1. Break off the expected short circuit current of SPD installation line;
2. Withstand the continuous opening of surge current through SPD;
3. Power frequency current that can not be disconnected by the internal thermal protection of the split SPD.
According to the above standard, the traditional miniature circuit breaker and fuse do not meet the requirements of surge special backup protection.