Why Is The Handle Of The Backup Surge Protector Designed To Be Non-interlocking?
SPD backup protectors are developed based on circuit breaker technology and are primarily used for overcurrent protection of surge protectors. However, unlike miniature circuit breakers, their handle design differs from that of small circuit breakers, employing a non-interlocking mechanism. Why is this?
When a surge protective device deteriorates or a transient overvoltage fault occurs in the power distribution line, a short circuit to ground will occur in the lightning protection branch. In this case, the surge protector may catch fire. To avoid this, clause 430.3 of IEC 61643-4-43 states: "A suitable overcurrent protection device should be used before the circuit becomes dangerous," preventing short-circuit current from causing the surge protection device to catch fire.
In the past, the backup protection device at the front end of the SPD mainly used circuit breakers (MCBs) as short-circuit protection devices. Circuit breakers are power distribution products. In power distribution requirements, due to the need to consider three-phase balance, the three-phase MCBs are designed in an interlocking manner; that is, if one phase is overloaded, the other phases will trip accordingly to prevent three-phase imbalance.
However, for surge protection, such a linkage design is actually flawed! Because surge protection branches do not participate in power supply and distribution, there is no three-phase balance issue when they trip. When a power line is struck by lightning, lightning may strike one line or all phases. If a short circuit to ground fault in one phase triggers the linkage to disconnect surge protection on all lines, lightning on other phases will have no discharge path, potentially damaging downstream equipment.
Therefore, SPD backup protection devices should not be linked on surge protection branches! Once a short circuit in one phase of our backup protection device trips, we can clearly and promptly identify which phase is faulty, allowing for timely replacement of the surge protection module and rapid restoration of surge protection functionality.
In summary, surge protectors are normally in a high-resistance state to ground and do not participate in power supply and distribution. Therefore, a single-phase SCB trip will not cause three-phase imbalance in the line, nor will it trigger the zero-sequence current protector, and it will not affect the power supply to normal lines. Therefore, from the perspective of lightning protection, the handle of the SPD backup protector does not need to be designed as a linkage mechanism; it must be designed as a non-linkage mechanism, otherwise it will cause serious lightning protection safety problems.
