Surge protective devices (SPDs) are often backed by fuses and small circuit breakers or molded-case circuit breakers. When these elements are coordinated with SPDs, they should remain unaffected during rated surge current conditions, ensuring the normal dissipation of surge current while maintaining a residual voltage (Ur) on branch circuits below the protection level of the electrical equipment (Up). This ensures the safety of the system and electrical devices.
Is It Reasonable to Add a Fuse in Front of the Surge Protector?
An SPD essentially represents a branch circuit, where protection devices (circuit breakers or fuses) are installed to prevent short-circuit breakdown (for voltage-limiting SPDs) or continuous current flow (for switch-type SPDs) from causing a wide-scale power outage due to tripping of the main power supply entry switch. The inclusion of fuses or circuit breakers in front of surge protectors is for safeguarding the surge protector, and engineers tend to favor circuit breakers over fuses because blown fuses are not easily detected.
What’s the Better Choice for the Front End of Surge Protectors: Fuses or Circuit Breakers?
Currently, the use of circuit breakers is more prevalent in this area, as engineers consider them more reliable. While circuit breakers are simpler to configure, they are prone to tripping, especially electronic ones. Fuses, on the other hand, do not have this issue but require regular inspection.
The Purpose of Adding a Fuse in Front of Surge Protectors
Surge protectorsfilter out strong interference signals, such as lightning signals, from both digital and analog signals used in industrial control. They protect control systems from damage, and their role is not to filter out interference signals.
The objectives of adding a fuse to the front end of surge protectors are as follows:
Prevent damage to SPDs and their circuits from the occurrence of continuous current flow due to lightning strikes (applicable to air gap discharge-type devices).
Facilitate maintenance and replacement of SPDs.
Guard against circuit faults resulting from the aging of SPDs, such as an increase in leakage current in MOV devices.
The selection of the SPD front-end fuse should be based on the parameters provided by the lightning protection device manufacturer. In cases where the manufacturer does not specify, the general principle is as follows:
Determine the cutoff current (C) of the switch or fuse basedon the maximum rated fuse strength (A) of the surge protector and the maximum supply current (B) of the connected distribution circuit. The method is as follows:
When B > A, C should be less than or equal to A.
When B = A, C should be less than A, or no C installation is required.
When B < A, C should be less than B, or no C installation is required.
The Role of Adding a Fuse in Front of Surge Protectors
If a surge protectormalfunctions, it may remain in a connected state for an extended period, potentially causing a short circuit in the power supply or system. In such cases, the front-end circuit breaker or fuse is needed to promptly disconnect the ground circuit and ensure the normal operation of the circuit.
But how can one distinguish whether this circuit breaker or fuse is dealing with a short circuit caused by a lightning strike (referred to as “A”) or a short circuit caused by the surge protector’s own damage (referred to as “B”)? This distinction is crucial because if “A” is mistaken for “B,” and the circuit breaker trips, the main circuit will be destroyed. Conversely, if “B” is mistaken for “A,” the main circuit will continue to short-circuit, potentially leading to circuit damage.
The surge protector we use is designed to protect against induced lightning, which is characterized by high peak voltages, large currents, and extremely short durations. Fuses require specific conditions for melting and breaking, and transient lightning strikes are clearly insufficient to melt the fuse during surge protector operation. With the inclusion of fuses, circuit damage is avoided, and the fuse is cut off, ensuring the electrical circuit and ground remain safely disconnected.