Load Switch Explained | Functions, Fuse Integration & vs Isolator Differences

What is a Load Switch?

A load switch is a control device equipped with a simple arc-extinguishing mechanism, capable of opening and closing circuits under load. It can interrupt a certain level of load current and overload current, but cannot interrupt short-circuit current. Therefore, it must be used in series with a high-voltage fuse, which relies on the fuse to clear short-circuit currents.

Functions of a Load Switch:

• Switching and Closing Function: Due to its certain arc-extinguishing capability, a load switch can be used to open and close load currents and overload currents up to a certain multiple (typically 3-4 times). It can also be used to open and close unloaded transformers with capacities larger than what isolating switches are permitted, longer unloaded lines, and sometimes large-capacity capacitor banks.

• Substitution Function: A load switch combined in series with a current-limiting fuse can replace a circuit breaker. The load switch handles the interruption and closing of smaller overload currents (within a certain multiple), while the current-limiting fuse handles the interruption of larger overload currents and short-circuit currents.

• Integrated Load Switch-Fuse Combination: A load switch integrated with a series-connected current-limiting fuse is referred to as a "Load Switch-Fuse Combination Apparatus" in national standards. The fuse can be installed on the power supply side or the load side of the load switch. When frequent fuse replacement is not required, it is preferable to install the fuse on the power supply side. This allows the load switch to also function as an isolating switch, isolating the voltage applied to the current-limiting fuse.

Differences between Load Switches and Isolating Switches

First Difference: The type of current they can interrupt is different.

Because an isolating switch lacks an arc-extinguishing device, it is only suitable for interrupting no-load current. It cannot interrupt load current or short-circuit current. Therefore, an isolating switch can only be operated safely when the circuit is completely de-energized. Operating it under load is strictly prohibited to prevent safety accidents. In contrast, a load switch has an arc-extinguishing device, enabling it to interrupt overload currents and rated load currents (though it still cannot interrupt short-circuit currents).

Second Difference: The presence of an arc-extinguishing device.

The presence or absence of this device makes a significant difference. An arc-extinguishing device is designed to facilitate the opening and closing operations of a switching device, effectively limit the arc, and help extinguish it. Having such a device makes the switching operation much safer. Consequently, most switching devices, especially those used in household applications, are equipped with arc-extinguishing devices.

Third Difference: Their functions are different.

Due to the lack of an arc-extinguishing device, an isolating switch is used in high-voltage installations to isolate energized sections from de-energized sections, ensuring the safety of personnel during maintenance and inspection of high-voltage circuits.

A load switch, on the other hand, is used in fixed high-voltage equipment and can interrupt fault currents and rated currents within the equipment. Therefore, their functions differ, although both are applied in high-voltage systems.