Fused or unfused interrupter switches are most often used on power feeder circuits where the continuous current capacity is relatively low (600 A). These circuits should only have infrequent switching, and must not require rapid service restoration after fault occurrences.
The basic disconnecting switch has no interrupting rating. It is intended to be operated only after the circuit has been opened by some other means. Transformer and magnetizing currents in this voltage range are handled with either a set of arc horns or an arc restricting device. With either of these arrangements, it is possible to interrupt 10 to 15 amperes.
The next step in upgrading a basic disconnecting switch is to add an interrupting mechanism to extinguish the arc formed when the contacts are opened. This permits full-load current interruption. Several methods are used by manufacturers, depending on the operating voltage. Load interrupter switches for medium-voltage circuits (2.4 to 13.8 kV) are available in continuous and interrupting ratings up to 1200 A. They may be of the air, fluid-immersed or oil-cutout type.
Medium-voltage power fuses with a variety of three-phase interrupting ratings (up to 750 MVA symmetrical) are used with interrupter switches to provide short circuit protection in the 2,400-13,800 Volt range. Remember that power fuses should only be applied for fault protection. Overload protection should normally be provided by low-voltage equipment on the secondary side of the transformer.
