FAULT PROTECTION DEVICES

Fuses are designed to melt and disconnect the circuit within which they are placed should the current in the circuit increase above a specified thermal rating. Fuses designed to be used in circuits operating above 600 V are classified as fuse cutouts. Oil-filled cutouts are mainly used in underground installations and contain the fusible elements in an oil-filled tank. Expulsion-type cutouts are the most common devices used on overhead primary feeders. In this class of device, the melting of the fusible element causes heating of a fiber fuse tube, which, in turn, produces de-ionizing gases to extinguish the arc. Expulsion- type cutouts are classified as:

• Open-fuse cutouts
• Enclosed-fuse cutouts
• Open-link-fuse cutouts

The automatic recloser is an overcurrent device that automatically trips and recloses a preset number of times to clear or isolate faults. The concept of reclosing is derived from the fact that most utility system faults are temporary in nature and can be cleared by de-energizing the circuit for a short period of time. Reclosers can be set for a number of operation sequences, depending on the action desired. These typically include instantaneous trip and reclose operation followed by a sequence of time-delayed trip operations prior to lockout of the recloser. The minimum pick-up for most reclosers is typically set to trip instantaneously at two times the nominal current rating.

An automatic line recloser is constructed of an interrupting chamber and the related contacts that operate in oil, a control mechanism to trigger tripping and reclosing, an operator integrator, and a lockout mechanism. An operating rod is actuated by a solenoid plunger that opens and closes the contacts in oil. Both single-phase and three-phase units are available.
The line sectionalizer is yet another overcurrent device. It is installed in conjunction with backup circuit breakers or reclosers. The line sectionalizer maintains coordination with the backup interrupting device and is designed to open after a preset number of tripping operations of the backup element. Line sectionalizers are installed on poles or cross arms in overhead distribution systems. The standard continuous current rating for sectionalizers ranges from 10 to 600 A. Sectionalizers also are available for both single-phase and three-phase systems.

The function of a circuit breaker is to protect a circuit from the harmful effects of a fault, in addition to energizing and de-energizing the same circuit during normal operation. Breakers are generally installed on both the incoming sub-transmission lines and the outgoing primary feeders of a utility substation. These devices are designed to operate as quickly as possible (less than 10 cycles of the power frequency) to limit the impact of a fault on the distribution and control system. At the same time, the arc that forms between the opening contacts must be quenched rapidly. Several schemes are available to extinguish the arc, the most common being immersion of the contacts in oil. Some circuit breakers have no oil, but quench the arc by a blast of compressed air. These are referred to as air circuit breakers. Yet another type encloses the contacts in a vacuum or a gas, such as sulfur hexa fluoride (SF₆).

Air circuit breakers are typically used when fault currents are relatively small. These devices are characteristically simple, are low cost, and require little maintenance. The fault current flows through coils, creating a magnetic field that tends to force the arc into ceramic chutes that stretch the arc, often with the aid of compressed air. When the arc is extinguished through vacuum, the breaker is referred to as a vacuum circuit breaker. Because a vacuum cannot sustain an arc, it can be an effective medium for this application. However, owing to imperfections present in a practical vacuum device, a small arc of short duration can be produced. The construction of vacuum circuit breakers is simple, but the maintenance is usually more complex than with other devices.