Low-Voltage Switchgear and Circuit Breakers:Air Disconnect Switches, Fuses, and Insulators

Air Disconnect Switches, Fuses, and Insulators

The low-voltage class electrical distribution system is comprised of equipment such as disconnects, fuses, insulators, lightning arresters, in addition to transformers, circuit breakers, and the like. The recommended frequency for maintenance of electrical equipment is a function of environmental conditions.

The frequency of maintenance for equipment in dirty, wet, and corrosive environments will always be more frequent than for equipment in clean areas. A general guide for maintaining this equipment is given in the following section.

Air Disconnect Switches

Air disconnect switches come in many varieties and ratings. The disconnect switches are normally not de-energized during routine maintenance of substations and therefore should be approached with caution. Also, service disconnect switches are seldom operated. However, the interrupter switches are specifically designed for making and breaking specified current. The function of the interrupter switch is similar to the circuit breaker, and the maintenance of this switch should be similar to the procedures listed under power circuit breakers.

The air disconnect switch should be inspected and maintained as follows:

• Close the switch several times to ensure the simultaneous closing of the blades and complete seating of the contacts. Check to see if the closing latch is in the fully closed position. Make adjustments if required in accordance with the instruction manual.

• Inspect the contacts for burns, pitting, pressure, and alignment. Also inspect arcing horns for excessive burn marks. If the contacts show minor damage, dress the contact surface with smooth sandpaper. Badly burned contacts and arcing horns should be replaced.

• Inspect the linkages and operating rod for bending or distortion.

• Check all safety interlocks for proper operation.

• Check for any abnormal conditions such as insulation cracks, chemical deposits if the switch is installed in a corrosive environment, flexible braids, and slip ring contacts.

• Perform special inspection and maintenance when the switch has carried heavy short-circuit current.

Power Fuses

The application of power fuses in electrical distribution system is quite common. There are many classes of fuses, such as current limiting or non- current limiting with various time–current characteristics, silver sand, liquid filled, or vented expulsion type. The frequency of fuse inspection and maintenance must be determined depending upon the environmental con- ditions of fuse location. Before fuses are removed or installed, de-energize the fuse holders (i.e., the total fuse assembly is disconnected from the power source). The following general procedures are suggested for inspection and maintenance of power fuses.

• Inspect the fuse unit and renewable element (if the fuse is a renewable type) for corrosion, tracking, and dirt. Replace those units that indicate deteriorated condition.

• Inspect for dirt, dust, salt deposits, and the like, on insulators for the holders to prevent flashover. Also look for cracks or burn marks on insulators.

• Inspect the seal on the expulsion chamber for vented expulsion-type fuses to ensure that no moisture has entered the interrupting chamber of the fuse.

• Check for any missing or damaged hardware, such as nuts, bolts, washers, and pins.

• Clean and polish contact surfaces of clips and fuse terminals that are corroded or oxidized.

• Tighten all loose connections and check to see if the fuse clips exert sufficient pressure to maintain good contact.

• Generally fuses that show signs of deterioration, such as loose connections, discoloration, or damaged casing, should be replaced.

Insulators

Insulators are used in all switchgear assemblies and equipment. Insulators separate the current-carrying parts from noncurrent-carrying parts. The integrity of insulators is therefore very important. The following procedures are recommended for maintaining insulators.

• Inspect insulators for physical damage such as cracks or broken parts. Replace those parts that have incurred damage.

• Inspect insulators for surface contamination such as dirt, grime, and dust. Wipe clean all contaminated insulators.

• Check for all hardware to ensure that the insulators mounting assembly is tight.

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