Electrical power equipment maintenance & testing: Low-Voltage Switchgear and Circuit Breakers (part 3)

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7 Low-Voltage Switchgear Maintenance and Care

The low-voltage switchgear discussed in this section involves power circuit breakers and enclosures of indoor or outdoor type. The frequency of inspection and maintenance should be 3-6 months when new equipment is installed and 1-2 years for existing equipment. However, if problems with switchgear are encountered, the frequency can be shortened. Similarly to medium- voltage switchgear, the conditions that call for frequent inspection and maintenance are high humidity and temperature, corrosive atmosphere, excessive dirt or dust, frequent fault interruption, and age of the equipment. The following guide is provided for the general maintenance of low-voltage equipment; where necessary it should be supplemented by the manufacturer's detailed instructions.

7.1 General Guidelines for Inspection and Maintenance of Switchgear

The ultimate long-term performance of switchgear depends on the reliability of its insulation system. An important factor in the insulation reliability is its regular switchgear inspection and maintenance program. The frequency of inspection should be based on the number of scheduled shutdowns, frequent emergency shutdowns, long periods of sustained overloading or abnormal operating conditions, numerous switching operations, number of fault occurrences, and extremes in atmospheric conditions. The following guide is offered on how to inspect and what to look for when inspecting switchgear.

This guide may also be used for inspecting medium-voltage switchgear.

On energized equipment:

Listen for popping, spitting, or cracking sounds produced by electrical discharges-also humming noises or vibration produced by resonance.

With lights out, look for blue or purple corona halos. Orange or red sputter arcs are created by intermittent sparking.

Ozone, produced by corona or overheating of organic materials, can usually be detected by their odors.

On de-energized equipment:

Look for physical damage-cracks, breaks, de-laminations, warping, blisters, flaking, or crazing of insulated parts.

Check for foreign objects and loose hardware, warped or distorted insulated bus, and rusty or bent structural framework.

Powdery deposits, carbon tracks, moisture stains or rust, flaking paint, or varnish are signs that moisture is, or has been, present; look for probable source of entry.

Specific areas to inspect:

Although the inspector should check the whole insulating structure, there are a number of specific areas where distress is more likely to occur.

Boundaries between two contiguous insulating members

Boundaries between an insulating member and the grounded metal structure

Taped or compounded splices or junctions

Bridging paths across insulating surfaces; either phase-to-phase or phase-to-ground

Hidden surfaces, such as the adjacent edges between the upper and lower members of split type bus supports, or the edges of a slot through which a bus bar protrudes

Edges of insulation surrounding mounting hardware-either grounded to the metal structure or floating within the insulating member

Physical damage

Broken or cracked insulating supports can allow the supported components to be subjected to mechanical stresses for which they were not designed, resulting in ultimate failure. Damaged or contaminated insulation materially reduces voltage striking and creepage distances, ultimately resulting in a flashover.

Physical damage can stem from several causes:

  • Mishandling of the switchgear during shipment, installation, over-loading, or maintenance
  • Mechanical forces induced by heavy faults:
  • Thermal cycling of insulating members
  • Strains induced by improper mounting of insulating members
  • Combinations of the above

Heat:

Temperatures (even slightly over design levels) for prolonged periods can significantly shorten the electrical life of organic insulating materials. A prolonged exposure to higher than rated temperatures will cause physical deterioration of organic materials, resulting in lower mechanical strength.

Localized heating (hot spots) sometimes occur. They are hard to detect because the overall temperature of the surroundings is not raised appreciably.

Loosely bolted connections in a bus bar splice or void spaces (dead air) in a taped assembly are examples of this.

Since power is usually removed prior to inspection, it is unlikely that apparatus temperature can be relied upon as an indicator of damaging heat.

But observed conditions can be used as the basis for determining heat damage. The signs of heating are as follows:

Discoloration-usually a darkening of materials or finishes

Crazing, cracking, or flaking of varnish coatings

Embrittlement of tapes and cable insulation

Delamination of taped conductors or laminated insulation

Generalized carbonization of materials or finishes

Melting, oozing, or exuding of substances from within an insulating assembly

Moisture:

The term moisture, usually associated with water, includes vapors which can readily conduct leakage currents. They are often present as air pollutants in industrial atmospheres. The main source of moisture is highly humid air which is subject to climatic type cycling. The drop in temperature between daytime and dark can cause relatively still air to pass through a dew-point, resulting in condensation. Sudden temperature drops can cause condensation, even inside of buildings housing switchgear. Detection of moisture usually depends on signs, rather than the presence of actual moisture. Look for these indications:

Droplet depressions (or craters) on a heavily dust-laden bus

Dust patterns, similar to those on an auto subjected to a light rain shower after driving on a dusty road Deposits which remain if a film of dirty water evaporates on a flat surface

Excessive rust anywhere in the metal housing

Actual condensation on metallic surfaces, even though the insulation is apparently dry

Tracking: Tracking is an electrical discharge phenomenon caused by voltage bridging insulating members-phase-to-phase or phase-to-ground. Normally considered to be a surface phenomenon, it can occur internally in some materials.

Materials that are known to track internally are never applied in metal-clad switchgear. Tracking may be detected in various ways:

Active streamers or sputter arcs may occur on insulating surfaces adjacent to high-voltage conductors. These arcs are very tiny, usually intermittent or random in nature, and of variable intensity. One or more irregular carbon lines (trees) eroded into the insulating surface is a sign that tracking has occurred.

Materials specifically designed for track resistance seldom, if ever, exhibit carbon lines. Instead, these materials usually develop erosion craters after extensive bombardment by electrical discharges.

Tracking can propagate from either the high voltage or ground terminal. It will not necessarily progress in a regular pattern or by the shortest possible path.

For tracking to occur, five conditions must exist simultaneously. Remove any one condition and tracking will cease. These conditions are:

• Appropriate temperature

• High local field intensity or gradient

• Contamination on the insulating surface

• Moisture on the insulation surface

• Susceptible insulating material, forming a bridging link over which leakage current can flow; phase-to-phase or phase-to-ground

Corona

[ Corona is an electrical discharge phenomenon occurring in gaseous substances. High electrical gradients, exceeding the breakdown level of the gas, lead to corona discharges.]

Pressure, temperature, humidity, and the type of gas affect breakdown levels. In metal-clad switchgear, corona (if it occurs) is usually localized in the tiny air gaps between the high-voltage bus bar and its insulation or between to contiguous insulating members or at sharp corners of the uninsulated bus bars. Corona can be detected in various ways without using instruments as follows:

A visible, pulsating, blue or purple haze (or halo) may surround the overstressed air gap. The halo is generally of low light intensity and invisible, except in the dark.

Popping, spitting, crackling, or frying noises may accompany the corona discharge.

Corona ionizes the surrounding air, converting the oxygen to ozone.

It has a distinctive penetrating odor.

Its presence may be indicated by erosion of the organic materials adjacent to an overstressed air gap. A white powdery deposit is often present along the edges of the eroded area. In some materials, corona deterioration has the appearance of worm-eaten wood.

Interference with radio reception may be a sign of corona. If the audible noise level increases as a radio is moved closer to switchgear, corona could be the cause.

* It should be noted that corona usually occurs in switchgear rated at 5 kV and higher. Corona is not a problem in 600 V switchgear. The inspection for corona is listed here only for completeness since this inspection guide may also be used for inspecting medium- and high-voltage switchgear.

Corona discharges create problems in a number of different ways:

Ionization of the air releases ions and electrons. These bombard nearby organic materials affecting their molecular or chemical structure.

Ozone, formed by corona, is a strong oxidizing agent; it can also react with many materials.

Nitrogen in the air will also react to ionizing. When ionized under humid conditions, it forms nitric acid which is harmful to insulation.

Insulations are generally selected from materials having acid resistance, but acids can become the conducting fluxids causing the tracking phenomenon.

Although switchgear is designed for corona-free performance, there have been cases, in specific applications, where corona has developed.

7.2 Maintenance of Power Circuit Breakers

It is generally recommended that low-voltage power breakers (see FIG. 3) should be maintained annually. Moreover, a breaker should be serviced after a severe fault interruption. The inspection, maintenance, and tests can be classified as mechanical and electrical and should be conducted on the breaker (the testing of protective devices is covered separately in Section 8.8) on a regular (such as annual) basis.

Mechanical maintenance factors:

• Operating mechanism

• Contact pressure and alignment

• Contact erosion

• Lubrication of the operating mechanism

• Lubrication of the current-carrying components

• Arc chute and interphase inspection

• Electrical maintenance factors:

• Primary circuit (contact) resistance test

• Insulation resistance test

• AC or DC dielectric withstand test

• The described mechanical and electrical maintenance factors are in Section7.2.1.

7.2.1 Mechanical Maintenance Factors

7.2.1.1 Operating Mechanism

The operating mechanism of a circuit breaker is typically checked by performing the following operations:

Closing and opening the breaker's interrupters several times to • verify consistency of operation.

Verifying the trip-free function (when applicable).

Adjusting the trip latch overlap (when applicable).

Adjusting the spring release or close latch overlap (when applicable).

Consistency of operation is defined by the mechanism's ability to successfully latch closed and trip open every time a manual or electrical signal is initiated. Trip-free operation is verified by attempting to close the interrupters while maintaining a trip signal at the same time. The main contacts should not touch as the stored energy of the breaker's mechanism discharges. The trip-free feature may not be a part of every circuit breaker. Some circuit breaker's main contacts will momentarily touch if a closing signal is initiated at the same time as a tripping signal. The specific procedure for adjusting trip latch overlap or close latch overlap is different for each model of circuit breaker. If these latches are not correctly adjusted, a circuit breaker might not latch when a close signal is initiated (a rapid close- open action) or might fail to trip when a trip signal is initiated. Some power circuit breakers have a trip latch or close latch adjustment.

FIG. 3 Low-voltage draw-out power circuit breaker. (a) Front view showing name plate of CH-DSLIFI and protective trip devices, (b) side view showing operating mechanism, and (c) back view showing disconnect stubs and finger cluster, and CLF fuses.

7.2.1.2 Contact Pressure and Alignment

Consult the manufacturer's instructional manual for specific procedures of inspecting and adjusting the pressure and alignment of the main and arcing contact of a power circuit breaker. Pressure inspections do not necessarily involve an actual measurement of force or pressure. More typically, dimensional measurements are specified that assure contact springs are compressed to an adequate amount. Additionally, springs are visually inspected to verify that they have a normal color. Discoloration indicates that the metallurgical properties of a spring are compromised. Alignment checks are typically dimensional measurements that assure sufficient penetration of moving contacts into the areas of fixed contacts.

7.2.1.3 Contact Erosion

Air circuit breakers and magnetic-air circuit breakers have separately replace able sets of arcing and main contacts. The arcing contacts are expected to erode at a rate that depends on the number of interruptions and the value of current that is interrupted. The main contacts are not expected to erode.

The arcing contacts should be replaced when they are eroded badly. The main contacts should be inspected for discoloration, pitting, burning, and deposits of foreign materials. The main contact should not be sanded but they could be dressed with a burnishing tool. If the main contacts are severely pitted, burned, or eroded, they should be replaced.

7.2.1.4 Lubrication of the Mechanism

Mechanism of all power circuit breakers need periodic renewal of lubrication.

There are several factors that influence how often breaker mechanism need to be lubricated. These factors are:

Continuous current rating of the circuit breaker

Number of operations (close-open) and time since last lubrication

Environment where breaker is installed

The ANSI/IEEE C37.16 establishes endurance requirements for low-voltage power circuit breakers and were discussed in Section 8.6.1. These requirements relate the minimum number of close-open operations that a breaker must be able to accomplish before requiring service. One of the limiting factors is the need to renew lubrication in a circuit breaker's mechanism. In general, the larger the breaker, the more frequent is the need for the required breaker service and lubrication. The manufacturer's manual for the breaker may suggest a higher number of operations than the number given in the ANSI/IEEE standards. For example, although an 800 A rated circuit breaker is required by the ANSI/IEEE standard to endure 500 operations before service is needed, the manufacturer's instruction book indicate that an 800 A would require renewal of lubrication after 1750 operations. If a circuit breaker operates only a few times each year, a 500- or 1750-operation count might never happen within the useful life of the breaker. However, a need to renew lubrication will be needed owing to the fact that lubrication materials deteriorate over time when exposed to environment. The deterioration of the replacement materials, such as lubricants, is accelerated by harsh environmental conditions such as elevated ambient temperature or the existence of airborne contaminants.

Most users establish programs to lubricate critical circuit breakers based on a fixed time interval. Many breaker malfunctions and/or failures have been attributed due to lack of, or dried, lubrication in as little as 5 years of normal service. The lubrication points that typically require critical attention are a circuit breaker's trip latch, spring-release latch, and cam- follow roller.

In all cases, the manufacturer's instructional literature should be consulted to determine which components require lubrication. There is a large variety of materials that are used to lubricate a circuit breaker's mechanism. More than one type of lubrication might be used in the same mechanism at different specific points. Additionally, the material that is recommended for renewal of lubrication is sometimes not the same material that was installed at the factory.

For example, many models of circuit breakers have molybdenum disulfide in a lithium base installed at the factory, but the breaker's instruction book recommends light machine oil to be applied to these same lubrication points for maintenance. In all cases, it is important to use the material that is specified in a circuit breaker's instruction book. Although newer and better lubrication materials are available on the market, circuit breaker manufacturers seldom re-qualify circuit breakers using new lubricants by conducting standard endurance tests after the time of initial introduction for sale.

7.2.1.5 Lubrication of Current-Carrying Components

A manufacturer's breaker manual sometimes recommends renewal of lubrication for specific current-carrying components of a circuit breaker.

These components include main contacts, primary-circuit finger clusters and bus studs. Care must be taken when assessing which current-carrying components should be lubricated and which components should not be.

Manufacturer's instruction book on the particular breaker type should be consulted for lubrication of current-carrying components.

7.2.1.6 Arc Chute and Interphase

The arc chutes and interphase barriers of a circuit breaker are inspected visually to detect broken or contaminated components. Broken components are replaced.

The contamination that is caused by arc products can be cleaned by various methods such as sand blasting or using a flexible aluminum-oxide coated paper disc. Soot and dust is typically removed with a pressure-regulated jet of air.

Some arc chutes may contain asbestos components such as rope, cement, or shields. Arc chutes having asbestos components should not be cleaned unless correct breathing apparatus is worn by the personnel cleaning the arc chutes.

7.2.2 Electrical Maintenance Factors

7.2.2.1 Primary Circuit Resistance Check

This test is also known as millivolt drop test or contact resistance (DC resistance) measurement test. This test is performed to assess the condition of the main contacts and connections of the current-carrying components of a breaker.

If desired, the DC resistance of the primary circuit may be measured by closing the breaker and passing DC current (at least 100 A) through the breaker. With a low-resistance instrument, measure resistance across the studs on the breaker side for each pole. The resistance should not exceed 60, 40, and 20 µ-Ohm for 1200, 2000, and 3000 A breakers, respectively. If no data is given in a manufacturer's instructional literature on primary circuit resistance (contact resistance) values, then evaluate by comparing readings of the past years test readings with present readings to detect a trend. A reading that increases by a factor of 2 is considered a significant sign of deterioration. Also, compare the primary circuit resistance of the three poles against each other and readings that differ by more than 50% should be investigated. Primary circuit resistance measurements are generally made before and after cleaning operations are performed on the main contacts.

7.2.2.2 Insulation Resistance Tests

Insulation resistance measurements of the primary insulation of a power circuit breaker can be used to detect deterioration such as absorption of moisture, contamination, or thermal aging. This test is performed to check the insulation integrity of the breaker, i.e., the insulation of the bushing, interphase barriers, and arc chutes. Measurements taken over a period of months or years can reveal a deteriorating trend. A change representing a factor of 10 over a period of 1 year is considered a reason to conduct additional inspections of an insulation system such as visual inspections or applied-potential tests. Insulation resistance tests are useful just before returning a circuit breaker to service is to confirm that moisture has not condensed on insulation surfaces and components of the breaker. A typical criterion for returning a breaker to service is that its insulation resistance should not be less than 90% of the value that was measured in previous years when the insulation was new and known to be dry. Refer to Section 2.10 and Table 2.8 for additional information on insulation resistance testing and quantifying good insulation resistance values.

7.2.2.3 AC Dielectric Withstand and Low-Frequency Withstand Tests

An AC dielectric withstand test, known as a low-frequency withstand test, an applied potential test, or a high-potential (hi-pot) test, can be used to detect a gross failure of an insulation, the presence of a foreign object within an insulation system, or insufficient clearance between energized components and ground. Breaker manufacturer's literature typically recommends that an AC dielectric withstand test should be conducted on a circuit breaker before initially placing it into service, after repairs, and on a periodic basis as part of a maintenance inspection. There are several ways for conducting a dielectric withstand test. The method outlined in ANSI/IEEE standard C37.50-2000,

recommends that with the circuit breaker in open position, apply the desired test voltage to (1) all upper and lower primary terminals (six breaker bushings) with respect to the metal parts (frame or ground), (2) all primary terminals with respect to the secondary terminals, and (3) all upper primary terminals with respect to all lower terminals of the breaker. The test procedure is similar to the procedure for conducting insulation resistance measurement test. Refer to Sections 2.6 and 7.4 for test method and connections.

7.2.3 Inspections Procedure

To conduct maintenance and inspection, withdraw the circuit breaker from its enclosure and perform the following:

Inspect alignment of movable and stationary contacts. Make adjustments as recommended in manufacturer's book. Do not unnecessarily file butt-type contacts. Silver-plated contacts should never be filed, if these contacts are in degraded condition they should be replaced.

Wipe bushings, barriers, and insulating parts. Remove dust, smoke, and deposits.

Check arc chutes for damage and blow-out dust (refer to Section 7.2.1.6 regarding arc chutes containing asbestos). Replace damaged, cracked, or broken arc chutes.

Refer to Section 7.2 for inspection of breaker-operating mechanism for broken, loose, or excessively worn parts. Clean and re-lubricate operating mechanism with light machine oil. Use nonhardening grease for lubrication of rollers, cams, latches, and the like. Adjust breaker operating mechanism if required.

Check control devices and replace if needed. Also replace badly worn contacts.

Check breaker control wiring and ensure that all connections are tight.

Operate breaker in fully opened and closed positions after it has been serviced. Check for any binding, and operate breaker manually and electrically before putting back in service.

Check other items, such as switches, relays, and instruments, during servicing of the breaker

7.3 MCCBs

The maintenance of molded-case breakers consists of inspection of the breaker, its mounting, electrical connections, and electrical tests. The reader is referred to NEMA standard publication AB-4-2003, Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers Used in Commercial and Industrial Applications, for more detail list on preventive maintenance of MCCB. Similar to the low-voltage power circuit breakers, the maintenance of MCCB can be addressed as mechanical and electrical factors. The following steps are recommended as a guide:

7.3.1 Mechanical Factors

7.3.1.1 Repair or Replacement of UL Listed Components

The majority of MCCBs have labels (paper sticker or silver-white stencil) that identify them as being listed or approved for use in a listed assembly by the UL. For listed or approved breakers, the kinds of repairs or component replacements that can be made by the user are limited. The presence of a paper label that would have to be broken to remove a cover or mounting screw of a breaker is an indication that no components under that label can be replaced or repaired without making the breaker's UL listing or approval void. This UL label is not the same as the factory warranty label that might also be present.

A hard putty sealant over any screw head has the same function. The basic set of components that can be replaced by the user typically includes replaceable types of terminals (including lugs), replaceable types of trip devices, and rating plugs. Some manufacturers and independent service organizations offer repair services that include a revalidation of the UL labeling. MCCBs that are not labeled have a greater variety of internal components that can be replaced.

Any component of a UL-listed panelboard or motor starter must be replaced with a component of the same manufacturer and same type.

7.3.1.2 Replacement Circuit Breakers

A replacement circuit breaker is a MCCB that is manufactured specifically to i t into an obsolete style electrical assembly without the need to physically or electrically modify the assembly. It is not permissible to install a replacement circuit breaker into a newly constructed assembly.

7.3.1.3 Replacement of an Automatic Trip Device or Rating Plug

Some MCCBs have a replaceable automatic trip device (thermal-magnetic types) or a replaceable rating plug (solid-state types). The ability to replace trip devices provides a flexible system for the application of circuit breakers according to National Electrical Code. An additional benefit is the ability to replace an automatic trip device that has become defective. Most MCCBs have built-in safeguards that prevents the installation of a trip device whose continuous-current rating is greater than the continuous-current rating of the breaker's frame.

7.3.1.4 Tightening of Connectors

The compression screws of the terminals (lugs) or bus connectors of a MCCB should be tightened periodically. Any terminal kit of recent manufacture is supplied with a paper label that lists the appropriate lb-ft or Newton-meter values of torque for each compression screw. This label has an adhesive back and is intended to be affixed onto the inside of the sheet metal cover of the compartment in which the circuit breaker is installed. Compression screws and mounting bolts are not intended to be tightened while a circuit breaker is energized.

7.3.1.5 Periodic Exercising

A MCCB must be operated open and closed with sufficient frequency to ensure that its main contacts are cleaned by wiping action and that the lubrication materials within its mechanism remain evenly spread. For any circuit breaker that is not operated in its normal service, a periodic open-close exercise should be implemented. Most of the failures and/or malfunction of MCCBs are due to lack of exercise of the operating mechanism.

7.3.1.6 Mechanical Operation

Before making any electrical connections, the circuit breaker should be checked by manually turning the breaker on and off several times. This check is to ensure that all mechanical linkages are operating.

7.3.1.7 Connection Test

This is a visual test that is conducted to assure that there is no overheating and/or that arcing is present in the electrical joints. An infrared (IR) gun may also be used to spot heated joints instead of visual observation. If signs of arcing or excessive heating are present, the connections should be removed and thoroughly cleaned. Also, during installation, proper attention should be given to electrical connections to minimize damage to the aluminum lugs and conductors.

7.3.2 Electrical Factors

7.3.2.1 Insulation Resistance Measurement Test

This test is made to verify the condition of the insulation of the circuit breaker.

A minimum of 1000 V test voltage should be used for low-voltage (600 V class) breakers for making this test. It would be preferable to use a DC test voltage that is at least 1.5-1.6 times the peak AC voltage of the circuit breaker.

Tests should be made between pole to ground, between adjacent poles with circuit breaker contacts closed, and between phase-to-load terminal with breaker in open position. A minimum value of 1 MO is considered safe to prevent a flashover. Resistance values below 1 MO should be investigated for possible trouble.

7.3.2.2 Millivolt Drop Test (Contact Resistance)

This is the primary circuit resistance test that was discussed in Section 8.7.2 for low-voltage power circuit breakers. This test consists of applying a DC across the closed circuit breaker contacts and measuring the voltage drop due to the contact resistance. Excessive voltage drop indicates abnormal conditions such as contact and/or connection erosion and contamination.

This test is similar to the circuit breaker contact resistance measurement test described in Section 7.4.5 for medium-voltage breakers. The manufacturers of MCCBs should be consulted in order to find the acceptable millivolt drop values for particular breakers being tested. It is recommended that large breakers be tested with DC of at least 100 A and smaller breakers be tested at rated (or below rated) currents. The measured values should be compared among three phases of the breaker under test, or with values of breakers of similar size or with manufacturer's recommended values to assess whether the contacts need to be replaced or dressed.

7.3.2.3 AC Dielectric Withstand Tests

This is the same test that was discussed in Section 7.2.2 for low-voltage power circuit breakers. This is a hi-pot test, and conducted to detect a gross failure of an insulation, the presence of a foreign object within an insulation system, or insufficient clearance between energized components and ground. This test maybe conducted to verify the MCCB withstand voltage capability.

7.3.3 Inspections Procedure

To conduct maintenance and inspection of MCCB, perform the following:

Clean all external contamination to permit internal heat dissipation.

Inspect all surfaces for cracks or damage.

Check for loose connections, and tighten circuit breaker terminals and bus bar connections. Use the manufacturer's recommended torque values.

Manually switch on and off the breaker in order to exercise the mechanism.

Check for high-humidity conditions since high humidity will deteriorate the insulation system.

Check for hot spots typically caused by overheating due to termination or connections being loose, high contact resistance, or inadequate ventilation.

7.4 Switchgear Enclosure

The following steps are recommended for servicing the switchgear enclosure, in addition to the maintenance recommendations made under medium-voltage switchgear:

Turn power off and ground the bus.

All dust and dirt should be vacuumed from the switchgear enclosure and components.

Wipe clean buses, insulators, cables, and the like.

Inspect bus work and disconnect for overheating.

Tighten all mounting and splice bolts. Examine all connections for tightness.

Check for alignment and seating of contacts of disconnecting devices.

Look for abnormal wear or damage.

Clean and lubricate draw-out mechanism.

Check operation of shutters, interlocks, and auxiliary devices.

Clean ventilating openings and filters.

After servicing, perform an insulation resistance test from phase to ground on each bus. Compare the results with previous tests to see any weakening tendency.

Refinish damaged paint surfaces.

7.5 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.

7.5.1 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.

7.5.2 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 conditions 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.

7.5.3 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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