Low-Voltage Switchgear and Circuit Breakers:Maintenance of Power Circuit Breakers

Maintenance of Power Circuit Breakers

It is generally recommended that low-voltage power breakers (see Figure 8.3) 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

Low-Voltage Switchgear and Circuit Breakers-0282

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 Section 8.7.2.1.

Mechanical Maintenance Factors

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.

ntact 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.

ntact 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.

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 sug- gest a higher number of operations than the number given in the ANSI/IEEE standards. For example, although an 800A 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 800A 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 requalify circuit breakers using new lubricants by conducting standard endurance tests after the time of initial introduction for sale.

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.

rc 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.

Electrical Maintenance Factors

rimary 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 100A) 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 μΩ for 1200, 2000, and 3000A 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.

Insulation Resistance Tests

Insulation resistance measurements of the primary insulation of a power cir- cuit 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 insula- tion 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 compo- nents 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.

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 bush- ings) with respect to the metal parts (frame or ground), (2) all primary termi- nals 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.

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 adjust- ments 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

8.7.2.1.6 regarding arc chutes containing asbestos). Replace damaged,

cracked, or broken arc chutes.

• Refer to Section 8.7.2 for inspection of breaker-operating mechanism for broken, loose, or excessively worn parts. Clean and relubricate 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.

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 Inspec- tion 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:

Mechanical Factors

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.

Replacement Circuit Breakers

A replacement circuit breaker is a MCCB that is manufactured specifically to fit 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.

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.

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.

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.

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.

8.7.3.1.7 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.

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