How to Carry Out the Maintenance of ACB?

The air circuit breaker (ACB) is a protection device and requires regular maintenance for a high level of performance and to avoid costly repairs. Necessary low-voltage air circuit breaker maintenance implies inspection and checking of the integrity of the mechanical and electrical components. Replacing the parts that have ended their life cycle will keep the performance level high.

Investing in the prevention of failure, rather than living with its consequences, such as loss of production and possible violation of health and safety legislation, is a primary concern for many businesses. Successful prevention of failure can make the difference as to whether or not a company manages to retain its long-term competitiveness.

The service life of the ACB depends on the working and environmental conditions. The ACB is exposed to mechanical and electrical stresses and thus suffers gradual degradation during use, which will increase the possibility of malfunctions. Preventive maintenance and periodical inspection are very important to avoid any functional degradation, prevent malfunctions, extend the service life, and ensure safe operation.

Air circuit breaker (ACB) Maintenance Checklist

Periodic inspections and associated maintenance are essential to the safe and reliable operation of air circuit breakers. To establish an exact schedule for a specific installation, use the following checklist:

1. Visual Inspection

Perform the following activities during the visual inspection:

Record operations counters
Check circuit breaker panels and insulation material for cracks and cleanliness.
Check the condition of enclosing cabinets including hinges, latches, locks, and door gaskets.
Check for loose or broken frame ground connections.
Check that all power and control circuit switches are closed and fuses are in place.
Inspect wiring for damaged insulation.

Reference – NFPA 70B-2016, Sections 11.10.6, 15.4, Table K.4(d), Table L.1.

2. Preventive Maintenance

Perform the following preventive maintenance of indoor and outdoor low-voltage draw-out air circuit breakers:

Check the frame, panel, and cabinet condition of the paint and repaint as necessary. Tighten bolts to recommend specifications. Clean the exterior of the cabinet.
Clean porcelain of bushings or insulators with water or a suitable cleaner. Repair chipped spots by painting with enamel such as 1201 glyptal. Tighten bolts to recommended specifications.
Perform in-depth bushing inspection for any carbon tracking, leaks, and cracks.
Clean and repair chipped spots, remove and clean interphase barriers.
Check main connections for loose or overheating terminals.
Remove arc chutes so that the contacts can be inspected. Dress contacts, if rough, with a fine file. It is necessary to carefully remove only the projecting beads. Pits on a flat, smooth surface are not objectionable.
Check contact springs for loss of temper, breaks, or rust deterioration.
Check flexible shunts at contact hinges for overheating and fraying. Tighten connections to recommended specifications.
Verify breaker fit and racking mechanism operation.
Check operating rods, shafts, and bell cranks for loose locknuts, setscrews, keys, bearings, bent rods, or twisted shafts, etc. Clean moving parts of rust, dirt, and accumulated grease and oil. Wash out bearings, pivots, and gears with a suitable cleaner. Lubricate with new grease or oil with the type required by the application being used. In cold climates, it is important to use a lubricant that will not stiffen with cold. Wipe off excess lube. Enclosed dust-tight bearings should require less servicing.
Observe the closing solenoid motor or spring during several closing operations to see that everything is in proper working order. Check the solenoid plunger for sticking in guides.
Check close coil resistance with an ohmmeter and insulation resistance with an appropriate insulation tester.
Observe the latch and trip mechanism during several tripping operations to see that everything is in working order.
Check latch and trip assembly pins, bearings, and latches for wear, binding, and misalignment.
Clean and lubricate latch and trip assembly. Check the latch carefully to see that it is not becoming worn so that it would unlatch from vibration or stick and fail to trip. Tighten bolts and screws to recommended specifications.
Observe tripping operation during electrical operation. See that the full energy action of the plunger is obtained. Check the plunger for sticking-in guides.
Inspect digital or electronic trip mechanisms for the power light and any possible error codes.
Check the condition of auxiliary contacts and refinish them with a burnishing tool if burned or corroded.
Check auxiliary contact springs, operating rods, and levers. Check the closing and opening position with respect to main contacts while the breaker is being slowly closed and opened manually. Certain auxiliary contacts used for special purposes may require adjustment for the closed position.
See that the position indicator or semaphore is properly indicating the breaker position. Check operating rods or levers for loose parts.

Reference – NFPA 70B-2016, Section 11, 15.

3. Manual Operation

Manually operate the indoor and outdoor low voltage draw-out air circuit breaker, ‘CLOSE’ and ‘OPEN’ three times.

Many breakers sit without any operation for long periods of time. This can be very harmful to the breakers operating mechanism and lead to faulty operations. This operation is best performed at the breaker with the local switch. Ensure the breaker is de-energized before operating.

Reference – NFPA 70B-2016, Section 11.10.6.

4. Breaker Timing Test

Complete timing tests for both indoor and outdoor circuit breakers.

Test all trip functions available. This may include long-time pickup and delay, short-time pickup and delay instantaneous, and ground-fault pickups.

The timing test for low voltage, draw-out and circuit breakers may require current injection for all trip functions unless an electronic or digital trip unit is used.

Note: If an electronic or digital trip unit is in place, the first trip test of each phase must be performed by current injection through the contact. It is then possible to test the remaining functions of the trip mechanism through secondary current injection directly into the trip mechanism.

Performing the current injection test for the initial test will ensure the CTs within the circuit breaker are operating as designed.

Reference – NFPA 70B-2016, Section 11.10.6.

5. Insulation Resistance Test

Complete an insulation resistance test for both indoor and outdoor low-voltage draw-out air circuit breakers.

Measure the insulation resistance of each pole for one minute at an applied voltage that is in accordance with the manufacturer’s O&M manual or at least 1,000 VDC. The test should be performed on individual poles with the breaker closed and the unused poles and frame grounded. An additional test should be performed with the breaker open and one side of the breaker grounded to measure the open poles. The insulation resistance test is a pass/fail test; however, at times the manufacturer may publish minimum resistance values within their O&M manual. If the O&M manual does not give a minimum value for its results, use a minimum value of 1 MΩ.

Reference – NFPA 70B-2016, Sections 11.10.6, 11.16.1.2.1-11.16.1.2.4, Table K.4(d), Table L.1.

6. Contact Resistance Test

Complete a contact resistance test for both indoor and outdoor low-voltage draw-out air circuit breakers.

Reference – NFPA 70B-2016, Section 11.16.1.2.2, Table K.4(d), Table L.1.

7. Equipment Rating Review

Perform equipment rating review and coordination study for both indoor and outdoor low voltage draw-out air circuit breaker.

Equipment rating review of AC and DC distribution systems is critical to ensure the proper operation of protective devices, including low-voltage circuit breakers. A coordination study shall also be included with the equipment rating review. The results of this review should be used to help procure properly rated low-voltage breakers. After installation and the as-built information are obtained, another equipment rating review must be performed. The equipment ratings of the breakers and the coordination study can easily be accomplished during the review of the Arc Flash Hazard Analysis.

Reference – NFPA 70B 2016, Section 9.

8. Infrared Scan and Thermal Analysis

Perform an infrared scan while the breaker is in service and closed.

Infrared scan or thermal scan/analysis allows for non-intrusive analysis of temperature and thermal characteristics or thermal patterns of circuit breakers and their components. The infrared scan may dynamically detect abnormal thermal conditions or changes in temperature that may indicate problems in circuit breakers in their incipient stages. Different technologies are available in the industry to detect dynamic changes in heating; however, using thermal imaging cameras is preferred. Identifying and remedying problems early by the infrared scan may avoid serious failures and costly outages. Early detection permits more effective maintenance planning and scheduled outages. Thermal analysis can be used to troubleshoot, perform pre-and post-outage thermal comparisons, verify successful installation or repair, and predict problems with the circuit breakers.

Note: Opening panel doors or removing panels which may expose maintenance personnel to live parts while the breaker is in service. This type of work may expose maintenance personnel to an arc flash hazard.

Reference – NFPA 70B, Section 11.17.

Parts replacement procedure

ACB maintenance, inspection, and parts replacement must be performed by competent persons.
Do not touch ACB current-carrying parts and ACB structural parts close to a current-carrying part immediately after the ACB trips open. The remaining heat may cause a burn.
Before commencing any work on the ACB, open an upstream circuit breaker or the like to isolate all sources of power/voltage from the main and control circuits. Otherwise, an electric shock may result.
Take care to avoid the adhesion of dust to the main and control circuit contacts. Dust on the contacts may result in a fire.
Before commencing maintenance, inspection, or parts replacement, make sure that the closing springs are released and the ACB is open. Otherwise, unintentional open/close operation may lead to fingers or tools being pinched by the open/close mechanism, resulting in injury.
Be sure to reinstall the arc chamber if removed. Failure to do so or incorrect installation of the arc chamber may result in a fire or burn.
When replacing an auxiliary, do not damage the control wire for the auxiliary or pinch the wire between the auxiliary and the breaker body. Doing so may cause a malfunction.

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