1. Overview The AKR low-voltage air circuit breaker is a product of GE (USA), conforming to ANSI standards, and is currently one of the most advanced low-voltage circuit breakers. The AKR series circuit breakers can break a maximum rated short-circuit current of 200kA and a maximum rated current of 4000A. It features large capacity, small size, comprehensive protection, compact design, and strong component interchangeability. The circuit breaker consists of a mechanism, conductive and supporting components, arc assembly devices, and various interlocking devices. The AKR series circuit breakers are available in fixed and withdrawable types. The AKR-60 series selected by our company is a withdrawable type, used in our company's low-voltage power distribution system. Because it is imported equipment, some maintenance personnel lack sufficient understanding of its performance and structure, and the most prominent fault is the circuit breaker's refusal to close, preventing normal operation. This section introduces common faults and troubleshooting methods for AKR low-voltage circuit breakers. 2. Main Mechanisms Related to Circuit Breaker Refusal to Close This mechanism is the key component that enables the circuit breaker to have closing, opening, and free-tripping functions, as shown in Figure 1. The closing spring releases energy, causing the circuit breaker to fully extend the closing elbow. The moving contact is pressed against the stationary contact by the elbow, and the elbow maintains its position (closing position) through the engagement of its cam roller (5) with the support (2) and the secondary hook with the trip hook (14 and 11). The process of changing from the closing position to the trip position is shown in Figure (2). The trip shaft (10) is rotated by manually (electrically) pressing the trip button or by other tripping devices, and the trip hook (11) is assembled onto the trip shaft. When the trip shaft rotates, the trip hook disengages from the secondary hook roller; the secondary hook rotates with the bearing, causing the elbow linkage to collapse and, together with the opening spring (15), cause the circuit breaker contacts to open (this is the trip position). The reset position mechanism (opening energy storage state) is shown in Figure (3). The cam (3) is mounted on the camshaft (4). It is rotated by the energy storage motor, the manual operation rocker arm or the maintenance rocker arm. The cam engages the cam roller and partially extends the elbow, which allows the secondary hook to rotate up and down against the front frame, as shown in Figure (1). A gap is left between the trip hook and the secondary hook roller. The secondary hook is now in the position of engaging with the upper hook and the cam roller. 2—Support 3—Cam 4—Camshaft 5—Cam Roller 10 Rotating Trip Shaft 11—Trip Hook 12—Insulating Arm 13—Main Shaft 14 Secondary Hook 15—Opening Spring 3 Fault Analysis and Handling Methods 3.1 Energy Storage Motor Fault Cannot Rotate or Runs Freely a. First, check whether the motor DC resistance is normal and whether the voltage is normal. If there is no voltage, check whether the circuit is open or broken, and whether the operating fuse is burnt out. If the voltage is normal, it may be that the motor winding is open or damaged. Replace or repair the motor. b. If the energy storage motor cannot store energy and runs idle, check if the ratchet of the energy storage mechanism is severely worn, so that the cam pawl of the motor cannot meet the teeth on the ratchet, causing it to run idle and not tighten the closing spring (energy cannot be stored). If the ratchet is severely worn, the ratchet should be replaced to make it work normally. c. If the cam pawl of the energy storage motor is severely loose and the gap is too large, it cannot meet the ratchet and cannot store energy. Check if the pawl position spring is properly fixed. 3.2 No response to closing operation a. No voltage in the closing coil (1) Check if the control circuit is open at some point, such as whether the PS electrical position interlocking circuit is in good contact; whether the interlocking device is heavily dusty, causing poor contact of the contact point, poor contact of the control circuit, or other reasons. (2) Check if the control operation power supply fuse of the circuit breaker unit is normal. If it is burnt out, a qualified fuse must be replaced. b. Closing coil not engaging properly or damaged. Check if the DC resistance of the closing coil is within the acceptable range, and if the armature of the closing solenoid coil is pushed into the winding normally without jamming. If the DC resistance is unacceptable, or if there is severe dust accumulation in the winding causing the armature to jam or burn out, a qualified closing solenoid coil needs to be replaced. 3.3 Automatic disconnection after closing operation a. Improper adjustment of the trip hook mechanism. If the trip hook mechanism is not properly adjusted and the gap is too small, the trip hook will always be in the reset state, causing the circuit breaker to trip automatically after operation, and the closing will fail. The reset position of the trip hook is set by the adjusting screw. Turn the adjusting screw clockwise three and a half turns to release the closed circuit breaker. After this adjustment is completed, the screw must be unscrewed, i.e., loosened three and a half turns in the opposite direction to fix it, ensuring that the trip hook position is normal. b. Interlocking device lever not reset. Check if the interlocking device lever is deformed, jammed, or not returning, causing the trip shaft to shift, keeping the circuit breaker in the tripped state and preventing closing. c. The circuit breaker is not in the correct operating position because the reliable interlocking device on the left side of the circuit breaker engages with a ramp cam in the bay. This cam raises the interlocking device lever assembly, causing the trip shaft to shift, and the circuit breaker remains in the tripped-free state. It cannot be closed in this bay position. Continue to rotate to the correct position to engage the trip hook. During operation, carefully check the circuit breaker to ensure it is in the correct position before operation. d. The trip button may be sintered or the auxiliary contacts may be faulty. When the circuit breaker closes, its auxiliary switch closes, energizing the shunt coil. The coil actuates, thus tripping the circuit breaker. During inspection, pay special attention to the closing and then reopening process. Use measuring instruments to check the control circuit status to quickly and accurately diagnose the fault. e. The crank mechanism cover of the circuit breaker is not properly reset. Because the rotating screw cover outside the mechanism engages with the small connecting plate, it is linked to the opening/closing indicators and also engages with the trip button. During operation, if the screw cover is opened but not reset, the trip button will be in the pressed (on) position, keeping the circuit breaker in a free-tripping state and preventing it from closing (this fault will not cause movement of the contact arm). Check if the rotating mechanism screw cover is deformed or has excessive dust accumulation, ensuring the screw cover moves freely and remains in the correct position. f. The gap between the shunt trip and the trip pedal is inappropriate. If the distance between the shunt trip arm and the trip pedal is too close, it will cause false tripping. To ensure the circuit breaker is in the closed state, ensure the gap between the trip arm and the trip pedal is within the range of 3-5mm. g. The undervoltage trip unit has voltage but cannot reliably engage. Check if the resistance value of the undervoltage trip unit is acceptable. If the voltage is normal, the gap between the armature extension and the pedal on the coupling shaft may be too large. Adjust the gap between the armature and the magnet to within the range of 5±1mm for normal operation. If the above checks are normal but the undervoltage trip coil still cannot operate, the undervoltage trip coil is open and needs to be replaced with a qualified undervoltage trip coil. h. The opening gap of the shock absorber assembly is too small. When the circuit breaker closes, the energy of the closing spring is transmitted to the main shaft through the mechanism, and then the main shaft drives the contacts to close. The end plate assembly on each end of the main shaft is driven to press against the shock absorber assembly. Because in the closed position, there should be a minimum gap of 1mm between the end plate assembly and the shock absorber bolt head; if there is no gap, the energy transmitted by the mechanism increases, the impact force is not absorbed well, and the trip hook roller and cam roller cannot mesh, and the circuit breaker fails to close. Adjust this gap to control it within the qualified range. 4. Precautions for replacing contacts Circuit breakers that frequently withstand the interruption of large currents may eventually need to have their contacts replaced. The general principle for determining the requirements for contact replacement is to discard half or more of the contact tip material. Roughness or slight pitting on the contact surface does not indicate a loss of current-carrying or interrupting ability. After careful handling, it can continue to be used. When replacing contacts, it is essential to ensure that an appropriate force is generated between the moving and stationary contacts when the circuit breaker closes. The quality of this force adjustment directly affects the normal arc transfer during short-circuit current interruption. "Arc transfer" is a forced arc movement in sequence, from the intermediate contact to the arc-starting contact, then to the arc path, and finally to the arc-extinguishing chamber, where the arc dissipates and is extinguished. It is recommended to regularly check the contact pressure during routine maintenance and inspection. 5. Summary After a circuit breaker malfunctions, its operation should be minimized to prevent deformation or breakage of the transmission mechanism, which could expand the scope of the fault. First, a comprehensive and visual inspection of the transmission mechanism parts is necessary to ensure smooth operation. If dirt, dried grease, or other debris remains on the parts, causing sluggish movement, these should be thoroughly cleaned, and a moderate level of lubrication should be applied to ensure normal operation of the mechanism parts. Adjustments should then be made according to the correct methods described above. Personnel who overhaul such circuit breakers should have a basic understanding of the structure and working principle of low-voltage circuit breakers, and possess certain electrical and mechanical maintenance and fault analysis capabilities to accurately diagnose and identify the causes of faults and eliminate them in a timely manner, thereby ensuring the correctness and reliability of the equipment operation.