Crane Maintenance Guidelines: Protective Measures for Crane Electrical Equipment

To protect the crane's electrical equipment and ensure the safety of personnel, the crane's electrical control system must be equipped with essential electrical protection measures.

1. Main isolating switch. When performing maintenance or inspections on electrical equipment, it is generally recommended to do so with the power disconnected. Therefore, there should be an isolating switch or other isolation measures between the electrical equipment and the power supply network. When the isolating switch is in the open position, it must provide adequate clearance distance and a clearly visible break point, allowing maintenance personnel to visually confirm that the power circuit has been safely disconnected. **Please note:** Air switches and molded-case circuit breakers, even when in the open position, lack a clearly visible break point and thus cannot be used as proper isolating switches.

2. Short-circuit protection for the main power supply: When insulation failure occurs in the crane's electrical equipment, leading to a case-to-case or phase-to-phase short circuit, the main power supply’s short-circuit protection device should immediately activate to cut off the faulty power source. This main power supply short-circuit protection can be implemented using either fuses or circuit breakers.

3. Undervoltage protection for the main power supply. This refers to the automatic disconnection of the main power circuit when the power supply is interrupted. When power is restored, the main circuit will not automatically re-energize without manual intervention. Typically, an automatically resettable button is connected in series within the coil circuit of the main power contactor or the undervoltage release coil circuit of the automatic circuit breaker to achieve this undervoltage protection feature.

4. Zero-Position Protection: The crane’s electric motor is controlled by a controller that does not allow automatic reset. This feature prevents the motor from accidentally starting if the power supply is momentarily interrupted and then restored while the controller handle is not in the zero position. To check the zero-position protection:

4.1 The motor of the mechanism is directly controlled by a cam controller; first, turn off the main power supply and move one of the controller handles away from the zero position. Then, attempt to re-energize the system—however, the main power should remain off. Only after all controller handles are returned to the zero position can the main power be safely restored.

4.2 The machine motor is controlled by a master controller and contactors. First, turn off the main power supply, then move the master controller away from the zero position, and finally, re-energize the main power supply. When the main power is turned back on, the machine motor cannot start simultaneously.

5. Overload protection for electric motors. Electric motors on cranes with a rated power greater than 1 kW must be equipped with individual overload protection. Currently, overcurrent protection is the most commonly used method for motor overload protection in cranes.

6. Emergency power-off switch. The crane must be equipped with an emergency power-off switch that can quickly disconnect the main power supply in case of an emergency, and it should be positioned in a location easily accessible to the operator. Importantly, the emergency power-off function must not be automatically resettable. There are two criteria for determining whether the emergency power-off switch meets the required standards:

6.1 Check whether the crane is equipped with a main power contactor. The crane must have a main power contactor, and all power supply cables for the mechanisms must be connected entirely to the output terminals of the power contactor.

6.2 Act the emergency power-off switch, causing the main power contactor or the circuit breaker with undervoltage release coil to trip, thereby cutting off the main power supply to all mechanisms and halting their operation. Additionally, meeting these conditions confirms that the emergency power-off switch is compliant.

7. Safety interlock protection for passageway entrances: The crane must be equipped with an interlock switch at the passageway entrance, which will cut off the main power supply when the entrance is opened. Additionally, the trolley of the bridge crane uses bare conductor rails for power supply, ensuring that personnel climbing onto the walkway cannot accidentally come into contact with electricity. This design also helps prevent injuries from crushing or falling accidents caused by sudden mechanism activation.

8. Limit Protection: To prevent accidents caused by operational negligence—such as the lifting device hitting the overhead support structure, snapping the wire rope, and resulting in a free fall—the hoist should be equipped with an upper limit switch. Common types include weight-based and screw (or worm gear) mechanisms. Additionally, electrical protection features like travel protection and grounding protection are also essential.