Based on the determined crane load, the overload limiter's alarm points should be set reasonably. The alarm points need to take into account the crane's design specifications, actual usage requirements, and safety standards. By setting warning alarm points and immediate alarm points, timely alerts will be triggered when the crane approaches or reaches an overload condition, reminding the operator to take appropriate actions.

In lifting operations, the crane sets a safety threshold, i.e., the critical load, based on theoretical calculations and actual measurement results. The overload limiter closely monitors the weight of the lifted object to maintain the safety of the entire operation process. The overload limiter setting is based on the crane's maximum lifting capacity. Once the actual lifted weight reaches or exceeds this limit, immediate action will be taken. The alarm levels for the overload limiter of the bridge crane are shown in Table 1.

Table 1: Alarm Levels of Overload Limiter for Overhead Cranes and Gantry Cranes

Alarm Level	Lifting Weight to Rated Load Ratio	Alarm Action/Indicator
Light Load Control	≤ 20% of Rated Load	Typically no alarm, but can be recorded or displayed
Warning Alarm	≥ 90% of Rated Load	Low-frequency audible and visual alarm
Delayed Alarm	≥ 105% of Rated Load (sustained for 2 seconds)	High-frequency audible and visual alarm, followed by disconnection of the lifting circuit after delay
Immediate Protection	≥ 110% of Rated Load	Immediate disconnection of the lifting circuit, high-frequency audible and visual alarm

When the lifted weight reaches 90% of the crane's rated load capacity, the instrument will immediately trigger the warning system, and the yellow indicator light will start flashing. However, at this stage, the control relay action is not triggered. If the weight continues to increase to 105%, both the yellow and red indicator lights will light up simultaneously, flashing a strong warning signal. Subsequently, the control relay will activate after a brief delay of 1 to 2 seconds, and the buzzer will sound, further reminding the operator to pay attention. When the lifted weight increases to the dangerous level of 130%, the control relay will respond instantly, triggering both the audible and visual alarm system, and immediately implementing safety measures to prevent potential accidents.

During the actual inspection process, different levels of overload conditions should be simulated, and the response of the overload limiter should be observed and recorded.

Table 2: Design of Overload Alarm Response Test Contents

Test Item	Test Condition	Verification Method
Warning Alarm Response	Crane load reaches the preset warning alarm point	Observe and record the flashing of indicator lights and the sound of the buzzer
Delayed Alarm Response	Crane load exceeds the warning alarm point and is sustained long enough to reach the delayed alarm point	Record changes in alarm sound, observe indicator light status, and check the disconnection of the lifting circuit
Immediate Protection Alarm Response	Crane load exceeds the preset immediate protection alarm point	Immediately observe the disconnection of the lifting circuit and the status of indicator lights and buzzer
Alarm Recovery Function Verification	After the overload condition is cleared, the crane returns to a normal safe state	Manually or automatically clear the overload condition, observe the reset status of the overload limiter
Multi-Point Alarm Test	Gradually increase the crane load to sequentially trigger the warning, delayed, and immediate protection alarms	Record the trigger sequence and response at each alarm point
Repeated Alarm Test	Trigger alarms multiple times within a short period to verify the stability of the overload limiter	Perform multiple repetitions of the test, observe and record the alarm response each time
Interference Test	Introduce external interference (e.g., vibration, electromagnetic interference) during crane operation	Conduct the test under simulated interference conditions, observe and record the alarm response

Through the design of these test contents, the system systematically verifies the accuracy and reliability of the overload limiter's response under overload conditions, thereby ensuring the safe operation of the crane.