When the SIS system was just put into use, the safety level was higher. However, with the passage of time, the possibility of safety failure, safety-related parameter values may gradually increase. If it is used for a long time, if it is not tested, the safety level will drop, SIL3-2-1. And the key component of the safety relay is the performance of the selected relay, and the relay has its mechanical life.

 Therefore, it is necessary to regularly perform fault diagnosis on the system. In the entire life cycle of safety-related systems, it is also pointed out that there must be a maintenance management plan. When doing PROOF TEST, it is to detect whether there is a safety failure in the safety system. If there is no fault, the safety level is still within the original range. If a fault is found, corresponding treatment including replacement is required. Therefore, the recommendation is 5 years of inspection and testing, or shorter, which is usually given in the "Safety Manual" of the product. Specifically, the maintenance inspection plan and plan can be determined in conjunction with the characteristics of the safety system, the overall maintenance management plan, and related circuits and components.

Note: The above are recommendations for the application of the process control industry.

For applications in the machinery industry, mechanical equipment is often in a state of human-machine collaboration, and it is more likely to have an impact on personal safety. Therefore, the test requirements are relatively frequent, and the "safety manual" proposes at least monthly test requirements.