What is Type A and B in IEC 61508

Introduction

In the field of industrial process control and safety systems, IEC 61508 is a well-established international standard that defines the functional safety requirements. This standard classifies devices into two types - Type A and Type B. These classifications help in determining the level of confidence one can have in a safety instrumented system (SIS) and play a crucial role in ensuring the safe operation of critical processes.

Type A Devices

Type A devices, as defined by IEC 61508, are those that possess a hardware fault tolerance capability. This means they are designed to handle random hardware failures and still maintain the required safety performance. The fault tolerance of these devices addresses the potential risks associated with potential failures or malfunctions in the equipment during its operation.

The fault tolerance design of Type A devices typically includes redundancy at both the hardware and software levels. Redundancy ensures that even if one component fails, the overall system continues to operate without compromising safety functionality. This enhances the reliability and availability of the safety instrumented system, minimizing the probability of dangerous failures.

Type B Devices

Type B devices also conform to the requirements stated in IEC 61508 but do not possess any hardware fault tolerance. They rely solely on software-based solutions for achieving the required level of safety integrity. Type B devices are often used in non-critical applications or in low-risk environments where the consequences of failure are less severe.

Compared to Type A devices, Type B devices are simpler and more cost-effective, as they do not require redundant hardware components. However, they still need to meet the necessary safety integrity levels (SIL) as specified by the standard, ensuring that they provide an acceptable level of safety performance.

Conclusion

In summary, the classification of devices into Type A and Type B in IEC 61508 plays a vital role in the field of functional safety. Type A devices offer hardware fault tolerance and are designed to handle potential failures, while Type B devices rely on software-based solutions without redundant hardware. Understanding these classifications helps in selecting the appropriate devices for specific applications, ensuring the safe operation of industrial processes.

It is important for individuals involved in process control and safety systems to familiarize themselves with the requirements outlined in IEC 61508, as it provides a comprehensive framework for managing functional safety and mitigating risks associated with critical equipment failure.