Imagine this scenario. You are asked to audit a machine and determine whether it is safe enough for use. Or, perhaps your firm is purchasing a new machine and wants to confirm its safety. The machine has two interlock switches on a guard door. Is it safe enough? To simplify the discussion, assume you determine that the switches are wired in parallel (Figure 1, p. 42). You are asked whether they should be wired in series to be safer.

What is your answer? The answer is important because it will affect safety and costs. If the wiring or components need to be changed, the company will incur significant costs and schedule de-lays. If the wiring is deemed acceptable, yet it is not, an employee could suffer a serious injury. The answer, of course, is "it depends." The application, reliability and quality of the components used, how the components are combined, and the system's ability to detect problems all play a role in determining the safety or adequacy of the control system.

This article highlights several factors of control system safety and discusses an important international machinery safety standard that applies to control systems, ISO 13849–1. Some significant areas addressed by the standard are reviewed, several controversies surrounding it are discussed, and guidance is provided to machinery suppliers and users on how to work effectively in the current situation.

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