This is the final installment in the series on understanding safety functions. When indicators and alarms come up in conversations between machinery controls engineers, large process plant control rooms like those shown at left often come to mind. While this is certainly true, there are many instances on smaller machines and assembly lines where alarms…
Read MoreUnderstanding safety functions: Fluctuations, loss and restoration of power sources
Control systems must be protected against fluctuations and power loss, whether electrical or fluidic. Fluctuations and losses are well defined in IEC, IEEE and ISO standards. Careful design of electrical systems, including the use of UPSs for electrical control systems and the careful design of pneumatic supply systems, including the proper sizing of pneumatic accumulators for compressed air-powered logic, is essential.
Understanding safety functions: Pressure or temperature
Pressure and temperature are important safety-related parameters. The two parameters are closely linked in systems where gases or vapours are present, with behaviour governed by the combined gas laws. The temperature is also critical in mechanical systems that can overheat, like motors or bearings. Monitoring temperature or pressure requires the correct selection of instruments.
Read MoreUnderstanding safety functions: Safe speed and safe standstill
In this post, I’ll discuss two safety-related parameters: safe speed and safe standstill. Speed control is a very common machine function. Conveyors, mixers, pumps, and many other applications rely on variable-speed drives. Some speed parameters are also safety-related because variations in speed can increase the risk to workers. See this post for more information on…
Read MoreUnderstanding safety functions: Response time
This post deepens the discussion of safety-related parameters. Safety functions are control functions whose failure causes an immediate increase in risk. ISO 13849-1 [1] defines several common safety functions to develop safety-related control systems. In this post, I will discuss the response time safety-related parameter.
Read MoreUnderstanding safety functions: Safety-related parameters
Safety-related parameters are control system variables whose incorrect setting immediately increases the risk to the user. Some of the most common include safe-reduced speed and safe standstill, but temperature, pressure and other process parameters can also be safety-related. Defining what machine parameters are safety-related and then designing control hardware and software to meet the functional…
Read MoreUnderstanding safety functions: Local control
The local control function is important to understand when designing machinery control systems. If the failure of the local control function could lead to increased risk to the user, it is a safety function. This article explains the local control function from a functional safety perspective.
Read MoreUnderstanding Safety Functions: the Safety-related stop function
The most used safety function on machinery is the safety-related stop function. The requirements discussed in this post are not generally applicable to process-related stop functions unless the process and safety stop functions share the same control system hardware and software.
Read MoreManual reset using an HMI
Update: Things have changed a bit since this post was first published in 2021. There are now some safety-rated HMIs available from big controls companies like Siemens, so if you are considering this approach, consult with your controls vendor to see if their HMIs can meet the requirements of ISO 13849-1. Ed. – 2022-05-10 Question: Can…
Read MoreUnderstanding safety functions: Manual Reset
Following the risk assessment, risk reduction is the next step. A safety function is needed to reduce the risk when the control system is called upon. Safety functions are defined in safety requirement specifications. ISO 13849-1:2015 [1] describes some of the more common safety functions. One of the most poorly understood is the manual reset…
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