Wiring, safety PLC programming critical to machine safety performance

July 11, 2013
Some common pitfalls in machine safety have to do with not wiring devices correctly. Many devices today can cover a range of safety levels, such as SIL 1 through SIL 3, but have different wiring methods. Diagnostic coverage is extremely important with safety and incorrect wiring or an incomplete understanding of the wiring requirements could lead to a lower safety level or a system that does not work at all.

Be sure to follow the manufacturer’s recommended wiring practices and understand the diagnostics that are performed on the circuit. Having two devices that perform diagnostics of the same type may cause the system to not work. An example would be cross-wire detection where the PLC card checks the circuit by sending out a pulse on the wire and detecting that pulse back on the input. If the device that it is wired to the PLC card does not pass the pulse through, then the I/O module may fault. This could be corrected by selecting the right devices or a simple hardware setup change. Either way, you need to understand how safety works. 

Once you have designed your system, you must program it. Pay special attention to what the controller allows you to do in the safety code. A safety PLC can add many benefits that can help the system be more flexible and increase production. On the other hand, it has to be programmed and must be tested for correct operation during the machine’s commissioning phase. This is a very important step and is often overlooked. Be sure to inject faults into the safety system to ensure that your configuration and code give you the desired response. 

A safety PLC can add great benefits to the automation project, but it also can add scan time that may affect the response time of the safety system. It is important to understand this and to select the right hardware. The response time is very important in safety and could make a machine that appears to have all its safety bases covered, not respond fast enough to a person working in a highly dangerous area. 

Make sure you document your safety system, from risk assessment to the testing and checkout phase. Integrated safety is very flexible and can give great benefits, but pay attention to the wiring and configuration of the devices. And remember, time is distance and the scan time may affect the response of your system on a high-speed machine with close approach boundaries. 

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