Advancement in machine safety devices
Operation of safety relay
The typical design of a safety relay is based on the classic 3 contactor combination. The design ensures that wiring errors do not lead to the loss of the safety function. Two relays (K1, K2) with positive-guided contacts – set of NO and NC contacts that operate together so NO and NC are never closed simultaneously.
The two input circuits CH1 and CH2 each activate one of the two internal relays. The circuit is started via the start relay K3. Y1 and Y2 connection points could also be used as feedback loop. This connection is used to check and monitor the position of actuators, which can be activated or shut down via the safety contacts. The device is designed in such a way that any faults in the input circuit are detected and The safety device stops the device switching back on and thereby stops the activation of relays K1 and K2.
Wiring classic safety relay on a machine.
Both channel 1 and channel 2 are wired with contacts from safety devices to close relays K1 and K2. Usually, safety relays have terminals labelled S11/S12, S21/S22, etc, then terminals labelled, perhaps 13/14,23,/24. I use a lot of different manufacturers but principle stays same. 
I wire a series loop of one set of E-stops/guard doors contacts beginning at S11 and returning at S12. I wire another series loop of set of of E-stop/guard doors contacts beginning at S21 and returning at S22. When these circuits are intact, we can reset the safety relay and this pulls in K13/14, K23/24 which are then connected to outputs in process for example plant motion, machine start etc.
Latest Electronic Safety Relay Technology
Latest safety relays are microprocessor based. These are compact and take less space. These electronics based relays output also have special inputs/outputs to communicate with other electronic safety relays. These special I/O are used to determine status of adjacent safety relays. Such design hugely save costs and wiring especially in those application where adjacent relays need to know status of each other to activate plant outputs.
Imagine the following situation on a machine: a safety gate is intended to prevent random access to a danger zone. Access is prohibited until the hazardous movement has been stopped and the machine is in a safe condition. However, the intention is for various drives to be operable at reduced speed so that the scheduled maintenance could be proceeded.
If these requirements are to be implemented, a substantial amount of wiring will be needed to connect the individual contact based safety devices!
Programmable Safety Controllers
Machine Safety technology has developed from classic contact-based safety to electronic microprocessor based safety devices and beyond to flexible, configurable small controllers. With this generation of devices, the safety functions are configured exclusively via the software tool. With the help of safe function elements and the logic connection between the elements, we can now create the safety-related application that would previously have implemented by wiring contactors and relays in a laborious, time-consuming process.
Programmable safety controllers also open a new era of machine safety control possibilities. With contact-based safety relays, we have LEDs flash sometimes with varying frequencies to distinguish between specific malfunctions. LCD displays indicate errors and/or operating states in plain text. Programmable small controllers offer a whole new set of options. Using field bus modules, they could be connected to almost any bus. These connections can be used to transfer data to automation systems and using Active X elements could be displayed directly to Human Machine Interface HMI screen via OPC servers.
Indeed advancement in safety devices has not only improved machine diagnostics but also reduced accidents and near misses that we might witness in manufacturing industry. If you require assistance with designing an appropriate level SIL machine, then feel free to comment or contact using the details on this website. Until next time…
