Safety relays are the logical development of electromechanical relays, during those applications where safety, availability, and their costs are the major concern. These relays have enabling contacts and mirror contacts in one or more linked pairs which change over when the relay is actuated. Safety relays are used for monitoring the dangerous outputs and provide superior failure mode analysis such as a light curtain to assure that it does not decrease harm. Safety relays compare the status of the controlled system comprising the verified circuits against an abstract of what should happen. If the comparison demonstrates a hazardous state, the safety relay will go to a down condition and extend the enable safety contacts. This will de-energize the controlled equipment and can be used to trigger an alarm. http://www.pilz.com
Safety relays monitor the faults in the interlock devices. The basic logic used is based on the conditions of the input and output wiring. Input wiring is the voltage that the safety relay will accept to energize the relay to switch the output contact. The output contact will turn the machine off using the interlock devices. Internal logic is represented in the truth table, which shows the relationship between the input and the output wiring. From the truth table, the next step is developing the functions and faults equations. These equations are very important to find the minimal sum of products, which later develop the ladder logic. The final considerations are to implement the functions equation into the ladder logic. The safety relay also must be user friendly to someone troubleshooting the system. A fault in the system can be determined through testing the machine and forcing the input wiring to simulate a fault condition. LED indicators are useful to show the system is functioning, off, and a fault in the system. The last consideration is the modification of the function block. If a function’s equation is changed, the ladder logic can also be changed using software and just download it to the safety relay. http://www.pilz.de