A Brief History of Safety

In the early days of control technology, the focus in the control system was on the function and therefore the process image. Relays and contactors activated plant and machinery. Where there were shutdown devices or devices to protect personnel, the actuator was simply separated from the supply when necessary. However, people gradually realized that this type of protection system could be rendered inoperational in the event of an error: the protective function would no longer be guaranteed. As a result, people began to consider the options
for safeguarding this type of separation function. Special relay circuits, such as the 3 contactor combination, were one of the initial outcomes of these considerations. These device combinations ultimately led to the development of the first safety relay, the PNOZ.

Safety relays, therefore, are devices which generally implement safety functions. In the event of a hazard, the task of such a safety function is to use appropriate measures to reduce the existing risk to an acceptable level. These may be safety functions such as emergency off/emergency stop, safety gate function or even standstill monitoring on a drive. Safety relays monitor a specific function; by connecting them to other safety relays they guarantee total monitoring of a plant or machine. The first safety-related control system ultimately came from the desire to connect functions flexibly through programming, similar to the way this is done on a programmable logic controller (PLC).