PNOZ (safety relay)

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PNOZ is a registered trademark of the company Pilz GmbH & Co KG. The product is a safety relay. In 1987 Pilz developed the first emergency stop relay to protect man and machine.[1]

Product history

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 realised 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.[2]

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. Excerpt from the patent EP0482437: For the purpose of initialising the channels (2, 3) after a disconnection process, a two-channel emergency shutdown device (1) is provided with a first initialisation circuit (4) which is activated with the aid of a manual key (21), which can be operated arbitrarily. In order that the emergency shutdown device (1) is brought to the switched-on state automatically, even after the supply voltage has been restored, there is a second initialisation circuit (5), which operates as a function of the restoration of the voltage and otherwise has no effect.[3]

Product description

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:[4]

  • Stop a movement in a controlled and therefore safe manner
  • Monitor the position of movable guards
  • Interrupt a closing movement during access

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).[5]

Structure and function

Today’s safety relays are distinguished primarily by their technological design:

  • Classic contact-based relay technology
  • With electronic evaluation and contact-based volt-free outputs
  • Fully electronic devices with semiconductor outputs

Safety relays must always be designed in such a way that – when wired correctly – neither a fault on the device nor an external fault caused by a sensor or actuator may lead to the loss of the safety function.[6]

The typical design of a first generation safety relay in relay technology is based on the classic 3 contactor combination. The redundant design ensures that wiring errors do not lead to the loss of the safety function. Two relays (K1, K2) with positiveguided contacts provide the safe switch contacts. The two input circuits CH1 and CH2 each activate one of the two internal relays. The circuit is started via the start relay K3. There is another monitoring circuit between the connection points Y1 and Y2 (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, e.g. contact welding on an emergency off / emergency stop pushbutton or on one of the safety contacts on the output relay. The safety device stops the device switching back on and thereby stops the activation of relays K1 and K2.[7]

References

  1. ↑ German patent 4033801
  2. ↑ Safety Compendium, Chapter 4: Safe control technology, page 115
  3. ↑ International patent EP0482437
  4. ↑ Safety Compendium, Chapter 4.1.1: Overview of safety relays, page 116
  5. ↑ Safety Compendium, Chapter 4: Safe control technology, page 115
  6. ↑ Safety Compendium, Chapter 4.1.2: Structure and function of safety relays, page 116 following pages
  7. ↑ Safety Compendium, Chapter 4.1: Safety relays, page 118 following pages

External links

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