Technical data

Working Out the Automation Concept
3.7 Establishing the Safety Requirements
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 67
3.7 Establishing the Safety Requirements
Decide which additional elements are needed to ensure the safety of the process - based on legal
requirements and corporate health and safety policy. In your description, you should also include
any influences that the safety elements have on your process areas.
Defining Safety Requirements
Find out which devices require hardwired circuits to meet safety requirements. By definition, these
safety circuits operate independently of the programmable controller (although the safety circuit
generally provides an I/O interface to allow coordination with the user program). Normally, you
configure a matrix to connect every actuator with its own emergency off range. This matrix is the
basis for the circuit diagrams of the safety circuits.
To design safety mechanisms, proceed as follows:
• Determine the logical and mechanical/electrical interlocks between the individual automation
tasks.
• Design circuits to allow the devices belonging to the process to be operated manually in an
emergency.
• Establish any further safety requirements for safe operation of the process.
Creating a Safety Circuit
The sample industrial blending process uses the following logic for its safety circuit:
• One emergency off switch shuts down the following devices independent of the programmable
controller (PLC):
- Feed pump for ingredient A
- Feed pump for ingredient B
- Agitator motor
- Valves
• The emergency off switch is located on the operator station.
• An input to the controller indicates the state of the emergency off switch.