Control elements and indicators must be designed so that they ensure safe and fast operation of the control system. The functions, arrangement and colour coding of the control elements and indicator lights are standardised. This allows the use of uniform operating procedures for different controllers and operating errors are prevented as far as possible.
13.4.1 Main switch
Every machine and system must have a main switch. This switch is used to switch off the electric power supply during cleaning, maintenance or repair work and for lengthy shutdown periods. The main switch must be manually operated and must have only two positions: "0" (OFF) and "1" (ON). The OFF position must be lockable in order to prevent manual starting or remote starting. If there is more than one incoming supply, the main switches must be interlocked so that no danger can arise for maintenance staff.
13.4.2 EMERGENCY STOP
The EMERGENCY STOP control switch is actuated by the operator in dangerous situations.
The EMERGENCY STOP operating device must have a mushroom button and is operated directly by hand.
Indirect operation by pull-wire or foot pedal is permissible. If there is more than one workstation or operating panel, each one must have its own EMERGENCY STOP operating device. The colour of the EMERGENCY STOP actuation element is a conspicuous red. The area beneath the control switch must be marked in contrasting yellow.
Once the EMERGENCY STOP device has been actuated, the drives must be shut down as quickly as possible and the controller should be isolated from the electrical and pneumatic power supplies where possible. The following limitations have to be observed, however:
• If illumination is necessary, this must not be switched off.
• Auxiliary units and brake devices provided to aid rapid shutdown must not be rendered ineffective.
• Clamped workpieces must not be released.
• Retraction movements must be initiated by actuation of the EMERGENCY STOP device where necessary.
Such movements should, however, only be initiated if this can be done without danger.
13.4.3 Control elements of an electropneumatic control system
An electropneumatic control system has other control elements in addition to the main switch and EMERGENCY STOP switch. An example of a control panel is shown below.
Main switch
Figure 13.7: Control panel for an electropneumatic controller (example)
A distinction is made between two different operating modes for electropneumatic controllers:
• Manual operation
• Automatic, i.e. program-controlled operation
Manual operation
The following control elements have an effect during manual operation:
• "Reset": the system is moved to the initial position.
• "Inching": each time that this pushbutton is pressed, the sequence is extended by one step.
• Individual movements: a drive is actuated when the corresponding pushbutton or control switch is pressed (example: "Open gripper" or "Close gripper").
Automatic operation
The following operating modes are only possible only with automatic operation:
• Single cycle: he sequence is executed once.
• Continuous cycle: the sequence is executed continuously.
Pressing the "Continuous cycle OFF" pushbutton (or a "Stop" button) interrupts the sequence. The interruption occurs either after the next step or after completion of the entire sequence.
The main switch and EMERGENCY STOP switch are effective in all operating modes. They must be available on every electropneumatic controller together with control elements for "Manual" and "Automatic",
"Start", "Stop" and "Reset". Which control elements are necessary in addition to these depends on each individual application.
Colour coding of control elements
The following table provides an overview of the colours of control elements and their meaning in accordance with EN 60204.
Colour Command Required operating status
Red Stop, Off Shut down one or more motors. Shut down units of a machine.
Switch off magnetic clamping devices.
Stop the cycle (if the operator presses the pushbutton during a cycle, the machine stops once the current cycle has been completed).
EMERGENCY STOP Stop in the event of danger (e.g. shutdown because of dangerous overheating)
Green or black
Start, On, Inching Energise control circuits (ready for operation). Start one or more motors for auxiliary functions. Start units of the machine. Switch on magnetic clamping devices. Inching operation (inching when switched on).
Yellow Start a return movement outside the normal work sequence, or start a movement to counteract dangerous conditions.
Return machine units to the starting point of the cycle, if the cycle is not yet completed. Actuation of the yellow pushbutton may deactivate other, previously selected functions.
White or black
Any function for which none of the above colours is used.
Control auxiliary functions which are not directly linked to the operating cycle.
13.1: Colour coding of control elements of machine controllers
Colour coding of indicator lights
To enable operating staff to immediately identify the operating status of a system, especially malfunctions and dangerous situations, indicator lights are colour-coded in accordance with EN 60204. The following table shows the meaning of the various colours.
Colour Operating status Sample applications
Red Abnormal status Indication that the machine has been stopped by a protective device (e.g. due to overload, overtravel or some other fault).
Prompt to shut down the machine (e.g. because of overload).
Yellow Attention or caution A value (current, temperature) is approaching its permissible limit. Signal for automatic cycle.
Green Machine ready to start Machine ready to start: auxiliaries operational. The (various) units are in their initial positions and the pneumatic pressure or the voltage of a transformer have reached the prescribed values.
The operation cycle is completed and the machine is ready for restarting.
White (colourless)
Electrical circuits energised.
Normal operating status
Main switch in On position. Selection of speed or direction of rotation. Individual drives and auxiliary drives in operation.
Machine running.
Blue Any function for which none of the above colours is used.
13.2: Colour coding of indicator lights for machine controllers