Fault avoidance

Dieses Thema enthält die folgenden Abschnitte:

Validation

Only you, the user, machine builder or system integrator can be aware of all the conditions and factors realized in the design of your application for the machine. Therefore, only you can determine the automation equipment and the related safeties and interlocks which can be properly used, and validate such usage.

WARNUNG

Unintended machine operation

Validate the overall safety-related function and thoroughly test the application.

Different (asymmetric) switching signals at the inputs

Different (asymmetric) states at inputs S_GuardSwitch1 and S_GuardSwitch2 after the time set at DiscrepancyTime has elapsed only produce an error message during the closing operation.

When the safety equipment is opened, the S_GuardOut enable output is switched to SAFEFALSE as soon as there is a SAFEFALSE signal at input S_GuardSwitch1 or S_GuardSwitch2. The defined safe state (S_GuardOut = SAFEFALSE) is also retained if the function block detects different states at S_GuardSwitch1 and S_GuardSwitch2 during the next closing operation after the time set at DiscrepancyTime has elapsed (this may be due, for example, to inoperable safety equipment, an inoperable switch, or damaged or disconnected cables).

Plausibility and connection errors

Plausibility errors are errors which occur, for example, when a range of values is exceeded or an impermissible connection is made. Such errors are detected and reported either by the function block itself or while the project is being compiled. However, this is not always possible in the case of connection errors.

This means that it is not possible, for example, to automatically verify whether:

Values or constants within the range of validity at inputs are, in fact, incorrect for the safety-related function executed.
This does not apply to a static TRUE signal at the Reset input. This is detected by the function block and reported as an error.
Inputs and/or outputs are incorrectly connected or are not connected when they should be.
Actual parameters/signals are incorrectly connected or are not connected when they should be.

WARNUNG

Unintended machine operation

Validate the signals, formulas (if applicable), variables or constants connected to the input and output formal parameters of the safety-related function block and thoroughly test the application.

Sporadically switching or toggling signal levels or impermissible signals

If no additional fault avoidance measures are taken, signal levels which switch or toggle sporadically have the following effects:

At the edge-triggered inputs, such signals cause the function block to interpret the signal as an edge and trigger a potentially undesirable corresponding action.
At the state-controlled inputs, such signals cause the signal to trigger a potentially undesirable corresponding action.

Impermissible signals at inputs can lead to an unintended start-up, prevent a requested action from being executed or cause an error.

These signals may be caused by:

Programming errors in the application program (user errors).
Cross circuit, short circuit, and cable break (user errors, wiring errors).
Errors in the standard (non-safety-related) application.

To prevent this, the following measures can be taken, depending on the safety-related function:

Using safety-related device signals.
Additional measures to prevent a hazard if a signal from the standard controller is used (e.g. execution of an additional function start after resetting a triggered safety function or after a fault has been rectified).
Using options for cross-circuit detection.
Suitable wiring when using standard (non-safety-related) signals from the standard application.
Verifying the safety-related code in the code editor followed by validation of all safety-related networks.

The measures listed above can also be taken in combination in order to help prevent errors.

Impermissible static signals when starting up the Sicherheitssteuerung

If a start-up inhibit following activation of the function block is specified by the setting S_StartReset = SAFEFALSE, a static TRUE signal at the Reset input when the Sicherheitssteuerung starts up leads to a function block error message (Error = TRUE).

If the start-up inhibit is not in use when the Sicherheitssteuerung starts up (S_StartReset = SAFETRUE), the signal state at the Reset input is not relevant at this time. In such cases, the signal at the S_GuardOut output depends exclusively

on the status of the connected safety-related equipment
and on the setting at S_AutoReset (restart inhibit).

Simultaneous edge change

In order to reduce the risk of an unintended start-up, it is essential to ensure that the Reset input is only connected to the signal of a manual reset device. The risk analysis determines how this signal is to be set up in practice.

Machine/system start-up without a function test for safety-related equipment

Inoperable or error producing safety-related equipment can only be detected by testing whether it is functioning correctly. The function block does not support function testing.

Possible causes of inoperable or error producing safety-related equipment are:

Inoperable devices (hardware errors)
Cross circuit, short circuit, and cable break (user errors, wiring errors)

WARNUNG

Unintended machine operation

Validate the safety-related equipment by performing function tests.

Prior to performing function tests, make certain that appropriate procedures and measures (according to applicable sector standards) have been established to help avoid hazardous situations if the safety logic operates in ways you did not intend.

Do not enter the zone of operation while the machine is operating.

Ensure that no other persons can access the zone of operation while the machine is operating.

Observe the regulations given by relevant sector standards while the machine is running in any other operating mode than "operational".

Use appropriate safety interlocks where personnel and/or equipment hazards exist.