Siemens SIMATIC S7 Functional Safety Manual page 113

In the presence of a single fault, the bit pattern tests can apply energy to the load for a
duration up to the configured "Maximum readback time switch on test".
If the load can respond dangerously within the configured readback time, it can respond to
bit pattern tests in the presence of a single fault, resulting in death or serious injury to
personnel and/or property damage.
Always choose a maximum readback configuration time that is guaranteed not to activate
the load.
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA
5.4 Configuring SM 1226 F-DQ 4 x 24 VDC DQ and channel parameters
WARNING
Fail-Safe signal module (SM) I/O configuration
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