Siemens SIMOCODE pro Manual page 106

List of abbreviations
A.1 List of abbreviations
Abbreviation Meaning
PFDavg Average probability of dangerous failure of a safety function on demand
PFHD Probability of dangerous failure per hour
PL Performance Level
PCS Process Control System
ProdIS Product Information System
PZ Pozidriv
RTU Remote Terminal Unit
SELV Safety Extra Low Voltage
SF Group error
SIL Safety Integrity Level
PLC Programmable controller
TIA Totally Integrated Automation
TN Terre-Neutre (ground neutral)
TT Terre-Terre (ground-ground)
T Total one fault delay time (maximum response time when error is present)
OFDT
T Total worst case delay time (maximum response time in error-free state)
WCDT
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SIMOCODE pro - Fail-Safe Digital Modules
Manual, 11/2017, NEB631679702000/RS-AA/002