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T₁ = Proof test interval (h)
MTTR = Mean time to restoration (hour)
MRT = Mean repair time (hour)
DC = Diagnostic coverage
β = The fraction of undetected failures that have a common cause
β_D = Of those failures that are detected by the diagnostic tests, the fraction that have a common cause
β_DU = Dangerous Failure rate (per hour) of a channel in a subsystem
PFD_G = Average probability of failure on demand for the group of voted Channels
PFD_S = Average probability of failure on demand for the sensor subsystem
PFD_L = Average probability of failure on demand for the logic subsystem
PFD_FE = Average probability of failure on demand for the final element subsystem
PFD_SYS = Average probability of failure on demand of a safety function for the E/E/PE safety-related system
PFH_G = Probability of failure per hour for the group of voted channels
PFH_S = Probability of failure per hour for the sensor subsystem
PFH_L= Probability of failure per hour for the logic subsystem
PFH_FE = Probability of failure per hour for the final element subsystem
PFH_SYS = Probability of failure per hour of a safety function for the E/E/PE safety-related system
λ = Total Failure rate (per hour) of a channel in a subsystem
λ_D = Dangerous failure rate (per hour) of a channel in a subsystem, equal to 0,5 λ (assumes 50 % dangerous failures and 50 % safe failures)
λ_DD = Detected dangerous failure rate (per hour) of a channel in a subsystem (this is the sum of all the detected dangerous failure rates within the channel of the subsystem)
λ_DU = Undetected dangerous failure rate (per hour) of a channel in a subsystem (this is the sum of all the undetected dangerous failure rates within the channel of the subsystem)
λ_SD = Detected safe failure rate (per hour) of a channel in a subsystem (this is the sum of all the detected safe failure rates within the channel of the subsystem)
t_CE = Channel equivalent mean down time (hour) for 1oo1, 1oo2, 2oo2 and 2oo3 architectures (this is the combined down time for all the components in the channel of the subsystem)
t_GE = Voted group equivalent mean down time (hour) for 1oo2 and 2oo3 architectures (this is the combined down time for all the channels in the voted group)
t_CE’ = Channel equivalent mean down time (hour) for 1oo2D architecture (this is the combined down time for all the components in the channel of the subsystem)
t_GE’ = Voted group equivalent mean down time (hour) for 1oo2D architecture (this is the combined down time for all the channels in the voted group)
T₂ = Interval between demands (h)
K = Fraction of the success of the auto test circuit in the 1oo2D system
PTC = Proof Test Coverage
1 = High demand or continuous mode only
2 = Low demand mode only