10
7 Consideration of the residual error probability to
IEC 61508
To calculate the PFH (probability of a dangerous failure per hour) of a safety-rela-
ted function the PFH values of all components used in this function must be taken
into account�
The probability of failure on demand (PFD) is 2�0 x 10
-5
�
The probability of a dangerous failure per hour (PFH) is 3�3 x 10
-9
/h (per hour)�
The indicated PFD and PFH values refer to the maximum switch-on time of 12
months�
The maximum service life (T) is 20 years� The unit can be used in applications up
to SIL 3 (PLe / EN ISO 13849-1 or cat� 4 / EN954-1)�
Explanation of the abbreviations:
PFD = probability of failure on demand
PFH = probability of a dangerous failure per hour
SIL = safety integrity level
T = life time (= service life)
The PFD / PFH values of the other components, especially of the AS-i safety
monitor, can be found in the corresponding documentation�
