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7
Determining PFH, PFD, and MTTF
GFK-2734
PFH
Probability of Failure per Hour
PFD
Probability of Failure on Demand
MTTF
Mean time to dangerous failure
d
7.1
Single-channel operation
7.1.1
Determining PFD for single-channel operation
The value always refers to one internal safety relay (see assignment of outputs to the
safety relay in Section "Terminal point assignment" on page 3-4).
This means that if several internal relays are used in a safety function, they should be
considered with 1% each of SIL 2.
The logic has been taken into account within 1%.
PFD = 1% of SIL 2 per internal safety relay
7.1.2
Determining PFH for single-channel operation
The value for PFH depends on the load for the contacts and the switching frequency.
In this section, the values are given for single-channel assignment, and these values only
refer to one internal safety relay. This means that if several safety relays are used for a
safety function, the values must be added together.
If the contacts of a safety relay have different loads, use the least favorable load for the
calculation (see assignment of outputs to the safety relay in Section "Terminal point
assignment" on page 3-4).
The value for PFH is determined using the following formula:
PFH = 0.01% + b x c
Where:
c
Switching frequency of the relay per hour [1/h]
b
Factor, see Table 7-1 [% x h]
Table 7-1
Factor for calculating PFH depending on the load for the contacts
Load for the contacts
(according to DIN EN 60947-4-1/
DIN EN 60947-5-1)
AC 15; 3 A
DC 13; 5 A
Chapter 7 Determining PFH, PFD, and MTTF
d
[% of SIL 2]
Factor b [% x h]
5.102
12.821
d
7
7-1
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