7
GFK-2734
Chapter 7 Determining PFH, PFD, and MTTF
d
7-3
7.2
Two-channel operation
7.2.1
Determining PFD for two-channel operation
The value always refers to two internal safety relays (two-channel assignment; see
assignment of outputs to the safety relay in Section "Terminal point assignment" on
page 3-4).
This means that if several safety relays are used in a safety function, they should be
considered with 1% each of SIL 3.
The logic has been taken into account within 1%.
PFD = 1% of SIL 3
per two internal safety relays in two-channel operation
7.2.2
Determining PFH for two-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 two-channel assignment, and these values refer to
two internal safety relays. 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:
Where:
c
Switching frequency of the relay per hour [1/h]
b
Channel 2 zero current: See alternative B in Figure 6-3 and Figure 6-4.
PFH = 0.04% + b x c
[% of SIL 3], per two internal safety relays in two-channel
operation
Table 7-3
Factor for calculating PFH depending on the load for the contacts
Version in
Load for the contacts (according to
DIN EN 60947-4-1/DIN EN 60947-5-1)
Factor B [% x h]
Channel 1
Channel 2
A
AC 15; 3 A
AC 15; 3 A
0.510
A
DC 13; 5 A
DC 13; 5 A
1.282
A
AC 15; 3 A
DC 13; 5 A
0.896
A
DC 13; 5 A
AC 15; 3 A
0.896
B
AC 15; 3 A
Zero current
0.505
B
DC 13; 5 A
Zero current
0.891
B
250 µF; DC 4 A
Zero current
3.583