7
GFK-2734
Chapter 7 Determining PFH, PFD, and MTTF
d
7-5
7.2.3
Determining MTTF
d
for two-channel operation
The value for MTTF
d
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.
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).
7.3
Other marginal conditions
1.
Operation with capacitive loads is only permitted for two-channel assignment (see
alternative B in Figure 6-3 and Figure 6-4).
2.
All relays of the second channel are enabled and disabled with a time delay, so that
zero current switching can be assumed for the first channel if the contacts of the first
and second channel are connected in series (see alternative B in Figure 6-3 and
Figure 6-4).
3.
The minimum switching frequency is 1 cycle in 6 months.
4.
The values are only valid if an appropriate external fuse is provided to protect the
contacts against overload.
Table 7-4
MTTF
d
depending on the load for the contacts and the switching fre-
quency
Load for the contacts (according to
DIN EN 60947-4-1/DIN EN 60947-5-1)
MTTF
d
>
10 years
MTTF
d
>
30 years
MTTF
d
>
100 years
Channel 1
Channel 2
Switching frequency [1/h]
AC 15; 3 A
AC 15; 3 A
< 214
< 65
< 13
DC 13; 5 A
DC 13; 5 A
< 85
< 25
< 5
AC 15; 3 A
DC 13; 5 A
< 159
< 48
< 9
DC 13; 5 A
AC 15; 3 A
< 159
< 48
< 9
AC 15; 3 A
Zero current
< 216
< 65
< 13
DC 13; 5 A
Zero current
< 161
< 49
< 9
250 µF; DC 4 A
Zero current
< 146
< 44
< 8