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Application 2: Wiring loads to L+ and M at each digital output
You can connect two relays using one fail-safe digital output. The following conditions should
be kept in mind:
● L+ and M of the relays must be connected with L+ and M of the F-DO module (reference
● The normally open contacts of the two relays must be connected in series.
A connection to each of the four digital outputs is possible. The figure below shows an
example of the connection to DO0. This configuration achieves safety class SIL3/Category
4/PLe (process status readback required).
Figure 7-52
When connecting two relays on one digital output, (as shown in the figure above), the
errors "wire break" and "overload" are detected only at the P-switch (not at the M-switch).
The controlled actuator can no longer be switched off should a cross circuit occur between
the P and M-switches of the output. To avoid cross circuits between the P and M-switches
of a fail-safe digital output, you should always wire the relay connection to the P and M-
switches separately, in order to prevent any cross circuits (for example with separately-
sheathed cables or using separate cable ducts).
ET 200S Distributed I/O System - Fail-Safe Modules
Installation and Operating Manual, 08/2008, A5E00103686-07
potential must be equal).
Wiring diagram for in each case 2 relays on 1 F-DO of the EM 4 F-DO DC24V/2A
PROFIsafe
WARNING
WARNING
7.7 4 F-DO DC24V/2A PROFIsafe digital electronic module
Fail-Safe Modules
169
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