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MM-TA-12B Muting Module
System Installation
Instruction Manual
3.7.4 FSD Interfacing Connections
Final switching devices (FSDs) can take many forms, though the most common
are captive contact, forced-guided relays or Interfacing Modules. The mechanical
linkage between the contacts allow the device to be monitored by the external
device monitoring circuit for certain failures.
Dependent on the application, the use of FSDs can facilitate controlling voltage
and current that differs from the OSSD outputs of the Module. FSDs can also be
used to control an additional number of hazards by creating multiple safety stop
circuits.
Safety Stop Circuits
A safety stop allows for an orderly cessation of motion for safeguarding purpos-
es, which results in a stop of motion and removal of power from the MPCEs
(assuming this does not create additional hazards). A safety stop circuit typically
comprises of a minimum of two normally open contacts from captive contact,
forced-guided relays, which are monitored to detect certain failures such that the
loss of the safety function does not occur (i.e. external device monitoring). Such
a circuit can be described as a "safe switching point".
Typically, safety stop circuits are either single channel (a series connection of at
least two N.O. contacts); or dual channel (a parallel connection of two N.O. con-
tacts). In either method, the safety function relies on the use of redundant con-
tacts to control a single hazard, so that if one contact fails ON, the second con-
tact will arrest the hazard and prevent the next cycle from occurring.
Interfacing safety stop circuits must be wired so that the safety function can not
be suspended, overridden, or defeated, unless accomplished in a manner at the
same or greater degree of safety as the machine's safety-related control system
that includes the Module.
The normally open outputs from an IM-T-9A or -11A interfacing module are a
series connection of redundant contacts that form safety stop circuits and can be
used in either single-channel or dual-channel control methods. (See Figures 3-18
and 3-19.)
Dual-Channel Control
Dual-channel (or two-channel) control has the ability to electrically extend the
safe switching point beyond the FSD contacts. With proper monitoring (i.e.,
EDM), this method of interfacing is capable of detecting certain failures in the
control wiring between the safety stop circuit and the MPCEs. These failures
include a short-circuit of one channel to a secondary source of energy or voltage,
or the loss of the switching action of one of the FSD outputs. The result could
lead to the loss of redundancy or a complete loss of safety if not detected and
corrected.
The possibility of a failure to the wiring increases as the physical distance
between the FSD safety stop circuits and the MPCEs increase, as the length or
the routing of the interconnecting wires increases, or if the FSD safety stop cir-
cuits and the MPCEs are located in different enclosures. Thus, dual-channel con-
trol with EDM monitoring should be used in any installation where the FSDs are
located remotely from the MPCEs.
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