Field Powered Devices; Sensor Configurations - Allen-Bradley AADvance T9110 Safety Manual

Chapter 4 AADvance Functional Safety System Implementation

Sensor Configurations

82

Field powered devices

The above circuit is appropriate for devices that are powered locally and expect
a current-controlled signal loop. Ensure that the loop is wired to pass current to
the Loop Plus terminal and return it on the Loop Minus terminal.
ATTENTION: In safety critical input applications using a single sensor, it is
important that the sensor failure modes be predictable and well understood,
so there is little probability of a failed sensor not responding to a critical
process condition. In such a configuration, it is important the sensor be
tested regularly, either by dynamic process conditions that are verified in the
AADvance system, or by manual intervention testing.
The function of a signal shall be considered when allocating the module and
channel within the system. In many cases, redundant sensor and actuator
configurations may be used, or differing sensor and actuator types provide
alternate detection and control possibilities. Plant facilities frequently have
related signals such as start, and stop signals. In these cases it is important to
ensure that failures beyond the system's fault-tolerant capability do not result in
either inability to respond safely or in inadvertent operation. In some cases, this
will require that channels be allocated on the same module, to ensure that a
module failure results in the associated signals failing-safe.
Sensor configurations should be considered. In most cases it will be necessary
to separate the signals across modules. Where non-redundant configurations
are employed, it is especially important to ensure that the fail-safe action is
generated in case of failures within the system.
Field loop power should be considered in the allocation of signals to input
channels and modules. For normally energized input configurations, field loop
Rockwell Automation Publication ICSTT-RM446N-EN-P - April 2018