If redundancy is required for the I/O pack, three (not two) I/O packs can be mounted on the I/O module to provide a reliable
two-out-of-three reference for each driven component. For general-purpose 4 - 20 mA outputs, the three I/O packs drive a
common, passive, current-sharing circuit on the I/O module that produces a single 4 - 20 mA output. This is the median signal
from the three I/O packs. There are also 0 – 200 mA outputs available to drive valve positioners.
A customized (application-specific) implementation of current outputs is used for servos. These outputs are similar to the 4 -
20 mA outputs and can have one or three I/O packs, for redundancy, that drive three bi-polar servo coils on the same control
valve actuator. The advantage of three coil servos is that there is no single component in the electronics that selects between
the redundant valve commands and is vulnerable to failure.
Relay outputs are available in three redundancy levels:
Level 1 provides dual redundant controllers, IONET switches, and Ethernet ports on a common I/O pack, which controls
a relay driver and a relay.
Level 2 also provides dual redundant controllers and IONET switches, but extends the redundancy to three redundant
I/O packs, which are voted by a common relay driver feeding a relay.
Level 3 extends the voting to three sets of mechanical relays, which vote with their contacts. This is available with 36
relays voting to create 12 contact outputs that are available as form "A" (normally open) and form "B (normally closed)
configuration. Application-specific versions of this are available for interface with hydraulic trip solenoids on turbines,
which vary in quantity, rating, and specific implementation
Many other factors should be considered when choosing the proper contact output circuit for reliability. For example,
magnetic relays and solid-state outputs are available. Magnetic relays have form "C" contacts (1 open and 1 closed with a
common point). This allows preplanning for the most common failure mode, de- energize, for a magnetic relay. In this
scenario, a normally-closed contact can energize a Motor Control Center starter, which is not available on solid-state outputs.
If the starter is for the lube oil pump, then it probably warrants the highest level of redundancy. However, if there is an
emergency lube oil pump to back-up the auxiliary pump, then there is less need for redundant electronics. Other
considerations are whether the relays are sealed for hazardous locations, leakage current in the case of solid-state relays,
suppression for solenoid applications and so forth.
Mark VIe Controls System Redundancy Options