Description
PM866A The Isolate-Not is a composite signal that represents a number of control unit states in the
control unit that should result in branch isolation. Isolate is negated (goes low) as a result of
system malfunction (see previous chapters on system diagnostics), a manual request for
maintenance purpose, or when a test reveals a fault that must be handled by disabling the failing
branch. This is particularly applicable to NE outputs.
The Isolate signal is timed so the loss of a control PM866A unit or an output board will always cause a
low Isolate signal to be recognized first. The shutdown output is timed to go low after isolate to
prevent it from requesting a shutdown when the isolate is still high.
In a similar fashion, the isolate is designed to return high after a valid signal is presented on the
shutdown output to prevent spurious action on PM866A system restart or isolate removal. This function
allows fault tolerant action to be taken on failure or during maintenance.
Shutdown-Not or Activate
The Shutdown-Not (Normally Energized) or Activate (Normally De-Energized) signal is a
combination of an application generated action, PM866A manual action and signals from the test
program. It is also affected by system errors, and is forced low (NE) or will stay low (ND) on a
control unit malfunction.
Master Vote 3000 automatic output tests are performed as a part of the Safeguard 400 Series
base software and consists of:
The output tests consist of:
- Channel test
- Isolate test
- Option for monitoring of read-back of the output status.
- Option for feedback of the output loop supervision signal.
The basic test uses the Isolate-Not and Shutdown-Not (NE) / Activate (ND) signals to control
the test, and receives information back on two inputs (Isolate Status and Test Status). These are
common to all seven outputs on one Master Vote 3000 unit. The standard tests cover all
functions of the Master Vote 3000. The PM866A normal action for a NE output on detection of a fault is
to isolate the branch by deactivating Isolate-Not, and report this as a safety message and on the
diagnostics displays. ND stays low.
2.1.7 Bypass Management
The functions of Bypass management are:
- Operator station access control
- On-Line Builder (engineering tool) access control
- Override control and reset
Override is generally used as a collective term for inhibit, input block, output block and manual
mode for the relevant signal types.
The use of override functions in safety related equipment introduces a potential hazard to the
installation and to the people it is designed to protect. Any inhibit of a safety critical input or
override on a safety critical output represents a degradation of the safety level and a possibility
for failure on demand.
Nevertheless, such functions are necessary to achieve a reasonable availability of the process.
All field equipment needs maintenance or replacement at regular intervals, and this is included
in the design of the safety system regarding e.g. number/wiring and placement of instruments.
In these cases the safety level may be maintained by other measures while necessary
maintenance operations are carried out.
There are also requirements for use of inhibits during start-up in certain applications.
The Bypass management function enables project/application specific configuration of the
appropriate level of restrictions regarding operation of the Safeguard 400 Series.
2.1.7.1 Operator station access control
Operator stations which shall be accepted for write access must be configured in the Safeguard
400 Series and a key switch shall be hard wired to the Safeguard 400 Series for control of
permission to operate the system.
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