The status of the inputs influences block behavior based on how the STATUS_OPT parameter is configured. The three choices of STATUS_OPT are:
-
Always Use Value — The value of the input is always used regardless of status. This is the default option.
-
Will Not Vote if Bad — The input value is not counted as a vote to trip if its status is Bad.
-
Vote to Trip if Bad — The input value is counted as a vote to trip if the input status is Bad.
The following table shows how several common voting schemes degrade when a single input has bad status based on the option chosen for STATUS_OPT.
|
Original Voting Scheme |
Resulting Voting Scheme for STATUS_OPT Values |
||
|---|---|---|---|
|
Always Use Value(1) |
Will Not Vote if Bad |
Vote to Trip if Bad |
|
|
2oo3 |
2oo3 or 1oo2 |
2oo2 |
1oo2 |
|
2oo2 |
2oo2 or 1oo1 |
Will Not Vote if Bad |
1oo1 |
|
1oo2 |
1oo2 or Tripped |
1oo1 |
Tripped |
|
1oo1 |
1oo1 or Tripped |
Trip Inhibited |
Tripped |
The DVTR block determines the status of OUT_D the same way no matter which status option is chosen. The status calculation is completely separate from the value calculation.
The status of OUT_D is Good if the number of non-bypassed inputs with Good status is greater than or equal to A_NUM_TO_TRIP or all inputs are bypassed, otherwise, the status is Bad. Uncertain status on inputs is treated as Good.
When any input has Bad status, the DVTR_ALERTS Input Bad becomes active.
TRIP_STATUS Indication
The TRIP_STATUS parameter indicates the state of the trip vote functions. The typical value for TRIP_STATUS is Normal, and less commonly, Tripped. As shown in the following figure, TRIP_STATUS can be delayed when TRIP_DELAY or NORMAL_DELAY is non-zero and a transition is occurring between normal and tripped states.
A fifth state, Trip Inhibited, occurs whenever a startup bypass is active or when it is not possible to trip because there are not enough inputs participating in voting. The latter case can occur when inputs are bypassed or when inputs have bad status and STATUS_OPT selected is Will Not Vote if Bad.
The solid lines in the figure show the common state transitions of TRIP_STATUS expected as the process value moves above and below the trip point. The dashed lines show less common state transitions.
