A signal reaching the DCS or ESD system correctly does not necessarily mean the system will perform the correct action.
The next question is:
“When the required conditions occur, does the configured logic produce the correct output?”
That is the purpose of a Logic Test.
What is a Logic Test?
A logic test is a functional test used to verify that the configured control or safety logic produces the intended result for defined input conditions.
In simple terms:
Inputs → Logic → Outputs
We apply or simulate specific input conditions and verify that the system responds according to the approved logic diagrams, Cause & Effect, control philosophy, and functional requirements.
Logic Test in DCS
Consider a cooling-water pump:
Low Flow → DCS Logic → Pump Start
The logic may be designed so that:
- Low flow detected
- Standby pump is available
- Auto mode is selected
- Start command is generated
During the logic test, different combinations are simulated to verify:
Correct input recognition
Correct AND/OR conditions
Correct timers
Correct interlocks
Correct permissives
Correct sequence
Correct output command
Example
Suppose a pump can start only when:
Tank Level > 30% AND Suction Valve Open AND Pump in AUTO
The logic test should verify not only the TRUE condition, but also the FALSE conditions.
If any required permissive is missing:
Pump should not start.
Logic Test in ESD System
ESD logic testing is more safety-critical.
Consider a high-high pressure shutdown:
PT → ESD Input → Safety Logic → Solenoid → Shutdown Valve
Suppose the Cause & Effect requires:
2oo3 Pressure Voting → ESD Trip → Close SDV
The logic test verifies that the configured safety logic behaves exactly as specified.
For example:
One transmitter high → No trip
Two transmitters high → Trip
Three transmitters high → Trip
The actual voting philosophy depends on the approved SIS design.
What is Checked During Logic Testing?
Input Conditions
Are the correct input tags being used?
Logic
Are the configured:
AND / OR / NOT / Voting / Latching
functions correct?
Timers
Are:
Time delays
On-delay
Off-delay
configured correctly?
Interlocks & Permissives
Does the equipment respond correctly when permissives are:
Present
or
Absent?
Outputs
Does the correct output operate for the specified input condition?
Latching & Reset
For shutdown functions:
Trip → Latch → Reset
The test should verify that the trip remains latched until the required reset conditions are satisfied.
DCS Logic Test vs ESD Logic Test
| DCS Logic Test | ESD Logic Test |
|---|---|
| Tests control/sequencing logic | Tests safety/shutdown logic |
| Interlocks & permissives | Trips, voting & shutdown logic |
| Start/stop sequences | Cause & Effect actions |
| Control outputs | Safety outputs |
| Process control focus | Risk reduction/protection focus |
| Based on control philosophy | Based on SIS safety requirements |
Very Important
A logic test is not the same as a loop test.
Loop Test:
Verifies the signal path and functional response from field → system → output/final element as defined by the test scope.
Logic Test:
Verifies that the configured logic makes the correct decision for defined input conditions.
Simple Interview Answer
What is a Logic Test in DCS/ESD?
A logic test verifies that the configured DCS or ESD logic responds correctly to specified input conditions and generates the intended outputs according to the approved control philosophy, Cause & Effect, and functional requirements.
Remember:
Loop Test = Is the signal path working correctly?
Logic Test = Is the programmed decision correct?
ESD Logic Test = Will the safety system take the correct protective action when the specified trip conditions occur?
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