The pump command is on, but the contactor does not pull in—or the PLC reports SC20 while the output circuit appears healthy. Treat those as different faults. On a CLICK PLC with a C0-04TRS relay output module, SC20 identifies an I/O communication problem; it does not prove that the module’s relay contacts switched or that the pump produced water flow.
Reject the quick fixes first
Do not bypass the output, force the contactor, or depend on the normally closed contact as the only protection for a critical pump. Those actions can restore motion without proving that the control path, contactor, motor, pump, and process response are healthy.
| Quick fix | Why it fails | Required check |
|---|---|---|
| Use the NC contact as automatic failover | The contact can pass current only while the relay contact and the rest of the circuit remain serviceable. A failed contact may remain open. | Measure the contact state and verify contactor pickup. |
Use SC20 as proof of output failure |
The bit responds to loss of I/O communication, not to burned, broken, or nonconducting relay contacts. | Compare the PLC command with electrical and process feedback. |
| Force the pump contactor | This bypasses interlocks and does not identify the failed component. | Trace the complete command path under the approved operating procedure. |
| Replace the module immediately | A fuse, breaker, field wire, interposing device, contactor, overload path, or process fault can produce the same symptom. | Prove voltage through each stage before replacing hardware. |
Stop here if starting the pump without its normal interlocks could damage equipment or create an unsafe process condition. The proof for moving forward is an authorized test plan that preserves every required permissive and protective function.
Set the required fail-state before wiring
A normally closed relay contact describes its mechanical state when the relay coil is de-energized; it does not guarantee operation after every module failure. Separate the two failure cases:
| Failure case | Expected NC-path behavior |
SC20 response |
|---|---|---|
| The PLC cannot communicate with the output module and the relay returns normally | The healthy NC contact can pass control current, provided the fuse, breaker, wiring, series contacts, and contactor are also healthy. | Turns on for the I/O communication failure. |
| The relay is commanded but its contact does not conduct | The pump contactor does not pull in through that failed contact. | Does not turn on while I/O communication remains healthy. |
| A field-circuit component fails | Depends on the failed fuse, breaker, wire, contact, or contactor. | Does not identify the field failure. |
For a process where loss of pumping could be catastrophic, define whether loss of PLC power, loss of I/O communication, a broken field wire, or a failed relay should start the pump, stop it, or transfer duty to another system. An NC contact alone cannot distinguish those events. The check before continuing is a documented response for each credible loss of command and loss of feedback.
Connect the command path and prove the relay contact
Trace the circuit from the control-power source through every device to the contactor coil. Include protective devices, PLC relay terminals, interlocks, overload contacts, terminal blocks, and the return conductor. Use the approved electrical drawing and observe the circuit’s rated measurement practices.
- Place the process in a condition where a test start cannot cause harm.
- Record the PLC output command and whether the
C0-04TRSrelay is expected to be energized or de-energized. - Measure control voltage at the source, on both sides of each series device, across the selected relay contact, and at the contactor coil.
- Command both output states. Verify that the NC path conducts in the relay’s de-energized state and opens in its energized state.
- Confirm that the contactor physically pulls in and that an auxiliary contact changes state.
A PLC status indication proves the logic’s requested state, not current flow through the relay contact. If the command changes but the measured contact state does not, isolate power under the site procedure and test the contact and wiring. The pass condition is correct voltage at the contactor coil and matching auxiliary-contact feedback for both commanded states.
Connect feedback that detects the real failure
Use separate feedback for each layer that matters. A contactor auxiliary input reports contactor position. Motor-current or motor-protection feedback reports whether the electrical load is operating. Pressure and flow instruments report whether the pump produces the required process result.
| Signal | What it proves | What it misses |
|---|---|---|
| PLC output command | The program requested an output state. | Relay contact, wiring, and load operation. |
SC20 |
The CPU has an I/O communication fault. | Relay-contact conduction and pump performance. |
| Contactor auxiliary contact | The contactor changed mechanical state. | Motor rotation, water flow, and pressure. |
| Pressure or flow feedback | The process responded to the pump command. | The exact electrical component that failed. |
Program a command-versus-feedback alarm: after a pump command, require the appropriate contactor and process feedback before declaring the pump available. No delay value is specified here; set it from measured contactor pickup, pump acceleration, and process response, then include enough margin to avoid nuisance alarms without masking a real loss of flow. The proof is an alarm when the output command exists but the selected feedback does not arrive.
Configure SC20 as a communication alarm
Use SC20 to report the failure it can see. Annunciate the I/O communication fault, inhibit any logic that depends on trustworthy remote output state, and initiate the defined transfer or shutdown response. Do not label SC20 as “pump failed” unless another diagnostic separates communication, electrical, and process failures.
- Monitor
SC20continuously in normal operation. - Latch an operator-visible I/O communication alarm when it turns on.
- Record the commanded pump state when the alarm occurs.
- Evaluate independent contactor, pressure, and flow feedback.
- Apply the documented fail-state or backup-pump sequence.
Test the logic with an approved simulation or controlled communication-loss test. The check passes when SC20 produces the communication alarm and the system takes its documented response without representing the bit as proof of a bad relay contact.
Run the end-to-end failure test
Test each fault separately because a single successful pump start proves only the intact path used during that start.
- Run a normal start and stop. Confirm the output command, relay state, contactor feedback, and pressure or flow response agree.
- Simulate a missing contactor response without defeating protective devices. Confirm the command-versus-contactor alarm operates while
SC20remains unaffected. - Simulate a missing process response. Confirm the pressure or flow alarm identifies failure beyond the contactor.
- Perform the approved I/O communication-loss test. Confirm
SC20, operator indication, and the defined fallback action. - Restore the system, clear latched alarms through the normal reset path, and repeat a complete automatic cycle.
For critical service, include an independent backup arrangement sized and controlled for the required water duty. Verify it by creating the initiating condition and proving actual pressure or flow, not merely a backup output bit.
Frequently asked questions
What happens if the CLICK output module loses I/O communication?
SC20 turns on. A healthy de-energized relay may return to its NC state, but the resulting contactor action still depends on the complete field circuit.
What happens if a C0-04TRS relay contact fails open?
The contactor will not pull in through that contact. SC20 remains off if CPU-to-module communication is still working.
What happens if the PLC output is on but the pump does not run?
Measure the relay contact and contactor-coil voltage, then check auxiliary-contact, motor, pressure, and flow feedback. The failed stage is the first point where command and measured response stop agreeing.
What happens if the contactor pulls in but there is no water flow?
The output circuit may be operating correctly while the motor, pump, valves, supply, or process path is not. Trip a separate no-flow or low-pressure alarm from process feedback.
What happens if the NC contact is used as the only fail-safe?
A contact failure, broken wire, open protective device, or failed contactor can still prevent pump operation. Add independent command, contactor, and process feedback plus a tested backup strategy.
Stop and escalate to AutomationDirect official support when SC20 persists after the module seating, power, and I/O path have been checked, or when the relay state does not match the commanded state with field wiring isolated under an approved procedure. Do not return a critical pump to automatic service until the normal path, each alarm, the defined fail-state, and the backup response have passed an end-to-end test.