The operator pressed the start button.
Nothing happened.
The HMI showed no active trip.
The interlock summary was clear.
The motor was available.
So the start button was pressed again.
Still nothing.
This is where troubleshooting often becomes confusing.
If there is no active interlock, why will the machine not start?
Because “no trip” does not mean “ready to start.”
A machine may have no active shutdown condition and still be missing one small permissive, sequence condition, control signal, or field status required for startup.
Trip, Interlock, and Permissive Are Not the Same
These terms are often used loosely in the field, but they do different jobs.
Trip
A trip stops equipment that is already running or prevents unsafe operation.
Examples:
- Motor overload
- Low lubrication pressure
- Emergency stop
- High bearing temperature
- Process shutdown signal
Permissive
A permissive must be healthy before the equipment is allowed to start.
Examples:
- Local/Remote selector in Remote
- Discharge valve open
- Lubrication pump running
- MCC ready signal available
- No maintenance lock active
Sequence Condition
A sequence condition confirms that the process is at the correct step.
Examples:
- Previous step completed
- Required timer expired
- Valve travel confirmed
- Tank level within starting range
- Another machine already running
The HMI may show the trip list clearly while hiding a missing permissive or incomplete sequence step.
That is why a clear interlock screen can create false confidence.
1. The HMI May Not Show Every Start Condition
An interlock summary is only as complete as the logic and graphics behind it.
Some start conditions may exist:
- Inside PLC logic
- In a sequence function block
- In another controller
- In the MCC
- In a hardwired safety circuit
- Inside a packaged machine panel
The operator may see ten green conditions while the eleventh condition is not displayed at all.
A useful question is:
What exact logic bit enables the final start output?
Do not begin with the alarm screen.
Begin with the output instruction or final start command, then work backward through the logic.
2. “Remote” on the Screen May Not Mean Remote in the Field
The HMI may indicate Remote based on one signal.
The motor starter or local control station may use another contact.
Common problems include:
- Selector switch contact failure
- One pole of a multi-contact selector not changing
- PLC status showing Remote while the hardwired circuit remains in Local
- Incorrect feedback mapping
- Remote selection lost after maintenance
The word “Remote” on the screen proves only that one Remote status signal is active.
It does not prove that the complete remote start path is healthy.
3. The Start Command May Be Too Short—or Never Reach the Output
Sometimes the start button works.
The PLC sees it.
But the final output never energizes.
Check whether the command is:
- A momentary pulse
- Latched internally
- Blocked by scan order
- Reset immediately by another condition
- Sent through communication to another controller
- Dependent on an acknowledgement signal
An HMI start command may remain active for only a short time.
If another controller scans slowly, communication is delayed, or the logic expects a maintained command, the signal can disappear before it is used.
Do not ask only:
Did the operator press Start?
Ask:
How far did the start command travel?
4. A Stale Feedback Signal Can Block the Next Start
A machine may refuse to start because the PLC believes it is already running, still stopping, or not fully reset.
Examples include:
- Run feedback stuck ON
- Zero-speed switch not reset
- Valve-open and valve-closed feedback active together
- Contactor feedback remaining latched
- Sequence step not returning to Idle
- Previous start-failure bit not reset
The problem is not always a missing signal.
Sometimes the problem is a signal that should have disappeared but did not.
Contradictory feedback deserves special attention.
A valve cannot be fully open and fully closed at the same time.
A motor should not be stopped and running at the same time.
When both states appear healthy, the logic may intentionally block the start.
5. The MCC May Not Be Ready
The PLC can be healthy while the starter is not ready to receive a command.
Check:
- Control power availability
- Breaker or isolator position
- Overload relay reset
- Contactor auxiliary contacts
- MCC drawer service position
- Protection relay ready status
- Communication health
- Local emergency-stop circuit
A motor may show no process interlock but still lack electrical readiness.
This is especially common when the DCS and MCC readiness signals are displayed on different screens.
The process system says:
No trip.
The starter says:
Not ready.
Both can be correct.
6. A Hardwired Contact May Be Missing from the PLC Display
Not every stop or permissive is programmed as a visible PLC signal.
A control circuit may contain hardwired contacts such as:
- Emergency-stop contacts
- Motor protection relay contacts
- Local stop pushbuttons
- Safety relay outputs
- Package panel permissives
- Field junction contacts
If one of these contacts opens, the contactor coil may never energize even though the PLC output turns ON.
This creates an important troubleshooting split:
PLC output is OFF
Investigate PLC logic, permissives, sequence conditions, and commands.
PLC output is ON, but the starter does not operate
Investigate the output module, interposing relay, control power, hardwired circuit, contactor coil, and starter readiness.
That single distinction can save a great deal of time.
Follow the Start Command Step by Step
When the machine will not start, trace the command in order.
Step 1: Confirm the Start Request
- Did the HMI or pushbutton generate the command?
- Did the PLC input or internal tag change?
- How long did the command remain active?
Step 2: Find the Final Start Output
- Which output actually commands the motor, solenoid, or package?
- Is that output ON or OFF?
- What condition is blocking it?
Step 3: Work Backward Through the Logic
Check:
- Trips
- Permissives
- Mode selection
- Sequence step
- Reset conditions
- Feedback contradictions
- Communication status
Step 4: If the Output Is ON, Move to the Field Circuit
Check:
- Output module voltage
- Interposing relay
- Control fuse
- Hardwired stop circuit
- Contactor coil
- MCC readiness
- Package panel input
Do not jump randomly between the HMI, PLC cabinet, and field equipment.
Follow the command until it disappears.
Field Checklist
When the interlock screen is clear but the machine will not start, ask:
- Is the machine actually in the correct operating mode?
- Are all permissives displayed—or only the trips?
- Is the sequence in the correct starting step?
- Did the start command reach the PLC?
- Did the final output energize?
- Is a feedback signal stuck or contradictory?
- Is the MCC or package panel ready?
- Is there a hardwired contact outside the displayed logic?
- Was a reset or acknowledgement condition missed?
Final Thought
A clear interlock screen does not prove that a machine is ready.
It only proves that the conditions shown on that screen are clear.
When nothing happens after the start command, do not keep pressing the button and do not search every screen at once.
Find the final start output.
Then follow the command path backward through the logic—or forward into the field circuit.
The fault is usually located where the command stops.