PLC-Controlled Pump Will Not Start After Sensor Replacement
A wastewater transfer pump is controlled by a PLC. The start condition is: high-level float AND low-level float NOT active AND no overload trip AND selector in AUTO. A proximity sensor was replaced on the pump coupling guard; since then the pump will not start in AUTO but runs in HAND. The sensor is a three-wire PNP inductive type, 24 V DC, normally open, wired brown to +24 V, blue to 0 V, black to input X4. The PLC input LED for X4 is off when the guard is closed and on when the guard is open. The maintenance technician wants to bypass X4 in the program.
1. Diagnose the likely cause of the failure to start, and evaluate the proposal to bypass X4.
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The PLC input LED behaviour tells us the sensor is working but inverted relative to expectation: the input is off with the guard closed and on with it open. A PNP normally-open sensor should switch +24 V to the input when its target is present. If the guard is closed (target present) and the LED is off, either the target is not being detected, or the sensor is the wrong type (NPN or normally-closed) for a PNP sinking/sourcing input configuration.
Check: sensor type marking, target distance versus the sensor's rated sensing range, whether the target is the correct material (inductive sensors derate on non-ferrous), and whether the input common is correct for PNP. Also confirm the program expects the guard closed signal to be true.
Do not bypass X4. It is a guard interlock; bypassing removes a safety function and could allow the pump to start with the guard open. If a temporary bypass is ever used for diagnosis, it must be controlled, documented, time-limited, and never defeat a safety interlock.
2. Describe how you would verify the wiring and input configuration without altering the program, and what you would record.
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Verification steps:
- With the machine isolated and locked off, confirm supply polarity: brown to +24 V, blue to 0 V, black to X4.
- Measure voltage at X4 with the guard closed and open. For a PNP sensor, expect about 24 V at X4 when the target is present and about 0 V when absent. If the readings are reversed, the sensor is NC or NPN.
- Check the PLC input common terminal is tied to 0 V for PNP (sourcing) inputs. A mismatch here explains the inverted LED.
- Test the sensor with a known ferrous target at the rated distance to confirm it switches.
- Confirm the program's X4 contact type (NO or NC) matches the intended logic.
Record: sensor part number and type, measured voltages at X4 in both guard states, target gap, the input common configuration, and the program contact type. Then correct the sensor type or wiring and retest in AUTO with the guard closed, confirming the pump starts and the interlock still stops it when the guard opens.