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A side-mounted liquid level switch can change state correctly at the vessel while the PLC reports the opposite condition. Probe coverage, relay energisation and the input program are three different things. During commissioning, record them together instead of accepting a green indicator as proof that the high-level alarm works.
Side-Mounted Liquid Level Switch: Write the State Table First
The METRAVON MV-SP601 is an RF admittance point detector with rod and cable arrangements. Confirm that the proposed probe and active region suit the side connection. Its public product information does not establish one universal relay assignment, supply voltage or power-loss response for every assembly. Obtain the terminal drawing for the actual output version.
| State | Local check | PLC check |
|---|---|---|
| Clear | Material is below the active region; record the actual output. | Display the intended normal condition, not an assumed contact convention. |
| Covered | Representative liquid reaches the sensing region. | The correct alarm tag changes and the approved action follows. |
| Power lost | Observe the output under an authorised isolation test. | Confirm the specified fault or trip response. |
| Wire open | Test at the agreed circuit point using a safe procedure. | Determine whether the input distinguishes failure from a normal state. |
Test Detection Separately From the Input Circuit
An electrical simulation at the panel checks the downstream wiring and program, not liquid detection. Conversely, an output change observed beside the tank does not prove that the cable reaches the intended input. Perform both checks and retain the terminal numbers, tag name and observed state.
For the wet test, use the normal liquid where practical and record the rising and falling transitions. Residue in the side nozzle can leave the probe covered after the main vessel drains. Investigate that physical condition before reversing software logic or increasing delays. The side-nozzle installation review covers retained material and removal space.
Close the Loop Without an Uncontrolled Trip
Agree temporary control arrangements before exercising a live interlock. Record any bypass, its owner and its removal. After restoring wiring and power, confirm the final configuration and normal operating state. Do not leave a simulated input in place as a substitute for an uncompleted material test.
Contact and Alarm Questions
Q1. Does normally closed mean safe for every high-level duty?
No. The contact designation alone does not describe the covered state, loss of supply or controller logic. Assess the complete circuit against the required failure response. An ordinary process switch is not automatically an approved independent protection system.
Q2. What should accompany the quotation request?
Send the nozzle section, liquid conditions, trip elevation and PLC input specification. Include the completed intended-state table so the offered output can be checked before wiring starts.
Summary
Accept the installation only when real probe coverage produces the intended receiving-system state. Keep the observed contact table with the MV-SP601 wiring record so a later replacement does not silently reverse the alarm.
