Level 8 · Free project

Commission the 4-20 mA level loop

Wire a 2-wire level transmitter into an analog input, scale the counts to percent, raise a high-level alarm, and see why a broken loop must read as a fault and not as an empty tank.

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The circuit diagram of the panel, as drawn in the lab: Commission the 4-20 mA level loop
The closed cabinet door with its lamps, buttons and isolator handle, as drawn in the lab: Commission the 4-20 mA level loop

What you will build

Work order WO-PS-801. Your site team, Commission the buffer tank level loop.

Narmada Dyestuffs, Vadodara. Buffer tank TK-801 is filled by transfer pump P-801 and drawn down by the process all day. A 2-wire level transmitter LT-801, 4 to 20 mA for 0 to 5 m, sits on the tank. The control panel +F801 has arrived with the power circuit, the 24 V supply and the analog input module AI1 powered. The control and loop wiring is yours.

What you will learn

  • Lock out and prove dead, live then dead then live
  • Land 26 control conductors from a wiring schedule
  • Read the supplied PLC program and match it to your wiring
  • Restore power safely, test, and run the FAT

How it goes

  1. Lock out the panel at the main isolator Q1: switch OFF, apply your lock and your tag.
  2. Prove it dead with the meter, live-dead-live.
  3. Wire the 26 conductors of the wiring schedule. W24 to W26 are the loop: the transmitter is powered by the loop, so +24 V goes to its + terminal, its − terminal goes to the module's 0+, and the module's 0− goes back to 0 V. Swap them and the transmitter blocks the current.
  4. Read the supplied PLC program. The module gives 0 counts for 0 mA and 27648 for 20 mA, so the live zero of 4 mA is 5530 counts. The program takes that off before scaling. It also treats anything under about 3.6 mA as a broken loop.
  5. Restore power: tick the checklist, remove your tag and lock, switch Q1 ON, put the PLC in RUN.
  6. Test: the pump runs between 30 % and 80 % after START, the HIGH LEVEL lamp lights at 90 %, and a cut cable raises SIGNAL FAULT and stops the pump. Then run the FAT.

Parts in the panel

  • Main isolator, 4-pole, 63 A

    Switches all three phases and neutral. The incoming cable lands on 1, 3, 5, 7: those terminals stay live when it is OFF.

  • MCB 2-pole, 6 A, C curve: control supply

    Feeds the 24 V power supply with 230 V (L1 and N).

  • MCB 3-pole, 16 A, D curve: motor circuit

    Protects the motor circuit against short circuits. The overload relay protects the motor against overload.

  • Contactor 3-pole, 9 A AC-3, coil 24 V DC, 1 NO aux

    Switches pump P-801. Its coil gets 24 V from Q0.0 through the E-STOP and the overload contact. The 13-14 auxiliary tells the PLC it pulled in.

  • Thermal overload relay, 5.5-8 A

    Trips on sustained overcurrent. 95-96 (NC) opens and 97-98 (NO) closes. Blue button: reset.

  • Power supply 230 V AC to 24 V DC, 5 A

    Its 0 V (−) is bonded to PE at the factory, so 0 V and earth are the same potential.

  • Analog input module, 2 channels, 4-20 mA (0 to 27648 counts)

    Channel 0 is IW64. The loop current enters 0+ and leaves 0-. 4 mA reads 5530 counts, 20 mA reads 27648, and an open loop reads 0.

  • PLC: compact CPU, 8 DI / 6 DQ, 24 V DC

    Inputs I0.0-I0.7 are PNP: 24 V on the terminal turns the input on, with 1M at 0 V. Outputs Q0.0-Q0.5 source 24 V from 3L+.

  • Terminal strip X2: 24 V DC distribution, 16 positions

    Positions 1 to 8 are bridged +24 V, 9 to 16 are bridged 0 V. Top and bottom of each position are the same point.

  • Terminal strip X1: motor

    The motor cable from the plant lands here.

  • PE bar

    Protective earth, bonded to the incoming supply's earth.

  • Push button START, green, 1 NO

  • Push button STOP, red, 1 NC

  • Emergency stop, 2 NC, twist to release

    Contact 11-12 is hardwired in the coil circuit; 21-22 tells the PLC.

  • Pilot lamp green 24 V: RUNNING

  • Pilot lamp amber 24 V: OVERLOAD TRIPPED

  • Pilot lamp red 24 V: HIGH LEVEL

  • Pilot lamp amber 24 V: SIGNAL FAULT

  • Motor 3-phase 2.2 kW, 415 V, 4.6 A, driving transfer pump P-801

  • Level transmitter LT1, 0 to 5 m, 4-20 mA, 2-wire, loop-powered

    Hydrostatic transmitter in the tank. It takes its power from the loop: 24 V on +, and the signal current leaves on -.

Questions

Is it free?
Yes. The lab is free to use with a free account. There is nothing to buy to open this scenario.
Do I need to install anything?
No. It runs in your browser with a 3D panel, a circuit drawing, a meter and a PLC.
What will I do in Panel Lab 8?
You wire a control panel, then check your work against an acceptance test.
How long does it take?
Plan for one sitting. You can stop and come back.
Do I need experience?
No. Each work order says what to do step by step, and hints are there if you get stuck.
Open in the lab