Commission the 4-20 mA level loop

The brief

Work order

WO-PS-801

Job

Commission the buffer tank level loop

Difficulty

Advanced

Type of work

Commissioning

Detail

26 wires, lockout, meter checks and a PLC program

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 do

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 3.47 mA (4800 counts) 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.

What the test checks

The factory acceptance test is marked against the panel you built. The level sets out:

• 26 conductors in the wiring schedule
• 6 dead checks before you touch the panel

In this lab

• Lock out and prove dead, live-dead-live
• Wire the backplate from the schematic
• Meter, tester, clamp and insulation tester