Commission the overhead tank pump (Hand/Off/Auto)

The brief

Work order

WO-PS-501

Job

Commission the overhead tank pump (Hand/Off/Auto)

Difficulty

Intermediate

Type of work

Commissioning

Detail

25 wires, lockout, meter checks and a PLC program

Sunrise Apartments, Bengaluru. Fill pump P-501 lifts water from the sump to overhead tank OHT-501, and the building draws the tank down all day. The motor is 3.7 kW, 415 V, 7.5 A. Two float switches in the tank, B1 at 20 % and B2 at 90 %, go to the PLC. The plant engineer wants a Hand/Off/Auto selector so that the caretaker can run the pump by hand if the PLC is out of service. The starter panel +P501 has arrived with the power circuit and the 24 V supply wired. The control 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 25 conductors of the wiring schedule. The selector S3 switches the source of K1's coil: AUTO takes it from PLC output Q0.0, HAND takes it from +24 V, OFF takes nothing. Both meet on the coil junction X3. Because the selector switches between them, the PLC output is never connected to the supply: a hand-wired link between them would feed 24 V back into the output.
4. After X3 the coil circuit passes the E-STOP and the overload contact. So HAND bypasses the PLC, but it can never bypass the E-STOP or the overload.
5. Read the supplied PLC program. The pump starts when the water falls below B1, fills until B2 closes, then waits.
6. Restore power: tick the checklist, remove your tag and lock, switch Q1 ON, put the PLC in RUN.
7. Test: OFF, AUTO through a fill cycle, HAND with the PLC stopped, the E-STOP in HAND and an overload trip. The tank starts half full and the building draws it down, so give it time to fall below B1. Then run the FAT.

What the test checks

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

• 25 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