Lab 14: Pump Proof-of-Flow Fault Trip

Application Scenario

A cooling-water pump at a treatment plant must never run dry or against a closed valve. Without water flowing through it, the pump heats up quickly, and its shaft seal can be destroyed in minutes. A flow switch in the discharge pipe closes (or opens) when water is actually moving.

When the pump starts, water needs a few seconds to begin flowing, so the flow switch cannot be checked instantly. The control circuit therefore gives the pump a proving time:

1. The operator presses START. The pump starts, and a proving timer begins counting.
2. If flow is proven before the proving time ends, the pump keeps running.
3. If there is still no flow when the time ends, the pump trips (stops) and a FAULT lamp lights.
4. If flow is lost later while the pump is running, the pump trips at once.
5. The fault is latched: the pump cannot be restarted until an operator acknowledges it by pressing STOP/RESET.

This proof-of-flow (or proof-of-operation) check is used on pumps, fans, compressors, and conveyors everywhere. In this lab:

Learning Outcomes

After this lab you will be able to:

• Use an on-delay timer as a proving timer that checks a feedback signal after a grace period.
• Build a latched fault that trips a motor and blocks restarting until it is reset.
• Use an N.C. contact of the fault relay as a running interlock.
• Explain the difference between a command (the pump was told to run) and proof (the pump is really doing its job).

Background

Command vs. proof. A motor contactor that is ON only proves that the motor was told to run. It does not prove that the motor is turning, or that the pump is moving water. A broken coupling, a closed valve, an empty tank, or a lost prime all leave the contactor ON while nothing useful happens. A feedback device, such as a flow switch, pressure switch, or speed switch, provides the proof.

How the circuit decides. The fault relay CR2 energizes when both of these are true:

• the proving time is over (TR1 has timed out: TR1A N.O.T.C. is closed), and
• there is no flow (the flow switch N.C. contact, PB4, is closed).

That is two contacts in series (AND logic). Once CR2 energizes, its own seal-in keeps it ON, and its N.C. contact in the pump rung trips the pump.

Why the fault must be latched. When the pump trips, the timer resets and TR1A opens again. Without a seal-in, the fault would disappear at once, the FAULT lamp would go out, and nobody would know why the pump stopped. Worse, if START were held, the pump would restart, run dry for another 5 seconds, trip, and repeat. A latched fault makes a person look at the problem before the pump runs again.

Timing Diagram

• First start (no flow): at the end of the proving time, TR1A closes while PB4 is still closed (no flow). CR2 latches, R1 lights, and CR2B trips the pump. The timer resets, but CR2 stays latched.
• Reset: pressing PB2 clears the fault.
• Second start (flow proven): PB4 is pressed (flow) before the time ends, so its N.C. contact is open when TR1A closes. No fault; the pump keeps running.

Functional Requirements

1. Pressing PB1 starts the pump (CR1, G1) and the 5-second proving timer.
2. If PB4 is pressed and held (flow) before 5 seconds, the pump keeps running.
3. If PB4 is not pressed within 5 seconds, the pump trips: G1 OFF, R1 (FAULT) ON.
4. If PB4 is released while the pump is running (flow lost), the pump trips at once and R1 turns ON.
5. While the fault is latched, PB1 cannot restart the pump. Pressing PB2 clears the fault.
6. Pressing PB2 while the pump runs normally stops the pump without a fault.

Ladder Diagram — Step-by-Step Drawing Guide

Draw on the Ladder Diagram Worksheet. Use pencil. This design uses 7 grid lines.

Step 1 — Rung 1: pump contactor

1. From L1: PB2 (N.C.), PB1 (N.O.), a CR2B N.C. contact (10–2), and the CR1 coil (2–7).
2. Mark the nodes: N2 (after PB2), N3 (after PB1), N4 (after CR2B, at the CR1 coil).

Step 2 — Rung 2: pump seal-in

1. Draw the CR1A N.O. seal-in (1–3) from N2 to N3.

Step 3 — Rung 3: proving timer

1. From N4, draw a line down to grid line 3 and draw the TR1 coil (2–7). TR1 is in parallel with the CR1 coil.

Step 4 — Rung 4: fault detection

1. From N2, draw a line down to grid line 4.
2. Draw a TR1A N.O.T.C. contact (1–3), then PB4 (N.C.), then the CR2 coil (14–13).
3. Mark the nodes: N5 (between TR1A and PB4), N6 (between PB4 and the CR2 coil).

Step 5 — Rung 5: fault seal-in

1. Draw a CR2A N.O. contact (9–5) from N2 to N6, in parallel with TR1A and PB4.

Step 6 — Rungs 6–7: lamps

1. Rung 6: from L1, CR1B N.O. (8–6), then G1. Node N7.
2. Rung 7: from L1, CR2C N.O. (11–7), then R1. Node N8.

Step 7 — Trace your drawing with a pencil

1. PB1 is pressed and PB4 is never pressed. At t = 5 s, trace the path to the CR2 coil. What happens to CR1, TR1, and TR1A next? Why does CR2 stay ON?
2. The fault is latched and PB1 is pressed. Is there a path to the CR1 coil?
3. PB1 is pressed and PB4 is held from t = 2 s. At t = 5 s, TR1A closes. Is there a path to the CR2 coil?
4. The pump has been running with PB4 held for 1 minute. PB4 is released. What happens, and how quickly?
5. What would happen if the CR2A seal-in were missing?

Step 8 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

List every terminal on each node (with device names), chain them, and fill in the wiring table on the worksheet. The N6 row is done for you.

Get instructor approval before wiring.

Key Wiring Points

1. Proof = timer done AND no flow. TR1A and the flow contact are in series in the fault rung.
2. Latch the fault with its own seal-in, fed from after the STOP/RESET button.
3. The fault trips the pump through an N.C. contact after the seal-in junction, so it works whether the pump is starting or already running.
4. The proving timer follows the pump coil (in parallel with CR1), so it resets whenever the pump stops.
5. PB4 is N.C.: pressing it means "flow is present." Keep it held for as long as you want to simulate flow.

Wiring Procedure

1. Power OFF. Set TR1 to 5 s.
2. Wire the circuit from your approved wiring table, one node at a time.
3. Meter check, power still OFF:

   Reading the meter: once the circuit is wired, the meter can find a path through coils and lamps. A reading of tens to thousands of ohms is normal and counts as high. Only a reading near 0 Ω means a direct connection.

   • L1 to CR2:9 should read near 0 Ω, and high while PB2 is pressed (CR2:9 is on N2).
   • CR2:10 to CR1:2 should read near 0 Ω (CR2B N.C. closed).
   • CR1:2 to TR1:2 should read near 0 Ω (both on N4).
   • TR1:3 to CR2:14 should read near 0 Ω, and high while PB4 is pressed.
   • CR2:9 to CR2:14 should read high (TR1A and CR2A both open).
4. Ask your lab partner to do a buddy check, then show the instructor.
5. Power ON and complete the test table below.

Test & Acceptance

Troubleshooting

Review Questions

1. Explain the difference between "the pump contactor is ON" and "the pump is working." Name three failures that the flow switch detects but the contactor does not.
2. Why does the circuit wait 5 seconds before checking the flow switch? What would happen if TR1A were replaced by a plain CR1 N.O. contact?
3. Why must the fault be latched? Describe what would happen without CR2A if the operator kept START held down.
4. After the pump has proven flow, losing flow trips the pump immediately. Real systems often allow a short delay here too. Why? How could you add one?
5. After a power outage, the latched fault is lost. Why might that be a problem in a real plant? How do PLC-based systems usually handle this?
6. Suppose the flow switch already shows "flow" before the pump starts (PB4 held before PB1 is pressed). What might that mean in a real plant? How could a circuit detect a stuck flow switch?

Applying This Circuit Pattern

The proof-of-operation pattern: after a command, allow a grace period; then, if the expected feedback is missing, latch a fault and remove the command.

Design question for the future: A dust-collector fan must prove airflow within 10 s, and the saw it serves (Lab 07's pattern) may run only after airflow is proven. Which contact would you use as the saw's running interlock: a fan contactor contact, or a contact that means "airflow proven"? How would you build "airflow proven" from this lab's circuit?

Extension

Operators complain that a short air bubble in the pipe (PB4 released for less than a second) trips the pump. Add a 1-second ride-through for loss of flow while running, without changing the 5-second proving time at start. You may use one more timer from the panel. Draw your solution and explain it; wire it if time allows.