How to Verify a Pump Loss-of-Prime Underload Trip
When a pump loses prime or runs dry, it stops moving liquid, and because there is no fluid load on the impeller the motor current drops below its normal running value. An underload trip watches for that drop and shuts the pump down before it damages itself running dry. This procedure is for the technician setting and verifying that underload protection, confirming the threshold sits below normal load but above a true dry-run condition, and proving the trip actually fires.
Verify Loss-of-Prime Trip in one line: To verify a pump loss-of-prime underload trip, confirm the underload threshold is set below the pump's normal running load but above the load a fully dry pump draws, add a short time delay so a momentary dip does not trip on a normal load swing, then simulate a loss of prime and confirm the pump trips and annunciates before it can run dry for long.
Establish the Normal Running Load
The underload threshold is meaningless without knowing what normal looks like, so first record the pump's motor current or power under normal, primed operation at the station's real operating head. A pump moving liquid draws a characteristic load; when it loses prime and pumps air, that load falls off because the impeller is doing little work. The gap between those two states is the window the underload trip lives in, and you cannot set the threshold sensibly without measuring the primed value on the actual installation, not the nameplate.
Note that the running load varies with operating conditions, so capture it across the real range: a pump running against a higher discharge head or a partly closed valve draws differently than one running freely. You want the threshold below the lowest normal running load you will see, so that ordinary operation never trips the underload, while still being high enough to catch a genuine loss of prime. This is the same load signature a monitoring platform trends as motor current, which is why a prime loss is often visible in the data before the trip even fires.
Set the Threshold and the Delay
Set the underload threshold below the minimum normal running load and above the load a fully unprimed pump draws, placing it in the clear gap between the two states. Too high and normal load swings, such as a flush of light flow or a head change, trip the pump needlessly; too low and the pump runs dry for a while before the load falls far enough to trip, defeating the protection. The exact value is site-specific and set from the measured primed and unprimed loads, not from a generic number.
Add a trip delay long enough to ride through momentary load dips that are not a real loss of prime, such as the transient at start or a brief air slug, but short enough that a genuine dry-run is caught before it damages the pump. The delay is a balance: too short and the trip nuisance-trips on transients, too long and the pump runs dry meaningfully before shutting down. Setting this delay is analogous to setting an alarm off-delay, and the reasoning in the guide on alarm on-delay and off-delay applies directly to why the delay exists.
Simulate a Loss of Prime and Confirm the Trip
Prove the trip by creating the underload condition safely. Depending on the installation you can close the suction to induce a loss of prime under supervision, or where the protection is implemented as a relay or logic on the motor current, inject a below-threshold current signal to confirm the trip logic and timing without stressing the pump. The goal is to confirm two things: that the pump actually trips when the load falls below the threshold for longer than the delay, and that the trip is annunciated so an operator learns the pump lost prime rather than just finding it stopped.
Confirm the trip latches or requires a deliberate reset rather than auto-restarting straight back into a dry-run, because a pump that trips on underload and immediately restarts into the same lost-prime condition just cycles itself to destruction. Where an auto-restart is used, confirm it is limited to a small number of attempts before locking out and calling for help. Route the underload trip into the station's alarm path so the loss of prime is reported, tying into the guide on how to configure an email alarm notification in SCADA.
Verifying the Result and Common Mistakes
A verified underload trip has a threshold sitting cleanly between the measured primed and unprimed loads, a delay that rides through transients but catches a real dry-run quickly, a proven trip on a simulated loss of prime, and an annunciation that tells an operator what happened. Record the primed load, the unprimed load, the threshold, and the delay as the as-left baseline, because these justify the setting and let the next technician confirm nothing drifted. This protection complements the diagnosis in the guide on the pump underload loss-of-prime trip.
The most common mistake is setting the threshold from a generic number rather than the measured loads on the actual pump, which either nuisance-trips or fails to protect. The second is omitting the delay, so ordinary load transients trip the pump. The third is letting the trip auto-restart unlimited times into the same dry-run condition, which turns a protective trip into a destructive cycle. The fourth is failing to annunciate the trip, so the pump is found stopped with no clue why.
Frequently Asked Questions
How does an underload trip protect a pump from losing prime?
When a pump loses prime or runs dry, it stops moving liquid, so the impeller does little work and the motor current drops below its normal running value. An underload trip watches for that drop and shuts the pump down before it runs dry long enough to damage its seal and bearings. The threshold is set in the gap between the pump's normal primed load and the lower load it draws when pumping air.
Where should the underload threshold be set?
Below the lowest normal running load the pump draws when primed, so ordinary operation never trips it, and above the load a fully unprimed pump draws, so a genuine loss of prime does trip it. The exact value is site-specific and comes from measuring the primed and unprimed motor current or power on the actual installation, not from a nameplate or a generic figure.
Why does an underload trip need a time delay?
To ride through momentary load dips that are not a real loss of prime, such as the start transient or a brief air slug, without nuisance-tripping. The delay must still be short enough that a genuine dry-run is caught before it damages the pump. It is the same balance as an alarm off-delay: too short and it trips on transients, too long and the pump runs dry meaningfully before shutting down.
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