Overcurrent and Undercurrent Monitoring Relays

What are overcurrent and undercurrent monitoring relays? A current monitoring relay measures load current through a built-in shunt or an external current transformer (CT), compares it to an adjustable threshold in amps or percent of rated current, and switches an SPDT/DPDT output when current crosses that threshold for longer than the set trip delay, per IEC 60255 and IEC 60947-5-1. Set the threshold wrong and the relay either nuisance-trips on normal starting current or misses a jam until the motor overheats. This article covers how the relay senses current, where overcurrent and undercurrent detection each apply, hysteresis and trip delay, shunt vs external CT selection, and wiring the output into a contactor or PLC.

What Overcurrent and Undercurrent Monitoring Catch That a Breaker Won't

A motor circuit breaker or overload relay trips on sustained overcurrent, protecting the winding insulation from thermal damage over minutes. It has no floor. Drop the load to zero and the breaker sits there unaware, while a snapped drive belt lets the motor spin free or a pump runs dry against a closed valve. A current monitoring relay adds both ends of the window: overcurrent for a jam, undercurrent for a lost load, checked continuously rather than on a thermal time-current curve. For background on where this device class sits against MPCBs and PLC-based monitoring, see the monitoring relay engineering guide.

Overcurrent detection on its own largely duplicates what a properly sized motor protection circuit breaker already does, so most panel builders add a dedicated current monitoring relay specifically for the undercurrent side, or for a fast alarm ahead of the breaker's thermal curve. What we see in the field: undercurrent protection gets specified for pumps and conveyors far more often than overcurrent, because the breaker already has overcurrent covered — the relay fills the gap the breaker cannot see.

How the Relay Measures Current: Built-In Shunt vs External CT

Compact relays rated for a few amps wire the load current directly through an internal shunt — simple, no extra hardware, but limited to the relay's own current range. Above that range, an external current transformer (CT) carries the load conductor through its window and feeds a scaled secondary current back to the relay, set either to a CT ratio (for example 100:5) or reading a 0-5A / 0-1A standard secondary directly. Clamp-on split-core CTs let you retrofit monitoring onto an existing feeder without breaking the circuit; feedthrough CTs cost less and read more accurately but need the conductor threaded through during panel build.

Sizing the CT matters more than most spec sheets suggest. A CT sized for a much higher current than the actual load reads a compressed, less precise signal near the low end of its range — exactly where undercurrent detection needs resolution. Match the CT primary rating to the motor's actual full-load current, not to the feeder's maximum ampacity.

Setting the Overcurrent Threshold

Set the overcurrent threshold above the highest current the load reaches in normal operation — including starting current, if the relay's trip delay does not already ride through it — and below the point where sustained current indicates a jam, a stalled rotor, or a mechanical bind. Too tight and every legitimate load swing trips the output. Too loose and the relay never fires before the upstream breaker or overload does its own job, making the monitoring function redundant. Some current monitoring relays offer a separate, longer delay just for the overcurrent side, specifically to ride through starting current without adding a second timer relay.

Undercurrent Monitoring: Catching a Broken Belt, Dry Pump, or Lost Load

A centrifugal pump running dry draws noticeably less current than one moving liquid, because the hydraulic load on the impeller disappears. Set the undercurrent threshold below the lowest current seen under normal minimum-flow conditions and the relay catches a dry-run condition before bearing damage, often faster than a mechanical low-flow switch. The same logic applies to a conveyor with a snapped chain, a mixer with a sheared shaft, or a belt-driven fan that has thrown its belt: torque and current both collapse, and the motor itself does not react — it just spins faster and lighter, current drops, and nothing downstream shows damage unless something is watching.

For pump and level applications specifically, undercurrent detection on the motor feeder complements, rather than replaces, direct liquid level monitoring at the tank or wet well — the current relay tells you the motor lost its load, the level relay tells you why.

Hysteresis and Trip Delay: Avoiding Nuisance Trips

Every motor draws a current spike on start — six to eight times full-load current for a direct-on-line induction motor, for a fraction of a second up to a few seconds depending on load inertia. Without an adequate trip delay, the overcurrent side of the relay fires on every single start. The undercurrent side has the opposite problem: a load with cyclical duty, like a reciprocating compressor or an intermittent mixer, dips below threshold repeatedly during normal operation, and a delay set too short turns a normal cycle into a nuisance alarm.

This depends heavily on the load's actual current profile, which is why bench-testing the setpoint against a current logger on the real motor beats guessing from a nameplate. Undersized hysteresis is the more common mistake — set the reset band too narrow and the output relay chatters every time current sits near the threshold, wearing the contacts and confusing whatever downstream device is watching the alarm.

Selecting and Wiring an Overcurrent/Undercurrent Relay

Confirm four things before ordering: the load's actual full-load and starting current, whether a built-in shunt covers that range or a CT is required, whether the application needs overcurrent, undercurrent, or both on one relay, and the output logic. Fail-safe (normally-energized) wiring means the output relay de-energizes on fault or on loss of the relay's own auxiliary supply — the safer default for anything feeding a contactor coil or a PLC input that should show a fault state on power loss. Latching (manual reset) output stops an automatic restart from masking a repeat fault. It also means someone has to walk to the panel to clear it, a trade-off worth discussing with the end user before commissioning, not after the third nuisance shutdown.

Where the application also needs phase loss, phase sequence, or voltage monitoring on the same feeder, a multifunction relay from the monitoring and control relays range often combines current sensing with those functions in one 22.5 mm module rather than stacking separate devices. For motor-specific protection where thermistor-based winding temperature also matters, see monitoring relays for motor protection. Where phase loss or single-phasing is the more likely fault mode on the same motor, cross-check against MPCB phase loss and single-phasing protection — the two failure modes often get confused during troubleshooting.

New to the broader monitoring relay category before narrowing to current sensing specifically? Start with what a monitoring relay is and how it works.

Conclusion

Overcurrent monitoring catches a jam; undercurrent monitoring catches a load that has quietly disappeared — a dry pump, a broken belt, a lost coupling — conditions a standard breaker cannot see because it only watches for too much current, never too little. Size the sensing method, shunt or CT, to the actual load current, set thresholds with enough hysteresis and delay to survive normal starts and duty cycles, and decide fail-safe versus latching output before wiring the control circuit. Get those four decisions right and the relay catches the fault on the first cycle, not the tenth service call.