Ground-Fault and Stall Protection in Electronic Overload Relays
What does ground-fault and stall protection add to an electronic overload relay? An electronic (solid-state) overload relay layers two functions on top of the IEC 60947-4-1 thermal-overload curve that a bimetallic relay cannot provide: ground-fault detection through vector-sum current sensing, and stall (locked-rotor) detection based on a current threshold and time delay applied after the motor's run-up window. Without them, a developing insulation fault or a jammed load can run well past the point a plain thermal element would catch it, since neither event necessarily pushes total phase current high enough, or for long enough, to move a bimetal strip. This article covers how vector-sum ground-fault sensing works, why a bimetal element can't see it, how stall detection differs from a trip-class curve, how to set both thresholds, and which Schneider, ABB and Siemens electronic series document these functions.
Why Electronic Relays Can Do More Than Bimetal Ones
A bimetallic relay has three strips, one per phase, each bending in proportion to the heat its own phase current generates. The only comparison it makes between phases is the differential trip bar used for phase-loss sensing — a mechanical linkage, not a calculation. An electronic overload relay replaces the strips with current transformers or shunts feeding a microcontroller, and a microcontroller can hold more than one number in memory at once. It tracks each phase current individually, it can sum all three as vectors, and it can time how long a value has persisted since the motor started. That processing layer, not a bigger current transformer, is what makes ground-fault and stall functions possible at all.
How Ground-Fault Detection Works: Vector-Sum Sensing
In a healthy three-phase circuit with no path to earth, the three phase currents sum to zero at every instant — Kirchhoff's current law applied as a phasor sum. A ground fault diverts part of one phase's current to earth outside the sensing loop, so that sum stops being zero. The relay's microcontroller computes this residual continuously and trips once it exceeds a set threshold for longer than a set delay. It is the same physics an RCD uses; the difference is that the sensing and trip logic sit inside the overload relay rather than in a separate device upstream.
Formula: Ground-Fault Trip Threshold — Source: vector-sum (residual) current principle, IEC 60947-4-1 electronic overload relay ground-fault add-on function
Igf = |IL1 + IL2 + IL3|, trip when Igf ≥ Iset(gf) for t ≥ tdelay(gf)
| Symbol | Description | Unit |
|---|---|---|
| IL1, IL2, IL3 | Instantaneous phase currents (phasor values) | A |
| Igf | Vector-sum (residual) current, zero in a healthy circuit | A |
| Iset(gf) | Ground-fault trip threshold set on the relay | A |
| tdelay(gf) | Trip delay, set to ride through normal transient imbalance | s |
Why a Bimetallic Relay Can't See a Ground Fault
A bimetal strip only responds to net heating on its own phase. A modest ground fault, one that leaks a fraction of FLC to earth, may not raise total phase current enough to bend any single strip past its trip point — especially with a resistive fault that develops slowly. What we see in the field: a slow ground fault on a submersible pump cable often shows up as nuisance tripping of an upstream RCD weeks before phase current itself climbs enough to trip a plain bimetal thermal overload relay. By the time the bimetal element reacts, the fault has usually progressed to a phase-to-phase or phase-to-ground short that the short-circuit protective device clears — at the cost of the cable and possibly the motor.
Stall (Locked-Rotor) Protection: Catching a Jam After Start-Up
Trip class already governs how long the relay tolerates elevated current — Class 10 trips in 4-10 s at 7.2x the setting from cold, Class 30 in 9-30 s. That curve is fixed once a class is picked and it does not distinguish a slow overload from a sudden mechanical jam. Stall protection is a separate function: after the start-up (run-up) delay elapses, if current stays above a stall threshold, typically set well above running FLC, for longer than a short stall delay measured in a few seconds, the relay trips independent of the thermal integrator. A conveyor jam or a pump impeller seizing mid-run can sit for several seconds inside a Class 10 curve's tolerance simply because the thermal model hasn't accumulated enough heat yet. The stall function closes that gap.
Setting the Ground-Fault and Stall Thresholds
The ground-fault threshold is set as a percentage of the relay's rated current or CT primary, with a short delay to ride through the transient imbalance every motor produces at the instant of start. Set it too sensitive and a VFD-fed motor with long cable runs will nuisance-trip on ordinary capacitive leakage current; set it too loose and a real fault develops for weeks before the relay reacts. The stall threshold is set above the current the motor draws during a normal, successful start, and the stall delay is set longer than the motor's normal run-up time — get the delay too short and every start looks like a stall. This depends on how long the motor actually takes to reach running speed under its real load, which is not always the number on the nameplate; a fan with a heavier-than-spec wheel runs up slower than the same frame size on a light load, so the delay has to reflect the installed condition, not the catalog figure.
Ground-Fault and Stall Across Schneider, ABB and Siemens Electronic Lines
All three of the priority brands sell an electronic overload tier alongside their bimetallic line, and the wider setting ratio and selectable trip class are consistent across them. Ground-fault and stall coverage is not uniform, and it is worth checking the specific model rather than assuming the whole electronic family carries both functions.
| Criteria | Schneider LR9 / TeSys T | ABB E-series (EF19-EF460) | Siemens 3RB30 / 3RB31 |
|---|---|---|---|
| Setting ratio | Wider than bimetal LRD (~1:1.5) | 1:3-1:4 | Up to 1:4 |
| Selectable trip class | Yes | Class 10/20/30 | Class 5/10/20/30 |
| Phase-loss sensing | Yes | Yes | Yes |
| Confirmed ground-fault function | Verify per model | Verify per model | Yes, on 3RB31 (not 3RB30) |
| Stall/locked-rotor function | Common on electronic tier — verify per model | Common on electronic tier — verify per model | Common on electronic tier — verify per model |
Siemens documents the split cleanly: the SIRIUS 3RB30 and 3RB31 share the same frame and selectable class range, but ground-fault detection is specifically a 3RB31 feature. That kind of frame-level split shows up across brands — two relays can look identical on the shelf and differ only in which add-on functions are enabled. Cross-check the datasheet, not the family name, before assuming a specific ground-fault or stall function is present. For coordination with the rest of the starter, see how these electronic relays fit the broader Type 1 vs Type 2 coordination tables published for each SCPD-contactor-relay combination.
Frequently Asked Questions
Does a bimetallic overload relay detect ground faults?
No. A bimetal strip responds only to heating from its own phase current and has no way to compare the vector sum of all three phases. Ground-fault protection requires either a separate residual-current device upstream or an electronic overload relay with a dedicated ground-fault function.
What's the difference between stall protection and a Class 20 trip curve?
Trip class is a fixed thermal curve selected once and applied continuously; it trips in 6-20 s at 7.2x the setting from cold for Class 20. Stall protection is a separate current-and-time threshold that only arms after the start-up delay elapses, aimed specifically at a mid-run jam rather than a general overload.
Where does the ground-fault current threshold usually get set?
As a percentage of the relay's rated current or the associated CT's primary rating, paired with a short delay long enough to ride through the current imbalance every motor produces momentarily at start.
Can a VFD-fed motor cause nuisance ground-fault trips on an electronic overload relay?
Yes. Long motor cables on a VFD output carry capacitive charging and common-mode currents that can register as residual current even with no actual fault to earth. A ground-fault threshold set too tight on a VFD-fed circuit will nuisance-trip on this leakage.
Which Siemens overload relay includes ground-fault detection?
The SIRIUS 3RB31. The 3RB30 shares the same frame sizes and selectable trip class range but is not documented with the ground-fault function — check the specific part number rather than the family name.
Conclusion
Ground-fault and stall protection are not extensions of the thermal trip curve — they are separate detection logic that only an electronic overload relay's microcontroller can run: a continuous vector-sum comparison across three phases for ground faults, and a current-plus-time threshold gated by a start-up delay for stalls. Neither function is automatic across every electronic relay on the shelf; Siemens' own family shows a 3RB30 without ground-fault detection sitting next to a 3RB31 with it. Set the ground-fault delay to ride through start-up imbalance and cable leakage, and set the stall delay to the motor's actual run-up time under its installed load, not the nameplate number. For the broader selection and setting logic these functions sit on top of, see the thermal overload relay engineering guide, the comparison of thermal vs electronic overload relays, and how phase-loss sensing fits alongside these functions in phase-loss and single-phasing protection. Both functions work only when the rest of the starter is sized correctly — pair them with the right contactors and motor protection circuit breakers for the declared coordination type.
