In this guide
The little dial on the overload relay, marked 7–10A, sits below the contactor in every DOL starter. Most students set it to the motor's nameplate current, wire it up, and move on. Then the motor trips on every second start — or worse, never trips while the motor slowly cooks. That dial is a thermal model of your motor, and setting it correctly is a five-minute job once you understand what it's modeling.
This guide explains how thermal and electronic overload relays work, exactly where to set the dial (including star-delta and CT-operated cases), the trip curves, and how to diagnose nuisance tripping vs real overloads.
What the overload relay protects against (and what it doesn't)
An overload relay protects the motor against sustained overcurrent: mechanical overload (jammed conveyor, overloaded pump), undervoltage (motor draws more current for the same load), single-phasing (one phase lost — remaining phases overcurrent), and phase imbalance. It models the motor's heating with a bimetallic strip (thermal type) or an electronic thermal model.
What it does not do:
- Short circuits — that's the MCCB/fuse upstream. The overload relay is far too slow.
- Earth faults — needs an earth-fault relay or RCD.
- Instantaneous protection — it deliberately ignores the 6–7× starting current for a few seconds.
Note: The contactor + overload relay + short-circuit device trio is a division of labor: contactor switches, overload relay protects thermally, MCCB/fuse clears faults. No single device does all three in a classic starter.
Thermal vs electronic overload relays
| Aspect | Thermal (bimetallic) | Electronic |
|---|---|---|
| Principle | Heater + bimetallic strip bends with I²t heating, trips the mechanism | Current transformers + microcontroller thermal model |
| Setting | Dial in amps | Digital, often with display |
| Trip class | Fixed by type (usually Class 10) | Selectable (Class 5/10/20/30) |
| Phase-loss sensitivity | Differential version trips faster on single-phasing | Standard — detects imbalance directly |
| Ambient compensation | Bimetallic ambient-compensated types exist; plain ones drift with panel temperature | Inherent — no drift |
| Extras | None | Jam/stall, undercurrent (dry-run), ground fault, communication |
| Cost | Low | 3–5× thermal |
For student panels and standard motors, thermal is fine and is what you'll meet in labs. Electronic earns its place on critical, expensive, or submersible motors.
The basic setting: dial = motor FLC
For a motor connected direct-on-line, set the overload relay's current dial to the motor's full-load current from the nameplate.
Example: 5.5kW, 400V motor, nameplate FLC = 11.2A → set the dial to 11.2A (choose an overload relay whose range brackets it, e.g. 9–13A or 10–16A — never use a relay whose range ends at your setting; keep the setting in the middle-upper of the range).
The relay trips at ~1.05–1.2× setting after the class time — so at 11.2A setting, sustained 13A trips in minutes, and 60A (starting) is ignored for the few seconds a normal start takes. That discrimination between "starting" and "overloaded" is the whole art.
Star-delta: the setting changes
In a star-delta starter, the overload relay is usually wired in the delta loop (in series with the windings), where it sees only 1/√3 of line current. Setting:
I_setting = FLC / √3 = 0.58 × FLC
Example: 15kW motor, FLC = 28A, star-delta starter with the relay in the delta circuit → set 16.2A (0.58 × 28). Setting it to 28A here leaves the motor protected at 172% — effectively unprotected. If the relay is instead in the supply line (before the star-delta contactors), set full FLC. Know where your relay sits — this is the single most-tested overload setting question.
CT-operated relays
On large motors the relay works through CTs: setting = FLC / CT ratio, and the relay range is in secondary amps. Example: 110kW motor, FLC = 200A, 200/5A CTs → relay sees 5A at full load → set 5A on a 4–6A relay.
Trip classes: how fast it trips
The trip class defines the maximum tripping time at 7.2× setting (the locked-rotor reference), from cold:
| Class | Trip time at 7.2× setting | Use |
|---|---|---|
| Class 5 | ≤ 5s | Fast — submersible pumps, motors that can't tolerate long starts |
| Class 10 | ≤ 10s | Standard — most general-purpose motors |
| Class 20 | ≤ 20s | High-inertia loads: fans, centrifuges with long run-up |
| Class 30 | ≤ 30s | Very long starting: large mills, crushers |
A high-inertia fan that takes 15s to run up will trip a Class 10 relay on every start — the fix is Class 20, not a higher current setting. Raising the current dial to survive starting leaves the motor unprotected while running; changing the class keeps running protection intact.
The trip curve shape matters too: at 1.5× setting a Class 10 relay takes ~2–4 minutes; at 2× about 30–60s; at 3× under 20s. Keep the manufacturer's curve in your project file — examiners accept "per the manufacturer's trip curve" as an answer.
Single-phasing and phase imbalance
Losing one phase is the classic motor-killer: the motor keeps running on two phases, drawing ~1.7× current in the remaining windings. A plain thermal relay eventually trips on the overcurrent, but slowly. Differential (phase-loss sensitive) thermal relays and all electronic relays detect the imbalance directly and trip in seconds. For any motor that matters — and for your project report's protection section — specify phase-loss sensitivity and say why.
Commissioning and testing
- Dial set to FLC (or 0.58× FLC in delta, or FLC/CT ratio) — photographed for the report
- Relay range brackets the setting comfortably
- Trip class matches the starting time (measure actual run-up time with a stopwatch during commissioning)
- Test the trip: most relays have a test button — verify the contactor drops out and the trip indicator shows
- Reset mode understood: manual vs auto reset. Manual reset for most applications (forces someone to investigate); auto-reset only where the process tolerates it — a motor that restarts itself after a trip is a safety hazard around people
- Stop-circuit wiring correct: the relay's NC trip contact (95-96) is in series with the contactor coil — a trip must drop the contactor
Warning: Testing overload relays means running the motor and deliberately creating trip conditions — rotating machinery, live panels. Keep guards on, hands clear, and work under supervision. Never wedge a tripped relay's reset closed to "keep production running" — that converts a protection device into a decoration.
Diagnosing trips: nuisance or real?
| Symptom | Likely cause | Check |
|---|---|---|
| Trips on every start | Class too fast for run-up time; or voltage dip prolonging start | Measure run-up time; check supply voltage during start; consider Class 20 |
| Trips after minutes of running | Genuine overload; or high ambient; or relay set low | Clamp-meter the running current vs setting; check driven load; check panel temperature |
| Trips randomly | Loose terminal heating the bimetal; phase imbalance; single-phasing upstream | Thermal camera or thermometer on terminals; measure phase currents |
| Never trips, motor burned | Setting too high; relay in wrong location (line vs delta); welded/stuck mechanism | Verify setting calculation; test button; replace relay |
| Trips only in summer | Ambient derating of thermal relay in a hot panel | Ventilation; ambient-compensated relay; re-check setting |
The clamp meter is the arbiter. If running current is genuinely above setting, the relay is doing its job — fix the load, not the relay. If current is normal and it still trips, suspect the relay, its environment, or the setting calculation.
Common mistakes
- Setting FLC in a delta-wired relay — 172% protection, the most common exam trap and a real field error.
- Raising the dial to cure starting trips — changes running protection; change the trip class instead.
- Wrong relay range — setting 11A on a 9–13A relay is fine; setting 12.8A on a 9–13A relay leaves no margin — pick the next range.
- Auto-reset on a manned machine — the motor restarts into someone's hands.
- No test after wiring — the 95-96 contact wired to the wrong place means the relay trips and the contactor stays in. Test it.
Where to go from here
- DOL vs Star-Delta Starter — the starter assembly the overload relay lives in.
- DOL vs Star-Delta Starter — the starter where the 0.58× setting applies.
- MCB vs MCCB vs ELCB Selection — the short-circuit protection coordinating with the same FLC.
- Single-Phase vs Three-Phase Motors — the loads being protected.
- More motor-protection topics in the Electrical branch hub.