An overload relay protects a motor against currents that are too high for too long: a jammed load, a heavy start, low supply voltage or a lost phase. It sits after the contactor, measures the motor current, and opens the control circuit so the contactor drops out before the motor windings overheat. This article compares thermal and electronic overload relays and explains what trip classes 10, 20 and 30 mean.
What an overload relay does
A motor can carry several times its full-load current for a short time, as it does at every start, but the same current for too long damages the insulation. An overload relay therefore has an inverse-time characteristic: the higher the current, the faster it trips. It is not designed to clear short circuits; that job belongs to fuses, a circuit breaker or a motor protective circuit breaker upstream.
Most overload relays have a normally closed contact (usually marked 95-96) wired in series with the contactor coil, and a normally open contact (97-98) for an alarm or a PLC input.
How the overload relay contact drops out the contactor coil.
Thermal (bimetal) overload relays
In a thermal relay, the motor current heats a bimetal strip in each phase, either directly or through a heater element. When the strips bend far enough, they release a trip mechanism. Characteristics:
- Simple, with no electronics and no supply needed.
- Adjustable over a relatively narrow range with a dial set in amperes.
- Older NEMA-style starters use replaceable heater elements chosen from the manufacturer's table according to motor current; the heater, not a dial, sets the trip point.
- They need time to cool after a trip before they can be reset.
- Because they work by heat, they produce some heat in the panel and react to the temperature around them.
Ambient compensation
A bimetal cannot tell heat from the motor current from heat in the enclosure. An ambient-compensated relay adds a second bimetal that bends with the ambient temperature only and cancels its effect, so the trip point stays close to the setting whether the panel is cold or warm. Compensation matters when the motor and the starter are in different environments, for example a motor outdoors and a starter in a heated room. Check the relay's data for its compensated temperature range.
Electronic overload relays
An electronic relay measures the current with current transformers or sensors and calculates the motor's thermal state. Depending on the model, it can offer:
- A wider adjustment range, so fewer models cover more motors.
- Selectable trip class.
- Faster and more reliable phase-loss and phase-imbalance detection.
- Additional functions on some models, such as ground-fault, jam or underload detection and network communication.
- Low heat produced in the panel and little sensitivity to ambient temperature.
Many self-powered electronic relays draw their energy from the motor current itself; others need a separate control supply. Check the model's data before wiring it.
Trip classes 10, 20 and 30
The trip class describes how quickly a relay responds to a heavy overload. Under IEC 60947-4-1, the class number is the maximum trip time, in seconds, when the relay carries 7.2 times its current setting, starting from cold. A Class 10 relay therefore trips in 10 seconds or less at 7.2 × setting, a Class 20 relay in 20 seconds or less, and a Class 30 in 30 seconds or less. The standard also sets minimum times and other test points (including a Class 10A); see the current edition of the standard and the manufacturer's curves for the full characteristic.
| Trip class | Maximum trip time at 7.2 × setting | Test current for a 2.5 A setting | Test current for a 5 A setting | Test current for a 12 A setting |
|---|---|---|---|---|
| Class 10 | 10 s | 18 A | 36 A | 86.4 A |
| Class 20 | 20 s | 18 A | 36 A | 86.4 A |
| Class 30 | 30 s | 18 A | 36 A | 86.4 A |
Values for guidance. The test current is 7.2 times the dial setting; trip times are maximum values from cold.
Which class to choose
- Class 10: the usual choice for standard motors with normal starting times, and the most common class for IEC-style relays.
- Class 20: for loads that take longer to accelerate. Many older NEMA-style relays are Class 20.
- Class 30: for high-inertia loads with long starts, such as large fans, centrifuges or loaded conveyors.
A slower class must still protect the motor: confirm with the motor manufacturer that the motor can withstand the longer start.
Trip classes: the maximum tripping time at 7.2 × setting.
Setting the relay to the motor FLA
- Read the full-load current on the motor nameplate at the voltage and connection used.
- Choose a relay whose range includes that value.
- Set the dial (or choose the heater) according to the nameplate FLA and the manufacturer's instructions; where service factor or the applicable code allows a different setting, follow them.
- For a star-delta starter, check whether the relay sits in the line or in the phase (winding) circuit; the setting is different.
For typical motor currents, see motor nominal current (UL / CSA) and motor nominal current (IEC).
Manual or automatic reset
Most relays have a selector or a reset button:
- Manual reset: someone must press reset after the relay cools. This is the safer choice in most machines, because it forces a person to look at the cause.
- Automatic reset: the contact closes again by itself once the relay cools. Use it only where an unexpected restart cannot create a hazard, for example with three-wire (start/stop push-button) control, where someone must press start again. With two-wire control (a maintained switch or a thermostat), automatic reset can restart the motor without warning.
Many relays also have a test button that simulates a trip and a stop button that opens the NC contact.
Thermal vs electronic at a glance
| Feature | Thermal (bimetal) | Electronic |
|---|---|---|
| Measuring principle | Heat in bimetal strips | Current sensors and calculation |
| Adjustment range | Narrow | Wider |
| Trip class | Fixed by model | Often selectable |
| Phase-loss detection | On differential models | Usually built in |
| Ambient influence | Reduced by compensation | Low |
| Extra functions | None | Ground fault, jam, communication on some models |
Key takeaways
- An overload relay protects the motor against sustained overcurrent, not against short circuits.
- The trip class number is the maximum trip time in seconds at 7.2 times the setting.
- Set the relay from the motor nameplate FLA, and choose a slower class only for long starts.
- Prefer manual reset; never use automatic reset with two-wire control where a restart could hurt someone.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions.
Related parts at 3JIndustry
Browse used overload relays and starters in our motor control and contactor collections, and read about contactor applications.
Looking for a discontinued part? Send the full part number to sales@3jindustry.com or use our contact page.
