Motor starter thermal overload relays · Trip class 10, 20 or 30 against motor start time, setting at motor rated current, 1.05/1.2 trip limits, phase loss, ambient compensation, reset
A thermal overload relay sits under a contactor in a motor starter and opens the control circuit if motor current stays too high for too long, using bimetal strips heated by the current in each phase. Ask three things first: which setting range covers the motor's rated current, which trip class allows the motor to start without nuisance trips while still protecting it, and which functions (phase-loss sensitivity, ambient compensation, manual or automatic reset, trip indication) are needed.
Bimetallic thermal overload relays for low-voltage motor starters. Electronic overload relays, motor protection circuit breakers, protection relays for medium-voltage motors and motor protectors built into motors are outside this page except where compared.
01
ABB's thermal overload relay data sheet states that standard trip classes are 10 A, 10, 20 and 30, and that under IEC 60947-4-1 the trip class gives the maximum tripping time in seconds at 7.2 times the setting current, with trip and non-trip times also specified at 1.5 and 7.2 times. Its table gives, from cold, 4–10 s for class 10, 6–20 s for class 20 and 9–30 s for class 30 at 7.2 times setting current, no tripping at 1.05 times and tripping at 1.2 times within 2 hours. The setting current is the motor's rated current, not the tripping current. [1]
Editorial calculation: a relay set at 10 A for a motor rated 10 A should not trip at 10.5 A and must trip at 12 A within 2 hours; at 72 A (7.2 times) a class 10 relay trips within 4–10 s and a class 20 relay within 6–20 s. If the motor's start takes, say, 12 s at about that current (an assumed figure), a class 10 relay could trip during starting and class 20 is the better match; check the motor and load start time before choosing. [1]
Editorial note: give the supplier the motor's rated current, starting current and start time, and how long the driven load takes to reach speed.
Give the supplier the motor rated current, voltage and frequency, starting method and start time, the contactor type and size it must fit, trip class needed, reset mode, auxiliary contacts, ambient temperature range, mounting (direct or separate), and the approvals required (IEC, UL, CSA).
02
ABB describes its relays as three-pole, with indirectly heated bimetals (one per phase) that bend under heating to trip the relay and change the auxiliary contacts; protection covers overload, phase-failure imbalance and phase loss, with ambient compensation from −25 °C to +55 °C, manual or automatic reset, a test function, trip indication, remote trip and reset options, NC 95–96 and NO 97–98 contacts, and close-coupled mounting to contactors or separate panel mounting. It notes the relays are self-protecting in an overload until the short-circuit protective device operates. [1]
ETEK lists class 10 adjustable-current bimetallic thermal overload relays for use with contactors in motor control centres and starters. [2]
Editorial note: confirm with the supplier that the relay is approved for the contactor it will mount on, and which fuse or breaker rating protects the relay and contactor against short circuits.
03
Schneider Electric's data sheet for the TeSys Deca LRD10 differential thermal overload relay (3-pole, 4–6 A setting, class 10A to IEC 60947-4-1, 1 NO + 1 NC auxiliary contacts) shows the figures to check for each setting range: tripping threshold, phase-failure sensitivity, temperature compensation and the contactors it fits directly under (LC1D09 to LC1D38). [3]
The 1.14 × Ir tripping threshold and phase-failure behaviour are what make the relay respond to sustained overload and single-phasing, while the trip class sets the time at locked-rotor current described above. The data sheet also lists EN 50495 among its standards and ATEX INERIS among its certifications. [3] [1]
Editorial calculation: with the dial at 5 A, the tripping threshold of 1.14 ± 0.06 × Ir lies between 5.4 A and 6.0 A.
| LRD10 (Schneider Electric) | Value |
|---|---|
| Thermal protection setting | 4–6 A |
| Trip class | 10A (IEC 60947-4-1) |
| Tripping threshold | 1.14 ± 0.06 × Ir |
| Phase-failure sensitivity | Trips at 130% of Ir on two phases with the third at zero |
| Temperature compensation | −20 to +60 °C |
| Rated operational / insulation voltage | 690 V AC (IEC); 600 V (UL/CSA) |
| Auxiliary contacts | 1 NO + 1 NC; 3 A at 120 V AC-15; 0.22 A at 125 V DC-13 |
| Controls | Red stop push-button, blue reset push-button |
| Size and weight | 45 × 66 × 70 mm; 0.124 kg; IP20 |
| Standards | EN/IEC 60947-4-1 and -5-1, UL/CSA 60947, GB/T 14048.4/.5, EN 50495 |
State motor full-load current, starting time, the contactor it mounts on and whether the motor is in a hazardous area, and ask for the setting range, trip class, threshold, phase-failure sensitivity and ambient compensation range.
04
CHINT publishes product pages for two bimetallic thermal overload relay series, NR2 and NXR. Both pages list compliance with IEC 60947-4-1, rated frequency 50/60 Hz, rated operational voltage up to 690 V AC, tripping class 10A, three-phase bimetal elements, temperature compensation, manual or automatic reset, a test and stop button, a trip indicator and 1 NO + 1 NC electrically isolated auxiliary contacts. The two series differ in the current span they cover and in the protection functions CHINT lists for each. [4] [5]
The CHINT ambient range of −5 to +40 °C is narrower than the compensated ranges in the other data sheets on this page (−25 to +55 °C for ABB, −20 to +60 °C for the Schneider Electric LRD10), so for a panel that runs hot or cold, ask which range applies to tripping accuracy rather than reading one figure across makers. [5] [1] [3]
Editorial calculation: the NR2 AC-15 rating of 1.58 A at 380 V is 58% of its 2.73 A rating at 220 V; check that the contactor coil circuit wired through contacts 95–96 draws less than the rating at your control voltage. [4]
Neither CHINT page states a rated insulation voltage; ask for it with the catalogue for the chosen frame size.
| Item (CHINT page) | NR2 | NXR |
|---|---|---|
| Current setting range | 0.1–150 A (depending on model) [4] | 0.1–630 A (varies by model) [5] |
| Tripping class | 10A [4] | Class 10A [5] |
| Standards listed | IEC/EN 60947-4-1 [4] | IEC 60947-4-1 / GB 14048.4 [5] |
| Protection listed | Overload and phase loss [4] | Overload; phase loss and phase imbalance; locked rotor [5] |
| Auxiliary contact ratings | AC-15: 2.73 A at 220 V, 1.58 A at 380 V; DC-13: 0.2 A at 220 V [4] | 1 NO + 1 NC; ratings not stated on the page [5] |
| Mechanical / electrical life | ≥ 1,000,000 / ≥ 100,000 operations [4] | ≥ 1,000,000 / ≥ 100,000 operations [5] |
| Ambient / storage temperature | −5 to +40 °C / −25 to +70 °C [4] | −5 to +40 °C / −25 to +70 °C [5] |
| Altitude; humidity | ≤ 2000 m; ≤ 95% RH non-condensing [4] | ≤ 2000 m; ≤ 95% RH non-condensing [5] |
| Mounting | Plug-in, or independent with MB-1 to MB-4 bases [4] | Plug-in on NC1, NC6, NC8 and NC2 contactors, or independent base mounting [5] |
For a CHINT relay, state the series (NR2 or NXR), the setting range that brackets the motor rated current, the contactor it plugs onto, and whether locked-rotor and imbalance protection are required.
05
The suppliers reviewed list thermal overload relays with contactors and motor protection breakers; ask each for setting ranges, trip class and tripping curves, phase-loss sensitivity, ambient compensation range, reset modes, contactor compatibility and approvals.
06
The dial is set to the motor's rated current, not the tripping current; the bimetals and trip mechanism then decide when the relay opens. [1]
| Part | Role | Fix in the order |
|---|---|---|
| Bimetals (one per phase) | Heat and bend with current. [1] | Setting range. |
| Setting dial | Set to motor rated current. [1] | Range in A. |
| Trip mechanism | Trip on overload and phase loss. [1] | Trip class; phase loss. |
| Ambient compensation | Limits temperature effect. [1] | Range. |
| Reset and test buttons | Manual or automatic reset. [1] | Mode. |
| Auxiliary contacts 95–96, 97–98 | Open control circuit; signal. [1] | Ratings. |
07
The setting range has to bracket the motor's rated current and the trip class has to outlast its start; the remaining rows come from the chosen relay's own data sheet. [1]
| Field | What it means and published values | Ask the supplier for |
|---|---|---|
| Rated Insulation Voltage (V) | Relay rating; e.g. 690 V AC (IEC) for the Schneider Electric LRD10. [3] | Value. |
| Rated Operational Voltage (V AC) | Supply; e.g. up to 690 V AC for CHINT NR2 and NXR. [4] | Value. |
| Current Setting Range (A) | Covers motor rated current, e.g. 1.3–1.8 A up to 60–80 A ranges [1]; CHINT NR2 series spans 0.1–150 A. [4] | Range. |
| Trip Class | 10 A, 10, 20 or 30; e.g. class 20 = 6–20 s at 7.2 × Ie. [1] [2] | Class; curves. |
| Frequency (Hz) | Supply; e.g. 50/60 Hz for CHINT NR2. [4] | Value. |
| Operating Temperature (°C) | E.g. compensated −25 to +55 °C. [1] | Range. |
| Degree of Protection | Terminals and front; e.g. IP20 for the LRD10. [3] | IP. |
| Contact Rating (A) | Auxiliary contacts; e.g. 3 A at 120 V AC-15 on the LRD10. [1] [3] | Ratings. |
| Reset Time (s) | Cooling before reset. | Value. |
| Mechanical Endurance (operations) | Life; e.g. ≥ 1,000,000 operations for CHINT NR2 and NXR. [5] | Value. |
| Electrical Endurance (operations) | Life; e.g. ≥ 100,000 operations for CHINT NR2 and NXR. [5] | Value. |
| Weight (kg) | Mounting; e.g. 0.124 kg for the LRD10. [3] | Weight. |
| Tripping threshold and phase failure | 1.14 ± 0.06 × Ir; 130% Ir on two phases for phase failure in one relay. [3] | Threshold and phase-loss behaviour. |
08
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Motor: [rated current; voltage; start time] Load: [type; inertia] Relay: [setting range; trip class] Functions: [phase loss; ambient compensation; reset] Starter: [contactor type; mounting] Protection: [fuse or breaker rating] Approvals: [IEC; UL; CSA]
ETEK lists class 10 adjustable-current relays; CNC Electric, Changan Group and VIOX Electric list them with contactors and motor protectors; HIITIO lists UL-approved overload relays; Wenzhou Kinee lists them with contactors and VFDs.
Lists class 10 adjustable-current thermal overload relays with contactors and MPCBs.
Product scope checked 2026-10-11
Lists thermal overload relays with contactors and motor protectors.
Product scope checked 2026-10-11
Lists thermal overload relays with contactors and MPCBs.
Product scope checked 2026-10-11
Lists UL-approved overload relays with IEC contactors and motor protection breakers.
Product scope checked 2026-10-11
Lists thermal overload relays with contactors and MPCBs.
Product scope checked 2026-10-11
Lists thermal overload relays with contactors and VFDs.
Product scope checked 2026-10-11
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
Under IEC 60947-4-1 the relay trips within 10 s (ABB gives 4–10 s from cold) at 7.2 times its setting current. [1]
The motor's rated current; ABB notes no tripping at 1.05 times and tripping at 1.2 times setting current within 2 hours. [1]
ABB lists protection against overload, phase-failure imbalance and phase loss. [1]
Schneider's LRD10 data sheet gives a tripping threshold of 1.14 ± 0.06 times the set current, per IEC 60947-4-1. [3]
Sources checked 2026-10-12. ABB's data sheet summarises IEC 60947-4-1 trip classes for its relays; Schneider Electric's LRD10 data sheet and CHINT's NR2 and NXR product pages give setting ranges, trip class, contact ratings and service conditions for specific series; ETEK describes its own range. Tripping curves for any relay must come from its maker.
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