Thermal vs Electronic Overload Relay: Which Should You Use?
Every three-phase motor starter needs overload protection, and for decades the default was the thermal (bimetallic) overload relay mounted under the contactor. Electronic overload relays are now replacing them in many panels. This article compares the two so you can decide which suits your motors.
How a thermal overload relay works
Motor current passes through heaters that warm bimetallic strips. When the current stays high for long enough, the strips bend far enough to trip a contact and open the contactor coil circuit. The design is simple and cheap, but it has some built-in limits:
- Ambient temperature affects it. A hot panel in an Indian summer makes it trip early; a cold one makes it trip late.
- Limited setting range. Each relay covers a narrow current band, so you need many variants.
- Weak phase-loss protection. Some models have differential tripping, but the response to single phasing is slow.
- No information. After a trip you do not know what caused it.
How an electronic overload relay works
An electronic overload relay measures current with internal current sensors (or external CTs for large motors) and uses electronics to decide when to trip. That allows:
- Accurate trip settings that do not change with panel temperature
- Separate adjustment of start delay and trip time
- Fast, reliable phase-loss and locked-rotor protection
- Wide current ranges, so fewer variants to stock
- On digital models: current display, fault indication, undercurrent, unbalance and earth fault
The EOCR SS, for example, has separate dials for current, start delay (D-Time, 0.2–30 s) and trip time (O-Time, 0.2–10 s), so you can allow a long start for a heavy load and still trip quickly on a running overload.
Side-by-side comparison
| Feature | Thermal overload relay | Electronic overload relay |
|---|---|---|
| Sensing | Bimetal heated by current | Electronic current measurement |
| Effect of panel temperature | Significant | Negligible |
| Setting range per model | Narrow | Wide |
| Phase loss / single phasing | Slow or limited | Fast and dependable |
| Locked rotor | Trips on the overload curve only | Dedicated, fast protection |
| Start delay adjustment | Fixed by trip class | Independently adjustable |
| Dry run / undercurrent | No | Yes on digital models |
| Fault indication | Trip flag only | LEDs or display showing cause |
| Upfront price | Lower | Somewhat higher |
When a thermal relay is still acceptable
For small, non-critical motors in a temperature-controlled room, with stable supply and easy replacement, a thermal relay can be enough.
When you should upgrade to electronic
- Pumps, especially submersible and borewell pumps, where a failure is expensive to fix
- Sites with frequent single phasing or poor supply quality
- Hot panels or outdoor kiosks
- Heavy-start loads such as crushers, mixers and conveyors
- Critical process motors where downtime costs more than the relay
If you also need dry run, unbalance or earth fault protection, go straight to a digital relay such as the EOCR 3DE / 3EZ. See also our digital overload relay features guide and the electronic overload relay buyer’s guide.
Replacing a thermal relay in an existing starter
Electronic relays can usually be retrofitted into DOL and star-delta starters. The relay’s trip contact is wired into the contactor coil circuit in the same way. Check the mounting, the supply voltage needed by the relay, and the current range against the motor’s full-load current. In a star-delta starter, decide whether the relay measures line current or phase current and set it accordingly.
Frequently Asked Questions
Is an electronic overload relay better than a thermal overload relay?
For most industrial and pump motors, yes. It is more accurate, is not affected by panel temperature, protects reliably against phase loss and locked rotor, and can show the cause of a trip.
Can I replace a thermal overload relay with an electronic one?
Yes, in most DOL and star-delta starters. Wire the relay’s trip contact into the contactor coil circuit and set it to the motor full-load current.
Does an electronic overload relay need a separate power supply?
Many do. The EOCR SS, for example, accepts a universal 24–240 V AC/DC supply, so it can usually take its supply from the existing control circuit.
Talk to Concord
Concord Automation & Controls has supplied motor protection relays and industrial relays to Indian industry since 1996 and is the exclusive ComatReleco (Switzerland) distributor for India, Bangladesh and Mauritius. Share your motor, pump or panel details and we will recommend the right model: call or WhatsApp +91 96670 40511, email contact@concordrelay.com, or use our contact page.