Overcurrent Relay vs Overload Relay: Differences Explained
The terms “overcurrent relay” and “overload relay” are often used as if they mean the same thing. They overlap, but they are not the same. Knowing the difference helps you choose the right protection for a motor feeder and set it correctly.
Overload: a modest rise over a long time
An overload is current somewhat above the motor’s rated full-load current (FLC), for example 110–150%, lasting minutes. Typical causes are a jammed conveyor, a pump working off its curve, low voltage or worn bearings. The motor keeps running but heats up. If nothing stops it, the winding insulation ages quickly and eventually fails.
An overload relay protects against this. It allows short overcurrents (such as starting) and trips when the heating effect becomes dangerous.
Overcurrent: any current above the set level
Overcurrent is a broader term for any current above a set threshold. That covers overloads, locked rotor (typically 6–8 times FLC) and, at the extreme, short circuits. An overcurrent relay trips when current exceeds its setting for its set time.
Short circuit is a separate job
Short-circuit currents can be hundreds of times FLC and must be interrupted in milliseconds by a device rated to break that current: an MCCB, MPCB or fuse. A motor relay opens the contactor coil circuit, and a contactor is not designed to break a short circuit. So every motor feeder needs both:
- Short-circuit protection (MCCB, MPCB or fuse) upstream
- Overload / overcurrent protection (a motor protection relay) controlling the contactor
Definite-time vs inverse-time tripping
| Definite-time | Inverse-time | |
|---|---|---|
| How it trips | After a fixed time once current exceeds the setting | Faster at higher currents, slower near the setting |
| Behaves like | A timer with a current threshold | A thermal model of motor heating |
| Good for | Jams, locked rotor, loads where you know the acceptable time | General motor overload, matching the motor’s thermal capacity |
The EOCR 3DE / 3EZ lets you select either characteristic: definite-time operation of 0.2–30 s, or thermal inverse curves from Class 1 to Class 30. A start (manoeuvre) delay of 0–200 s keeps it from tripping during a long start. Read more in our article on definite-time vs inverse-time motor protection.
What an “electronic overcurrent relay” does
An electronic overcurrent relay (often called an EOCR) combines overload and overcurrent protection in one device and usually adds phase loss and locked-rotor protection. Digital versions add undercurrent, unbalance, reverse phase and earth fault. In practice, a modern motor protection relay does everything a separate overload relay and overcurrent relay used to do.
Setting tips
- Set the current to the motor’s actual FLC (from the nameplate), not to the relay’s maximum.
- Set the start delay a little longer than the measured starting time.
- Use a shorter definite trip time on loads that jam (crushers, conveyors) to limit mechanical damage.
- Check that the upstream MCCB or fuse coordinates with the relay so each device handles its own range of faults.
Frequently Asked Questions
Is an overload relay the same as an overcurrent relay?
No. Overload is a moderate, sustained rise above rated current; overcurrent is any current above a set level. Modern motor protection relays handle both.
Does a motor protection relay protect against short circuit?
No. It controls the contactor, which cannot safely break short-circuit current. Always fit an MCCB, MPCB or fuse upstream.
Which is better for a motor, definite-time or inverse-time?
Inverse-time follows motor heating and suits general overload protection. Definite-time suits jams and locked rotor where you want a predictable trip time. Relays like the EOCR 3DE let you choose.
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.