Motor Protection Devices: MPCB vs Overload Relay vs Motor Protection Relay
A motor feeder can include several protective devices, and buyers often ask which one they actually need. The short answer: most motors need one device for short circuits and another for everything else. This guide explains the common motor protection devices and how to combine them.
The faults a motor needs protection from
- Short circuit and earth short
- Overload (sustained overcurrent)
- Locked rotor / stall
- Single phasing (phase loss)
- Phase unbalance and reverse phase
- Dry run / loss of load (undercurrent)
- Earth leakage developing slowly in the insulation
No single device covers all of these well, which is why devices are combined.
The main devices
Fuses and MCCBs
These interrupt short-circuit currents. They are essential, but their overload protection, where present, is coarse and not tuned to the motor.
MPCB (motor protection circuit breaker)
An MPCB combines short-circuit protection with an adjustable thermal overload element in one unit, sized for small and medium motors. It is compact, but the overload element is thermal, it has limited phase-loss and no undercurrent or earth fault functions, and it gives no indication of the fault.
Thermal overload relay
Mounted with the contactor, it gives basic overload protection. See our comparison of thermal vs electronic overload relays.
Electronic / digital motor protection relay
This measures motor current electronically and controls the contactor. Depending on the model it covers overload, locked rotor, phase loss, unbalance, reverse phase, undercurrent and earth fault. Digital models display the current and the cause of a trip. Examples: EOCR SS, EOCR DS, EOCR 3DE / 3EZ.
What each device covers
| Fault | Fuse / MCCB | MPCB | Thermal OLR | Digital motor protection relay |
|---|---|---|---|---|
| Short circuit | Yes | Yes | No | No |
| Overload | Coarse | Yes (thermal) | Yes (thermal) | Yes (accurate) |
| Locked rotor | Limited | Limited | Limited | Yes |
| Single phasing | No | Partial | Partial | Yes |
| Unbalance / reverse phase | No | No | No | Yes (model-dependent) |
| Dry run / undercurrent | No | No | No | Yes (3DE / 3EZ) |
| Earth fault | Only heavy faults | No | No | Yes with ZCT (3EZ) |
| Fault indication | No | No | Trip flag | Display / LEDs |
Recommended combinations
- Small, non-critical motor: MPCB + contactor, or fuse + contactor + thermal overload relay.
- General industrial motor: MCCB or fuse + contactor + electronic overload relay (EOCR SS or DS).
- Pumps, borewells, critical process motors: MCCB or fuse + contactor + digital relay with undercurrent and earth fault (EOCR 3DE / 3EZ with ZCT).
- Large motors: add external CTs sized to the motor current.
Concord also supplies complete DOL and star-delta starters fitted with the right protection relay.
Frequently Asked Questions
Can an MPCB replace a motor protection relay?
For small, non-critical motors it can be enough. It does not provide dry run, unbalance, earth fault or fault indication, so for pumps and critical motors an electronic motor protection relay is the better choice.
Do I still need an MCCB if I use a motor protection relay?
Yes. The relay does not interrupt short circuits. Keep a correctly rated MCCB or fuse upstream.
What is the best motor protection device for a submersible pump?
An MCCB or fuse for short circuit, plus a digital relay with undercurrent and earth fault such as the EOCR 3EZ with a ZCT.
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.