Stop Using MCBs for Motors (The "Hidden" Danger)

One of the most common issues in industrial panels is a burnt-out motor that was supposedly "protected." When opening the panel, a standard C-Curve Miniature Circuit Breaker (MCB) often feeds the motor contactor.

Using standard lighting/distribution MCBs (like the Acti9 xC60) to protect industrial motors risks equipment failure. Standard breakers fail to protect motors properly, and the correct alternatives are the Thermal Overload Relay (LRD) and the Motor Protection Circuit Breaker (MPCB, GV2).

The Problem: The "Inrush" Trap

AC motors draw an inrush current of 6 to 8 times their rated Full Load Amps (FLA) during startup.

A standard C-Curve MCB trips instantly at 5-10 times its rated current, causing nuisance tripping on motor starts.

To avoid this, technicians often upsize the MCB (e.g., 20A for a 10A motor), allowing operation but providing no overload protection—if the motor pulls 18A due to a seized bearing, the windings melt while the breaker remains closed.

Solution 1: Contactor + Thermal Overload Relay

The traditional setup uses a contactor (like TeSys D) for switching and a Thermal Overload Relay (like TeSys LRD) mounted below it.

The LRD uses bimetallic strips that heat proportionally to motor windings, tripping the control circuit on prolonged overloads (e.g., heavy conveyor load).

LRDs provide overload and phase failure protection but require upstream short-circuit protection like fuses or a breaker.

Solution 2: MPCB (TeSys GV2/GV3)

The MPCB (e.g., TeSys GV2 or GV3) combines short-circuit, overload, and phase loss protection in one device, ideal for direct replacement of MCB + overload setups.

It handles high inrush without nuisance trips via magnetic settings (13-18x Ir typically), offers adjustable thermal overload, and trips on phase imbalance or loss.

GV2 suits motors up to about 15kW; GV3 for larger.

Setting the Dial

Set the overload dial on LRD or GV2 to the motor nameplate FLA (e.g., 4.5A).

Schneider engineers tripping curves (e.g., Class 10A/20) to account for service factors automatically—avoid adding margins.

Comparison Table

Leave MCBs for distribution boards. Use GV2 for small motors or LRD + contactor for complex setups—your motors and maintenance team will benefit.