When a low-voltage circuit breaker trips during a motor start, the underlying cause is almost always a mismatch between the motor’s starting profile and the breaker’s thermal characteristics. In simple terms—the motor’s starting time at high inrush current exceeds the breaker’s inverse-time overcurrent protection delay. Once this limit is breached, a nuisance trip becomes inevitable.

The Core Issue: Motor Starting Duration vs. Breaker Thermal Curve

During startup, an induction motor draws a current several times its rated full-load current—typically 6 to 8 times higher. This surge, known as the starting current, persists until the motor reaches its nominal speed. Standard motor protection circuit breakers, particularly thermal-magnetic types, are designed for motors with starting times shorter than 10 seconds.

If the starting period approaches or surpasses this threshold, the breaker’s thermal element (the bimetal strip) heats up and activates the tripping mechanism, mistaking the extended inrush current for an overload condition.

This explains why, in most cases, a standard breaker is unsuitable for motors with heavy starting loads or prolonged acceleration times. The protection device simply interprets the normal start-up phase as a fault.

Can You Simply Increase the Overload Setting?

A tempting solution might be to raise the overload adjustment current to “bypass” the problem. However, this approach is ill-advised. While increasing the thermal threshold may prevent unwanted tripping during startup, it also delays the breaker’s response to true overloads. This compromise can allow excessive thermal stress on the motor windings, accelerating insulation aging or even leading to motor burnout.

The more prudent approach is to analyze the motor’s actual starting current multiple and acceleration time, or measure the precise moment of tripping during startup. Only with these values can an appropriate protection class or device configuration be selected.

Solution A: Adjustable Trip Class Circuit Breakers

Certain motor protection circuit breakers allow the trip class (Class) to be modified. Examples include:

• Schneider NSX Mic2.2M

• ABB Tmax PR222MP

These advanced breakers feature a dial or selector switch on the front panel. If the current setting is at Class 10 and the motor cannot complete its startup sequence before tripping, the selector can be turned to Class 20 or higher.

Increasing the class extends the permissible thermal delay, allowing longer startup times while maintaining overload protection integrity. This method is safe, precise, and specifically intended for motors with heavy or delayed startups.

Solution B: Non-Adjustable Trip Class Breakers

Many common motor protection breakers, such as:

• Schneider GV Series

• ABB MS Series

• Siemens 3RV Series

are factory-set to Trip Class 10A (equivalent to approximately 10 seconds). These breakers cannot be tuned to accommodate longer starting durations. For motors requiring 12, 15, or even 20 seconds to reach full speed, these devices will almost certainly trip prematurely.

In such cases, a more effective approach is to separate short-circuit and overload protection functions. Replace the thermal-magnetic breaker with a magnetic-only breaker, responsible solely for short-circuit protection. Then, install an electronic overload relay with an adjustable trip class to manage thermal overload conditions.

Unlike traditional bimetal thermal relays—whose trip class is fixed at 10A—electronic overload relays offer precise programmability, allowing engineers to fine-tune response times to match the motor’s startup behavior.

Solution C: For Distribution-Type Circuit Breakers

In scenarios where the breaker used is a standard miniature circuit breaker (MCB) or molded-case breaker intended for general distribution rather than motor protection, mis-tripping during motor startup is even more probable. These devices are not designed to tolerate extended inrush currents or high starting torque conditions.

Here, the same principle applies: employ a dedicated magnetic short-circuit breaker in conjunction with an electronic overload relay. This division of duties ensures reliable coordination between short-circuit and overload protection without compromising the motor’s starting process.

Conclusion

A low-voltage breaker tripping during motor startup is not a sign of poor product quality—it is a symptom of mismatched protection coordination. Motors with high inertia loads, soft-start conditions, or long acceleration periods require appropriately rated protection devices.

By selecting circuit breakers with adjustable trip classes, or pairing magnetic-only breakers with intelligent electronic overload relays, engineers can eliminate nuisance trips, extend motor life, and maintain both safety and operational stability.

In modern motor control systems, precision protection configuration is not an option—it is a necessity.