One of the most critical devices protecting an electric motor is the motor protection circuit breaker (MPCB). An MPCB combines both overload (thermal) and short-circuit (magnetic) protection in a single device and is adjustable to the motor rated current. A correctly selected and correctly set MPCB protects the motor from burnout, the contactor from welding and the wiring from fire; an MPCB that is wrongly selected or left unset will either trip needlessly or burn the motor. In this guide we cover, from a practical installation perspective, what an MPCB is, selecting the setting range by rated current, thermal and magnetic settings, trip class, coordination with the contactor and comparison with a thermal relay.

Motor supply panel with motor protection circuit breaker (MPCB) and contactor

What Is a Motor Protection Circuit Breaker (MPCB)?

An MPCB is an adjustable thermomagnetic breaker that protects the motor against overcurrents. It contains two protection elements: a thermal (bimetal) element detects slowly developing overloads, and a magnetic (electromagnetic) element interrupts sudden short-circuit currents very quickly. The most important feature of the MPCB is that it can be adjusted to the motor rated current via a dial. This way a single breaker can serve motors across a wide current range; you read the motor nameplate current and set the dial to that value.

In the motor supply chain the MPCB typically sits after the main switch and before the contactor. For general protection-equipment planning and which devices to request together, our electric motor protection devices article offers a checklist. To compare with the classic thermal relay and fuse approach, see electric motor protection: thermal relay and fuse selection.

What Problems Does an MPCB Protect Against?

An MPCB mainly protects against three problems: overload (mechanical stress on the motor, jamming, current rise on phase loss), short circuit (insulation breakdown in the winding or cable) and, in some models, phase loss/phase imbalance. Overload is often the real cause of motor burnout; that is why it is vital for the thermal setting to match the motor's true rated current. For why a motor heats up and the signs of overload, the electric motor failures: symptoms and causes article guides you.

Selecting the Setting Range by Rated Current

The first step in MPCB selection is choosing the correct current setting range. MPCBs are sold in ranges such as "4-6.3 A", "9-14 A", "17-23 A". The range of the device you choose should leave the motor rated current around the middle of the range. For example, if the motor rated current is 11 A, an MPCB with a "9-14 A" range is ideal; "8-12.5 A" works too, but a device that pushes 11 A to the top of the range leaves no setting flexibility.

To read the rated current correctly, look at the motor nameplate. The current on the nameplate varies with voltage and connection (star/delta); so selecting the correct value matters. For reading the nameplate, our reading the IE3 motor nameplate article, and for star/delta voltage selection the electric motor terminal connection article are directly applicable. To select cable size, fuse and contactor together by rated current, the IE3 motor rated current: cable, fuse and contactor selection content is a comprehensive reference. If you need HP-kW conversion, see HP or kW.

MPCB in Star-Delta and Inside-Delta Connection

If the motor starts star-delta, the MPCB can be selected by the current in the delta winding leg (1/√3 of the line current); this requires attention to the wiring diagram. For the star-delta diagram see star-delta wiring diagram and for a comparison of starting methods see star-delta vs soft starter. For MPCB placement in soft-starter systems, the soft starter compatibility with IE3 motors article is useful.

Thermal and magnetic setting dials and trip class marking on an MPCB

Thermal (Overload) and Magnetic (Short-Circuit) Settings

The two protection functions of an MPCB are set differently:

Thermal (overload) setting: This is set on the dial to the motor rated current. The thermal element trips if the current exceeds the rated value and stays high for a certain time. It is essential to set it to the motor's true full-load current: set too high, the motor is not protected under overload and burns; set too low, it trips needlessly in normal operation. In practice the thermal setting is brought slightly above the measured actual running current, without exceeding the rated current.

Magnetic (short-circuit) setting: This element trips within milliseconds on sudden, very high currents (short circuit). In most motor MPCBs the magnetic threshold is fixed at about 12-13 times the rated current; because the motor inrush can reach 6-8 times and the magnetic protection must not trip at startup. In some models the magnetic threshold is adjustable. You can find why the inrush current is high in the starting current in an asynchronous motor (LRA) article.

Trip Class: Class 10 and Class 20

MPCBs and thermal relays are labelled with a trip class that states how long they take to trip under overload. The most common classes are Class 10 and Class 20. This number indicates, at a current of 7.2 times the motor rated current, in how many seconds the device trips:

  • Class 10: For normal-start motors. Trips in about 10 seconds at 7.2 times current. Suitable for most fast-starting loads such as pumps, fans and compressors.
  • Class 20: For high-inertia, long-start motors. Trips in about 20 seconds at 7.2 times current. Prevents needless tripping during the start of long-starting loads such as large fans, centrifuges and conveyors.

If the trip class is wrong, a Class 10 device on a long-starting motor may trip on every start. So the trip class must be chosen according to the motor's starting time. To assess duty type and start profile, see electric motor duty type (S1-S6) selection. For high-inertia impact loads, the motor selection under impact load content guides you.

Coordination with the Contactor

An MPCB is not used alone to switch the motor on and off; that job belongs to the contactor. The MPCB takes on protection and isolation. A correct motor supply group usually consists of an MPCB + contactor combination: the MPCB provides short-circuit and overload protection, while the contactor performs the normal switching. The coordination of these two describes how, in a short circuit, the contactor and MPCB behave as expected without damaging each other.

Coordination is defined at two levels: in Type 1 coordination the contactor or MPCB may be damaged after a short circuit but no hazard arises; in Type 2 coordination the devices remain usable after a short circuit (apart from light contact welding). In critical plants Type 2 coordination is preferred. In contactor selection the motor rated current and duty category (AC-3) are decisive; the rated current: cable, fuse and contactor selection article gives detail. To verify protection settings during commissioning and first start, use the motor commissioning and first-start checklist content.

MPCB Compared with a Thermal Relay

The classic motor protection method is a separate thermal relay + contactor + fuse trio. The MPCB gathers thermal and magnetic protection in one device. The main differences between the two approaches are:

  • MPCB: Both overload and short-circuit protection in one device; no separate fuse needed (if the short-circuit breaking capacity is sufficient). More compact panel. Easy setting via the dial.
  • Thermal relay + fuse: The thermal relay only protects against overload; a separate fuse or breaker is needed for short circuit. More parts, but can be economical at high powers.

On small and medium power motors the MPCB both saves space and shortens installation time. At high powers the fuse + thermal relay combination is still common. For a detailed comparison of the two methods, see our thermal relay and fuse selection article. If you want to strengthen protection further by monitoring winding temperature directly, the temperature monitoring with PT100 and PTC thermistor content is complementary.

Practical Installation Tips and Common Mistakes

The most common mistakes in MPCB installation on site are: never making the thermal setting (leaving the device at the factory value), setting to an approximate value instead of the motor's measured actual current, choosing the trip class without regard to the starting time, and using the MPCB frequently as an on-off device instead of the contactor. An MPCB is not designed for frequent switching; daily operation should be done with the contactor. In correct installation, the actual running current is measured with a clamp meter during commissioning and the thermal setting is fine-tuned accordingly. This step both prevents needless tripping and protects the motor against real overload. For all the steps of commissioning, follow the motor commissioning and first-start checklist article, and to avoid pairing the device with the wrong current, the nameplate-matching steps in avoid wrong motor delivery are very valuable.

Frequently Asked Questions

What should I set the MPCB to relative to the motor rated current?

Set the thermal adjustment to the motor's full-load (rated) current on the nameplate. If the motor's measured actual running current is below the rated current, you may set it to the measured value with a small margin; but never set it above the rated current. Setting above the rated current renders the overload protection ineffective.

Why does the MPCB not trip at startup but trips in normal operation?

Because the magnetic threshold is fixed above the inrush current (usually 12-13 times), the MPCB does not trip at startup. If it trips in normal operation, either the thermal setting is too low, the motor really is overloaded (mechanical jam, phase loss, bearing fault), or the trip class is wrong for the starting time.

Does an MPCB eliminate the need for a separate fuse?

If the MPCB's short-circuit breaking capacity (kA) covers the possible short-circuit current at the site, in most applications a separate short-circuit fuse is not needed. But if the short-circuit current exceeds the MPCB capacity, a back-up fuse is required ahead of it. This value must be verified from the MPCB label and the site short-circuit calculation.

Get a Quote

Let us determine the right MPCB setting range, trip class and contactor coordination for your motors together. Contact us to plan a protection solution suited to your motor rated current and duty type. To get a quote now, visit our contact page or call us at +90 (532) 345 49 86. You can review our entire high-efficiency electric motors range, and for geared applications take a look at our worm gear reducers category and our homepage.

Purchasing and Installation Checklist

Before selecting and installing an MPCB, verify the following:

  • Has the motor rated (full-load) current been read correctly from the nameplate?
  • Does the selected MPCB setting range leave the rated current around the middle?
  • Is the thermal adjustment set to the motor's true rated current (not above)?
  • Is the trip class (Class 10/20) suitable for the motor's starting time?
  • Is the magnetic threshold above the inrush current (no needless tripping)?
  • Is the contactor selected by the motor's AC-3 duty category?
  • Is the MPCB-contactor coordination (Type 1/Type 2) defined?
  • Does the MPCB short-circuit breaking capacity (kA) cover the site short-circuit current?

After completing this list, you can obtain a correct quote with the steps in our information to provide when requesting a quote article, and procure the protection equipment together with the motor.