Electrical Motor Circuit Protection

Part 3(b). Circuit breakers

By Ed Butts, PE, CPI

Figure 1. Trip mechanism and curve for an inverse time circuit breaker.

We began a two-part series on circuit breakers used for protection of motor electrical circuits in last month’s edition of Engineering Your Business and wrap things up here by discussing the basic operational function of circuit breakers and the role they play as electrical motor protection devices.

Circuit Breaker Types

There are two fundamental types of circuit breakers:

  1. Inverse time circuit breakers, which are also called thermal-magnetic circuit breakers
  2. Instantaneous trip circuit breakers, which are also known as magnetic-only circuit breakers.

Inverse Time Circuit Breakers

There are two trip units in an inverse time circuit breaker, as this circuit breaker type possesses both thermal and instantaneous (magnetic) trip characteristics and is preset to trip at preset or adjustable settings.

Figure 2. Shunt trip breaker wiring.

Figure 1 illustrates the tripping mechanisms and regions for both types of currents. The magnetic region trip unit is used to instantaneously respond as protection against short circuit current. It includes a solenoid that produces a strong magnetic field due to high short circuit current to instantly trip the circuit breaker.

The thermal region trip unit is used for protection against overloading. It uses a bimetallic contact that distorts with a time change in temperature.

Based on the device’s specific settings, at lower temporary overcurrent levels such as those that occur during motor starting, the circuit breaker will delay tripping for some time period— usually three to five seconds—to verify if this temporary fault current continues.

After passing the overcurrent for a specific period, if the circuit is still experiencing higher current, the circuit breaker will trip to break the circuit and disconnect the load from the power supply. This is called the inverse time characteristic.

Inverse time circuit breakers are generally designed to trip at 100% to 125% of their continuous current rating. Their characteristic inverse time trip settings under overload conditions are ideally suited for many applications, ranging from typical residential loads to heavy industrial motor loads.

When the circuit develops an overload as the pass-through current becomes greater, the heat generation is also increased; thus, the bimetallic element deforms to a certain extent to push the mechanism to move and ultimately trip. This means the greater the current, the shorter the operating time.

Many in the industry use the slang term “thermal mag” for inverse time breakers. The National Electrical Code (NEC 430.52) requires inverse time circuit breakers to be sized to a maximum of 250% of a motor’s full-load amperes.

Instantaneous Trip Circuit Breakers

Figure 3. Solid-state circuit breaker components.

Instantaneous trip circuit breakers trip when the current in the circuit reaches a specific value that the breaker trip mechanism is set to. Therefore, it trips instantaneously with no added time delay.

Instantaneous trip circuit breakers are designed and primarily used for one specific purpose—to provide branch-circuit short circuit protection for motor circuits.

They do not possess a thermal trip function and will not protect from an overcurrent due to motor overload alone. An instantaneous trip circuit breaker should be used only if adjustable, generally between 500% to 1300% of its full load current rating, and as part of a listed combination motor controller having coordinated motor overload, short circuit, and ground fault protection in each conductor with the setting adjusted to no more than the value specified in NEC Table 430.52.

When there is a shunt fault or short circuit, the magnetic field generated by the large current (generally 10 to 13 times) overcomes the reaction spring compression, the trip unit rapidly pulls the operating mechanism, and the breaker trips instantaneously. They are also used as motor circuit protector breakers (MCPs) discussed in greater detail in a little bit.

A shunt trip circuit breaker is a device used to protect circuit breakers from nuisance tripping. For example, in the event of a short circuit, the shunt trip breaker senses the excess current and disconnects the power supply by shunting it to ground instead of allowing it to pass through the circuit breaker.

Figure 4. Solid-state circuit breaker typical adjustments.

A shunt trip breaker is always installed in conjunction with a circuit breaker since it does not replace the need for a standard circuit breaker but works as an auxiliary safety device that protects the main circuit breaker from nuisance tripping. This is often due to high voltage or high current levels caused by momentary overload conditions, such as arc flash protection response or switching events that may cause fault currents above allowable levels.

The shunt trip breaker is a combination of the shunt trip accessory and the main circuit breaker and is installed on the main breaker to add protection to the electrical system. This also adds security to the electrical system as it manually or automatically cuts the electric supply in the circuit. This accessory can help prevent short circuits and avoid electrical damage should a disaster occur in the electrical system.

The shunt trip breaker wiring comprises two wires. One is connected to the ground and the other to a control system (Figure 2). The control system can be connected to a sensor or to a manual switch. When activated, the shunt trip accessory will cause the main breaker to trip.

There are also electronic, solid-state circuit breakers (Figure 3), which use current transformers or sensors and solidstate electronics to monitor the currents of each phase and compare them with the adjustable or set values.

When the current is abnormal, the microprocessor sends out a signal to cause the electronic trip unit to trigger the magnetic latch and the operating mechanism to open the contacts. Most electronic circuit breakers operate using mechanical pressure applied to the contacts from a spring with current sensing from either a relay or electronic sensor.

Figure 5a. Two examples of a motor circuit protector (MCP) breaker.

Although the basic function of all solid-state circuit breakers is reasonably uniform, different manufacturers will use unique circuitry and adjustments for their breakers.

Solid-state circuit breakers generally possess the ability to be adjusted for parameters such as trip current, thermal and instantaneous pickup, long and short time delay trips, and ground fault detection settings (Figure 4). These breakers are often used or NEC-required to protect circuits for large services more than 1000 amps and large motors, particularly if ground fault protection is required or desired.

Motor Circuit Protector Circuit Breakers

A motor circuit protector (MCP) (Figure 5a) is usually an instantaneous-trip type of circuit breaker, generally used to control and protect electrically connected electric motors. MCPs are typically used in combination starters for motors and for the motor’s branch circuit, short circuit, and ground fault protection in conjunction with a motor starter with overload protection using heaters or electronic overloads.

MCPs typically have an adjustable dial on the front side of the breaker that allows the installer to define and set the trip current in direct amps or as a percentage of the full-load rating. When using MCPs for motor circuit protection, it’s important to consult the breaker manufacturer’s data sheet to determine the trip threshold setting as the NEC has limits for the setting. Not all breakers include this information on the breaker itself, or the information may be covered with an operator lever such as those found in a motor control center.

Figure 5b. Typical MCP circuit breaker adjustments.

Unless specifically designed for overload functions, it’s important to note that most MCPs will not protect the motor from overload or overcurrent. This is the reason that standard MCPs can only be used as part of a listed combination starter assembly. The overload unit will protect the circuit conductors and the motor from overcurrent and overload, respectively.

This type of circuit breaker possesses a carefully calibrated internal mechanism that expands in response to temperature and is calibrated to interrupt an electrical current immediately when the rated current of the breaker is exceeded through a short circuit, while integral thermal protection will eventually interrupt a marginal overcurrent condition or overload.

The reason why the thermal protection mechanism is designed with a slower response time is to allow passage of a short duration overcurrent, which is a normal part of the operation in many types of motor-driven equipment. Electric motors often draw between five to eight times their rated current during initial startup, but only for a few seconds.

Magnetic protection interrupts any fault current which possesses much higher magnitudes than an overload and occurs during line-to-line or line-to-ground faults or downstream short circuits.

As implied by its name, magnetic protection is based on induction onto a coil inside the circuit breaker that produces a strong magnetic field when there is a fault current. This results in an immediate tripping of the internal contacts.

Figure 6a. Typical circuit breaker operating mechanism.

Since faults represent a high-risk condition and are never part of normal operation, they must be cleared immediately. To provide motor overload protection, it is necessary to add an overload relay and contactor below an MCP breaker.

The main advantage of MCPs is their trip response can be fine-tuned according to the expected motor inrush current that may vary according to the motor horsepower and the type of motor starter used.

When feeding a main or feeder circuit or supplying a branch circuit with more than one live conductor, each individual live conductor must be protected by a breaker or fuse. To ensure that all live conductors are simultaneously interrupted when any single pole trips on a circuit breaker, a common trip-style breaker must be used for 230-volt, single-phase, and all three-phase circuits.

These may either contain two or three tripping mechanisms within one case, or for smaller breakers the poles may be externally tied together through their operating handles. Single-pole circuit breakers are commonly used for 120-volt loads only. Single-phase, 230-volt loads, including those for 1/2 HP to 15 HP motor loads, can use both 120 VAC phases with a two-pole, common trip breaker. Three-pole common
trip breakers are typically used to supply three-phase electric power to large motors or to distribution and sub-feed panels.

Phase unbalance and loss can severely damage a three-phase motor, so an MCP will disconnect the motor in either case as soon as the fault is detected. A thermal delay prevents the motor from being immediately restarted after an overload, giving the motor adequate time to cool as an overheated motor can be permanently damaged if it is prematurely restarted. They may be used for group motor protection only when the circuit breaker is tested, listed, and marked for use in a group installation (see NEC 430.53[A] and [B] for exceptions).

Figure 6b. “Closed” circuit breaker.

There are no circuit breakers listed for group motor protection except for HVAC applications, in which case they are marked as HACR-rated (heating, air conditioning, refrigeration). They are suitable for use as a motor disconnecting means per NEC 430.109, as a motor controller (i.e., On-Off function) per NEC Article 430, Part VII, and as both a motor disconnecting means and motor controller (NEC 430.111).

The NEC states provisions of Article 430, Part III shall not apply to motor circuits rated more than 1000 volts nominal. Thus, this column’s focus applies to typical motors that operate below 1000 volts or primarily low voltage applications. These are circuit breakers without overload (thermal) protection capability and are intended to provide only branch circuit, short circuit, and ground fault protection for individual motor branch circuits.

Because they are UL-recognized, but not UL-listed, they cannot be used with external control as NEC 430.52 requires that they shall only be used as part of a listed combination controller. MCPs are short circuit tested only in combination with a motor controller and approved overload device (i.e., a motor starter). They are not labeled with an interrupting rating by themselves. Per NEC 430.109-Exception 7, they may be used as a motor disconnecting means when part of a listed combination motor controller.

Some specific types of MCP circuit breakers include both overload protection using thermal or electronic sensing, and fault protection using a magnetic trip function. This type of motor protection circuit breaker can be considered a subtype of a thermal magnetic circuit breaker, but with additional functions that are specially designed to protect electric motors.

The trip characteristics of this type of MCP are specifically designed for the protection of motors, allowing sufficient time for the motor’s inrush current to pass but preventing any extended or excessive overcurrent condition that exceeds its setpoint. These circuit breakers are intended to provide branch, feeder, and main circuit protection, with interrupting ratings from 5000 up to 200,000 amps.

Figure 6c. “Open” circuit breaker.

This type of MCP is often equipped with an adjustable overload setting and selector switch to provide a specific NEMA or IEC class of overload protection (Figure 5b). Properly sized inverse time circuit breakers can also provide motor branch circuit, short circuit, and ground fault protection.

Many models of MCP circuit breakers include a test button and LED display that is displayed whenever the breaker has tripped. This is a visual indication for nearby personnel that a fault has occurred, and the electric motor or circuit must not be restarted until the fault is addressed.

Some MCP models also allow a cool-down time in case of an overload, after which the motor will attempt to automatically restart. This function is not recommended unless proper safeguards are in place to prevent possible injury to personnel and equipment damage due to an inadvertent motor or load restart.

Fundamental Operation of Circuit Breakers

The primary functional requirement of a circuit breaker’s operation is fault detection and circuit protection. Proper and rapid fault detection is a paramount function of both circuit breaker types to prevent potential equipment damage and possible burns or electrocution to humans and animals.

The operation of these devices is made possible by the heating or magnetic effects of an electric current. The circuit breaker terminals must open to interrupt the flow of current through the circuit, using mechanically stored energy contained within or attached to the breaker once a fault condition due to an overload or short circuit is sensed.

This energy is normally facilitated and stored by an internal compressed spring (Figure 6a) or through an actuator using compressed fluid or air for larger breakers. Small circuit breakers normally have a manual control lever to switch the load on or off or to reset it after an interruption caused by a breaker tripping.

Figure 7. Circuit breaker internal mechanism.

On the other hand, heavy-duty (high amperage) low voltage and medium/high voltage circuit breakers often use voltage and current sensors to activate solenoid-driven actuators to trip the mechanism, with electric motors used to restore the breaker to normal operation, often activated by remote control for safety.

The electric contacts used in a circuit breaker must carry the load current without excessive heating, pitting, or distortion as well as withstand the heat intensity of the arc produced when opening the circuit, particularly during a high current flow as a high intensity arc is generated when a high current or voltage is suddenly interrupted.

The length of the arc depends entirely on the voltage while the heat developed is proportional to the broken current. To prevent damage and limit possible crossover between phases, this arc must be adequately cooled and extinguished in a controlled and contained manner so that the gap between the contacts can withstand the voltage present in the circuit and the arc cannot extend beyond the confines of the arc extinguisher to an adjacent phase.

An insulating media such as a physical barrier, vacuum, air, or non-conducting gas or oil are normally used to contain the arc. Two essential contacts are used in a circuit breaker: a stationary or fixed contact arm and a movable contact arm.

When the circuit is closed, the contacts engage and carry the current under this closed contact condition. Under a closed breaker (Figure 6b), the current-carrying contacts, known as electrodes, engage each other due to the applied and constant pressure of a spring within the operating mechanism. The switching of the system occurs using a snap-action engagement or disengagement of the contacts and is accomplished by either opening or closing the activating arm of the circuit breaker.

The circuit breaker is opened or tripped by applying pressure to an over-center trigger switch due to manual operation or from an overload or short circuit event occurring through the breaker (Figure 6c). This over-center trigger action provides the sudden and familiar popping or clapping sound of the circuit breaker during an opening or closing operation.

These two contacts used in a circuit breaker are indicated in Figure 7.


This wraps up the second part of this series on circuit breakers. Next month, we will complete the series on electrical protection methods with a discussion on circuit breaker and fuse trip curves, proper NEC sizing of each, and an example of selective coordination.

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Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.