Part 3(a). Circuit breakers.
By Ed Butts, PE, CPI
We continued a series on the devices used for protection of motor electrical circuits in last month’s edition of Engineering Your Business with a discussion on fuses and overloads. This month, we continue as we turn to circuit breakers—the various types, advantages and disadvantages, their components, and the role they play as an electrical protection device.
Fuses vs. Circuit Breakers
Fuses were originally recognized as the most reliable and predictable method of short circuit protection and were used regularly throughout the first half of the 1900s. However, increased sophistication in current sensing and response time, improvements in plastics and electronic circuitry, and the inherent advantages of resetting and reusing circuit breakers have enabled circuit breakers to largely overtake fuses in use.
There are, though, distinct advantages and disadvantages associated with each method—especially when it comes to short circuit protection, response time, and withstand ratings. Thus, designers must carefully weigh both the pros and cons before deciding which method to use for a specific design.
The primary function of the device is to interrupt the current flow, if necessary, to protect the equipment from damage as well as prevent the risk of personnel injury and fire.
Two distinct advantages a circuit breaker offers are (1) their two- or three-pole types of integral construction for multi-phase service and (2) their built-in short circuit and overload protection combined with an external means of circuit disconnect and reset. Most modern motor circuits generally use circuit breakers for their compactness, flexibility, adjustability, accessible disconnecting means, and reuse and reset capability.
Fuses are often preferred for protection of control transformer primary and secondary circuits and similar control circuits due to their smaller physical size ranges; wide range of voltage, current, and trip time choices; predictable operating nature; ambient heat tolerance; rapid blow capability to protect external control circuits; and the ability to custom select a certain type and size of fuse for a specific application.
Basics of Circuit Breakers
A circuit breaker is an inline electrical safety device that protects an electric circuit and equipment from potential damage caused by an overcurrent or short circuit. The primary function of the device is to interrupt the current flow, if necessary, to protect the equipment from damage as well as prevent the risk of personnel injury and fire.
The basic three functions of a typical circuit breaker are:
- Protection from an overload
- Protection from a short circuit fault or ground fault
- As a reset and switching device for an On/Off circuit.
Circuit breakers are designed and constructed to comply with various industry standards, including IEEE C37, UL 489, and IEC 62271 standards and NEC Sections 240 and 430 (motors).
The standard ampere ratings for inverse time low voltage (≤600VAC) circuit breakers are: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes.
An electrical circuit breaker can be operated both manually as a switching device and tripped automatically as a protective device to control and protect the electrical power system. As a circuit reset and switching device, circuit breakers are operated directly using a 90-degree travel toggle switch mounted on the front of the breaker or externally operated from the enclosure door.
Types of Circuit Breakers
There are multiple types, styles, ratings, and operating methods of circuit breakers, including:
- Miniature circuit breaker (MCB)
- Molded case circuit breaker (MCCB)
- Earth leakage circuit breaker (ELCB) and residual current circuit breaker (RCCB)
- Ground fault circuit interrupter (GFCI)
- Arc fault circuit interrupter (AFCI)
- Motor circuit protector breaker (MCP) (discussed in January 2023 column)
- MVAC medium voltage AC circuit breaker
- HVAC high voltage AC circuit breaker
- LVDC low voltage DC circuit breaker
- MVDC medium voltage DC circuit breaker
- HVDC high voltage DC circuit breaker.
Some circuit breakers are listed and rated for dual voltages (AC/DC). Commonly used circuit breakers for low voltage power and motor applications include the first six listed types above.
A miniature circuit breaker (Figure 1a) is generally used for applications of 100 amps or less. An MCB is used for low voltage control, power, and light industrial applications and works as a combination of a switching, overcurrent, and short circuit protection device.
There is a bimetallic strip to protect from an overload condition and an internal solenoid to protect against a short circuit. MCBs are generally non-adjustable devices that can be constructed for 240VAC and 480VAC voltages for both single- and three-phase applications with two, three, and four poles and a neutral pole, if necessary. This breaker type can typically work between 0.50 amp up to 100 amps with a short circuit current range of 3 to 10,000 amps.
A molded case circuit breaker (Figure 1b) is used when the load current and duty exceeds the capability of a miniature circuit breaker and is the most common type of circuit breaker used for power distribution and motor service applications.
This breaker type is often simply referred to as a generic circuit breaker or “CB.” The main difference between an MCCB and MCB is that the current range of an MCCB extends between 15 amps up to 2500 amps and its trip setting is adjustable. This wide range of current and adjustability allows MCCBs to be used in various applications in both low power as well as medium and high power applications. They are also available in multiple pole configurations and voltage ratings.
In addition to standard circuit breakers, an earth leakage circuit breaker (Figure 1c) is widely used in many residential, commercial, and industrial applications. The main purpose of an ELCB is to detect AC power to earth leakages and protect personnel and animals from electrical shocks and fires that are caused by short circuits.
Early earth leakage circuit breakers used voltage detecting devices, but most now use current sensing devices. Voltage sensing ELCBs monitor the voltage on the earth wire and disconnect the supply if the earth wire voltage is more than 50 volts. They are no longer recommended over current sensing ELCBs and no longer are they available because of outdated technology.
A current sensing ELCB, also called a residual current circuit breaker, detects the leakage current and isolates the electrical power flow by tripping the circuit breaker.
Current types of ELCBs consist of a three-winding transformer, which has two primary windings and one secondary winding, called the core balance current transformer (CBCT) (Figure 2).
When a fault occurs, a small amount of current will flow to the ground. This causes an imbalance between the phase and neutral currents, which creates an unbalanced magnetic field. This induces a current across the secondary winding, which is connected to the relay’s sensing circuit. This fault current sends a signal to the tripping mechanism, opening the breaker contact.
In an electronic circuit, such as an inverter, if the rectifying circuit fails, an earth leakage current with a half-wave rectified waveform or a phase-controlled waveform might be generated. In this case, Type-A earth leakage protection characteristics are needed to detect the half-wave rectified or half-wave phase-controlled earth leakage current waveform.
A ground fault represents any electric path between a source of current and a grounded surface. A ground fault occurs when current is leaking and escaping to the ground. If someone’s body provides a path to the ground for this leakage, the individual could be injured, burned, severely shocked, or even electrocuted. Since water conducts electricity, ground faults are especially common in areas where water can provide a conduit for electricity to escape and find an alternate path to the ground.
A ground fault circuit interrupter is also used to prevent injury from electrical shock to the human body. Figure 3a illustrates a GFCI-protected receptacle while Figure 3b illustrates a GFCI circuit breaker. Basically, it isolates the electrical power supply in a fraction of a second when a fault condition occurs.
The working principle of a GFCI is simple, as Kirchhoff’s Current Law states: the incoming and outgoing current to the load must be equal in the circuit. This circuit breaker type identifies the unbalanced current (i.e., current difference between the incoming and outgoing line due to the fault condition).
The circuit breaker is designed to comply with UL Standard 943 in such a way that it directly compares the value of the incoming and outgoing circuit currents. In a Class-A rated GFCI, whenever there is a sensed difference of 6 milliamps (6 mA or 0.006 amps) or more, the residual current is basically determined to be the difference between the two currents which is assumed to flow to ground, actuating the circuit to trip the breaker (Figure 3c). Low voltage (120 and 240VAC) GFCIs are available in circuit breaker and receptacle styles for circuit protection in bathrooms, garages, swimming pools and spas, and outdoor applications.
An arc fault circuit interrupter, also called an arc fault detection device, is a UL 1699-listed circuit breaker that is required by the NEC to be installed in most dwellings. It breaks the circuit when it detects the electric arcs that often accompany loose connections in wiring.
Loose connections which can develop over time, and repeated cycles of heating and cooling, are more commonly associated with residential settings, and can sometimes become hot enough to ignite fires.
An AFCI selectively distinguishes between a harmless arc incidental to normal operation of switches, plugs, and brushed motors, and a potentially dangerous arc. The electronic module inside an AFCI breaker detects electrical currents alternating at characteristic arcing frequencies, usually around 100 kHz for more than a few milliseconds.
A combination AFCI breaker provides protection against parallel arcing (line to neutral), series arcing (loose, broken, or an otherwise high resistance segment in a single line), ground arcing from line or neutral to line to ground, overload, and a short circuit. The AFCI will open to protect the circuit if dangerous arcing is detected.
Circuit breakers are also available for medium voltage (1 to 35 kilovolts [kV]) and high voltage (>35 kV) AC and DC systems. Some larger low voltage molded case and power circuit breakers may use electric motor operators to allow them to open and close via remote control techniques. Remote operation may also form part of an automatic transfer switch (ATS) system for standby power conversion.
A motor circuit protector breaker is a specialized type of circuit breaker that is designed specifically for electric motors, just as their name implies. This MCP breaker type will be discussed in greater detail in our April 2023 column.
Components of Circuit Breakers
There are five primary components that play an important role in the modern electrical circuit breakers (Figure 4).
Frame. The frame protects all the internal parts of the circuit breaker and supports the components. The frame provides the required insulation needed to contain the arc. Various increased frame sizes are used for increasing ampere service. Circuit breakers for 15 amps through 2500 amps are available in the frame sizes shown in Table 1.
Operating Mechanism. The operating mechanism opens or closes contacts of the circuit breaker. These typically include spring-loaded, mechanically activated switches or solenoid, hydraulic, and pneumatic actuators.
Contacts. The contacts allow the current to flow through a circuit breaker when it is closed. Generally, a circuit breaker has two electrical contacts: a stationary or fixed contact and a movable contact.
Arc Extinguisher. It extinguishes or quenches the arc while in fault conditions. When the contacts are disconnected, electricity can jump through the gap between the end parts of the contact. This causes an arc of electricity that can reach high temperatures. A circuit breaker uses an arc suppression mechanism (i.e., arc extinguisher) to prevent the damage and arc from recreating itself.
Arc extinguishing is one of the important applications of circuit breakers because the arc will not only cause damage to the equipment circuit, but also affect personal safety. The most common methods of arc extinguishing for low voltage service using ambient air means are mechanical arc, magnetic arc blowing, narrow slit, and grid. Medium and high voltage arc extinguishing high methods include air blast, vacuum, oil, oilless, compressed air, gas, sulfur hexafluoride, and magnetic blow.
Trip Unit. The trip unit senses the abnormal current flow in the event of an overcurrent, overload, or short circuit and causes the operating mechanism to open the contacts. The trip unit is designed and calibrated for different currents and voltages but primarily operates in an instantaneous function to sense a short circuit or an inverse-time function to sense an overload.
This means the higher the current in relation to time, the quicker the circuit breaker responds to the overload and trips. This is illustrated in Figure 5 for instantaneous and pickup (minimum) tripping functions. Certain trip units can also sense and respond to underloads. Circuit breaker trip characteristics will be explained in greater detail in an upcoming column.
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We will continue our series on circuit breakers next month with an expanded discussion on circuit breaker types plus those specifically designed for motor circuit protection with proper circuit breaker sizing and selective coordination factors wrapping up this topic in the May issue of Water Well Journal.
Until then, work safe and smart.
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.