What Is a Circuit Breaker? Definition, Types & How It Works

Circuit breakers are vital safety devices that protect electrical systems from overloads, short circuits, and faults. By automatically cutting off electricity during an issue, they help prevent fires, equipment damage, and shocks. This guide covers the basics of circuit breakers, including their types, how they work, and installation tips.

What is a Circuit Breaker?

In simple terms, a circuit breaker is an automatic device that provides safety. It is a device that keeps electrical and electronic equipment safe. When there is an excessive flow of electricity in a line, it can cause accidents in the equipment, which the circuit breaker protects against.

In other words, a Circuit Breaker is a type of device that can open or close a circuit under all conditions (no-load condition, full-load condition, and faulty or defective condition).

Alternatively, a Circuit Breaker is a device that automatically disconnects a faulty line from the source.

What does a Circuit Breaker do?

A circuit breaker automatically disconnects the circuit when the current exceeds the safe rating of the circuit wiring. A standard breaker responds to current, not voltage. It trips:

• In case of a short circuit in the AC line (line-to-line or line-to-neutral).
• When there is an excessive load.
• On a ground fault or arc fault, if it is a GFCI or AFCI breaker.

Types of Circuit Breakers

Based on arc or fire extinguishing

1. Oil Circuit Breaker (how an oil circuit breaker works).
2. Air Circuit Breaker.
3. Sulfur Hexafluoride Circuit Breaker.
4. Vacuum Circuit Breaker.

Based on service

1. Outdoor Circuit Breaker.
2. Indoor Circuit Breaker.

Based on operating mechanism

1. Spring-operated Circuit Breaker.
2. Pneumatic Circuit Breaker.
3. Hydraulic Circuit Breaker.

Based on voltage level installation

1. High Voltage Circuit Breaker
2. Medium Voltage Circuit Breaker
3. Low Voltage Circuit Breaker

Additionally, they can be categorized as follows

• Magnetic Circuit Breaker.
• Common Trip Circuit Breaker.
• Thermal Circuit Breaker.
• Disconnecting Circuit Breaker.
• Carbon Dioxide Circuit Breaker.

And others.

Construction and Operation of Circuit Breakers

A Circuit Breaker essentially consists of two main parts:

1. Fixed Contact.
2. Moving Contact.

We generally use Miniature Circuit Breakers (MCB) in homes. The image below shows a Miniature Circuit Breaker.

This is a commonly used circuit breaker in homes. The image shows the operating lever, arc splitter, moving contact, fixed contact, trip coil, etc.

Operation:

Here are some basic operational principles of a circuit breaker:

• In normal conditions, a Circuit Breaker is operated manually (remote control or others).
• In fault conditions, the Circuit Breaker operates automatically.
• In normal conditions, the moving and fixed contacts remain connected and electricity flows through the circuit.
• Both the CT (Current Transformer) and relay work for automatic interruption.

Working Steps

• When the current flowing in the line exceeds the design current (due to fault), the Current Transformer (CT) reduces the excess current and sends it to the relay coil.
• When the relay coil experiences peak current (more than the specified current), it activates and the relay’s moving contact disconnects the circuit.
• Then, the trip coil is energized through the battery source of the trip circuit, and it breaks the contact.

The CT-and-relay steps above describe medium- and high-voltage breakers in substations and industrial panels. A home miniature circuit breaker (MCB) has no CT or battery: a thermal bimetal strip bends and trips it on a sustained overload, and a magnetic coil trips it instantly on a short circuit. A shunt trip breaker adds a coil so it can also be tripped remotely.

Circuit Breaker Wiring Diagram:

Typically, the installation of a circuit breaker depends on the type of load used and the maximum current rating of the load.

The image above shows different ratings of circuit breakers used for each room in a complete house based on load and power. Using separate circuit breakers for each room helps in fault detection. Additionally, separate circuit breakers should be used for high-power loads such as microwave ovens, air conditioners, refrigerators, irons, etc.

What rating of Circuit Breaker should be used?

A breaker is sized to protect the wire, not just the load. Under the NEC, the rules are:

• Load: the breaker rating must be at least 100% of the non-continuous load plus 125% of the continuous load (a load running 3 hours or more), per NEC 210.20(A).
• Wire: the breaker must not be larger than the ampacity of the conductor it protects, per NEC 240.4. Check the wire with the wire size calculator.
• Motors: motor circuits follow NEC Article 430, which allows a larger breaker than the running current so the motor can start without tripping. See MCP vs circuit breaker.

Inductive and capacitive loads draw a short inrush current at switch-on. That is handled by the breaker’s trip curve (see the B, C and D types below), not by oversizing the breaker.

Rules for Installing Circuit Breakers:

We use circuit breakers in homes, industries, educational institutions, and various places. However, there are several rules to follow for proper installation.

1. Determine the Current Rating: First, calculate the current rating of the load you will be using the circuit breaker with. You can use the formula I = W/V for this. Typically, the wattage is mentioned on the equipment, from which you can determine the current by considering the supply voltage.
2. Identify the Type of Load: Determine what type of load is connected to the circuit breaker. In homes, different types of loads are used, so for safety, consider the maximum rating.
3. Select the Appropriate Circuit Breaker: Size it to at least 125% of any continuous load and no larger than the wire’s ampacity. For motors and other high-inrush loads, choose the breaker per NEC Article 430 and a suitable trip curve rather than simply multiplying the load current.
4. Use Separate Circuits for High Ampere Loads: For each high ampere load, use a separate circuit. Examples of high ampere loads include high-power motors, water pumps, air conditioners, electric heaters, etc.
5. Use a Closest Available Rating: If the calculated circuit breaker rating is not available in the market, use the closest available rating. For example, if the load current is 10 amperes but a 10-ampere breaker is not available, a 15-ampere breaker can be used, but only if the wire is rated for at least 15 amperes.

Advantages of Using Circuit Breakers:

1. Resettable: After a trip, a breaker is switched back on; a blown fuse must be replaced.
2. Visible Status: The handle position shows which circuit has tripped, which makes fault finding quicker.
3. Extra Protection: GFCI and AFCI breakers add shock and arc-fault protection that a plain fuse cannot give.
4. Testing Capability: Many circuit breakers have a test button to ensure they are functioning properly.

When Does a Circuit Breaker Trip?

Under the IEC 60898 standard (used for MCBs outside North America), miniature circuit breakers come in three magnetic trip types:

1. Type B Circuit Breaker: Trips when 3 to 5 times the rated current flows.
2. Type C Circuit Breaker: Trips when 5 to 10 times the rated current flows.
3. Type D Circuit Breaker: Trips when 10 to 20 times the rated current flows.

The specific conditions for tripping are usually mentioned in the manual or datasheet.

What is Surge Voltage?

In an AC circuit, the presence of excessive voltage for half a cycle or less is known as surge voltage. Circuit breakers usually trip due to reasons such as connecting a load higher than the specified rating or short circuits. Therefore, if a circuit breaker trips, the fault should be identified before turning it back on.

Conclusion:

In summary, a circuit breaker is a critical safety device for protecting electrical circuits and equipment. It helps prevent electrical accidents by automatically disconnecting faulty circuits. By understanding the types, operation, and proper installation of circuit breakers, you can ensure the safety and reliability of your electrical systems.

Frequently Asked Questions

What does a circuit breaker do?

A circuit breaker automatically cuts off power when the current in a circuit gets too high, from an overload, a short circuit or (for GFCI/AFCI types) a ground fault or arc fault. This stops the wiring from overheating and protects against fire and shock.

What are the main types of circuit breakers?

By arc-quenching medium: air, oil, vacuum and SF6 circuit breakers. In homes and small buildings, the common types are miniature circuit breakers (MCB), molded case circuit breakers (MCCB), GFCI breakers and AFCI breakers. Breakers are also grouped by voltage level: low, medium and high voltage.

What is a miniature circuit breaker (MCB)?

A miniature circuit breaker (MCB) is the small breaker used in home and office panels, usually rated up to about 125 A. It trips on overload with a thermal bimetal strip and on short circuit with a magnetic coil, and it is reset by switching the handle back on.

What is the difference between a fuse and a circuit breaker?

Both protect a circuit from overcurrent. A fuse has a metal link that melts and must be replaced after it blows. A circuit breaker opens a mechanical switch and can be reset and reused. Breakers are more convenient; fuses are simple, cheap and can interrupt very high fault currents.

Why does a circuit breaker trip?

A breaker trips because of an overload (too many appliances on one circuit), a short circuit (hot wire touching neutral or another hot), or a ground fault (hot wire touching ground). Find and fix the cause before switching it back on. If it trips again immediately, call an electrician.

What is the difference between an AFCI and a GFCI breaker?

A GFCI breaker protects people from shock by tripping when current leaks to ground. An AFCI breaker protects against fire by tripping on dangerous arcing in damaged wiring or loose connections. Dual-function breakers provide both, and the NEC requires each in different locations.

How do I choose the right circuit breaker size?

Match the breaker to the wire and the load: at least 125% of the continuous load and no larger than the wire’s ampacity (NEC 210.20(A) and 240.4). For example, 14 AWG copper is normally protected at 15 A and 12 AWG at 20 A. Motors follow NEC Article 430.