What is a Circuit Breaker?

What is a Circuit Breaker?

A circuit breaker is a device that may manually or automatically break a circuit under all circumstances, including no load, full load, and short circuit.      

A circuit breaker is a device that can perform any of the following functions:

  • Under typical circumstances, manually or by remote control, makes or breaks a circuit.
  • When a circuit is tripped, it automatically breaks the circuit.
  • Under fault conditions, makes the circuit trip either manually or remotely.
  • A circuit breaker is utilized to incorporate both manual and automatic switching control.
  • Relays are used to control the circuit breaker automatically.
  • Only in the event of a fault, automated control used.

Working Principle:

A circuit breaker is made up of electrodes, which are both fixed and movable contacts. These contacts remain closed under normal operating conditions and will not open automatically unless the system fails. These contacts can be manually or remotely opened.

When a failure arises in any side of the system, the circuit breaker’s trip coils get electrified, pulling the movable contacts apart and opening the circuit. An arc is formed between the contacts of a circuit breaker when they are separated due to a fault situation. Arc generation delays current interruption and generates a lot of heat, which might harm the system or the circuit breaker.

As a result, the fundamental challenge in a circuit breaker is to extinguish the arc as quickly as possible so that the heat generated by it does not reach a harmful level.


Schematic Diagram of the Circuit Breaker:



Contacts in Circuit Breaker:

1. Fixed Contact:

The fixed contact in the main circuit is the one that conducts current when it is closed.

2. Movable Contact:

A mechanically operated movable contact is included in the auxiliary circuit. It’s a remote indicator contact.

3. Normally Open (NO) Contact:

Normally Open (NO) Contact is an auxiliary contact that is floating. If the main contact is also closed, it is closed.

4. Normally Closed (NC) Contact:

Normally Closed (NC) Contact is an auxiliary contact that is floating. If the main contact is closed, it is open.


Properties of the Circuit breaker:

The basic properties of the circuit breaker are:

  • High system availability
  • High fault tolerance
  • Preferential protection
  • Broad range of applications
  • Reliable load protection
  • Stable supply capability

Arc Phenomenon:

When contacts are separated, an arc is formed. As a result, the current can continue. As a result, a circuit breaker’s exact property is to differentiate the arc in the smallest amount of time. The arc provides a low-resistance channel for the current, allowing it to flow freely across the circuit.

But, when a short circuit occurs, however, a large current run through the circuit breaker contacts before the safety mechanism opens them.

When the contacts split, the contact area shrinks quickly, and a strong fault current causes an increase in current density. The temperature between the contacts rises as a result of the higher current density. The ionization of the air is caused by the heat generated in the medium between contacts. Ionized air acts as a conductor, forming an arc between the contacts. The potential difference between the contacts is very small, and it’s just enough to keep the arc going. The arc resistance determines how much current flows between the contacts.

Factors influence arc resistance:

The following factors influence arc resistance:

  • Degree of ionization
  • Length of the arc
  • Cross section of the arc

Factors are responsible for maintaining the arc between the contacts:

1. Potential difference between the contacts that could exist:

When the contacts are close together, the potential difference between them is enough to keep the arc going. The arc can be extinguished if the contacts are separated by a large enough distance that the potential difference is insufficient to keep the arc alive.

2. Ionised particles between contacts: 

The arc can be deionized by cooling it or physically removing the ionised particles from the gap between the contacts.

Arc Extinction Techniques:

They are two types of arc extinction techniques:

  • High Resistance Method
  • Low Resistance Method

1. High Resistance Method:

Arc resistance is increased over time, reducing current to a level insufficient to maintain the arc. The arc’s resistance can be improved by:

  • Length of the arc: by widening the spacing between contacts.
  • Arc cooling: by directing a gas blast along the arc.
  • Reduce Cross section of the arc: allowing the arc to pass through a tiny opening or having a reduced contact area.
  • Creating a split in the arc: by attaching some conductor plates between the contact.

2. Low Resistance Method:

       In AC circuits, this method is used to extinguish arcs. After every half-cycle in an AC system, current drops to zero. The arc extinguishes for a brief moment with each current zero. Because the medium between the contacts is made up of ions and electrons, it has a low dielectric strength and is readily broken down by the rising contact voltage, also known as re-striking voltage. If there is a breakdown, the arc will continue for another half cycle.


If the dielectric strength of the medium between contacts builds up faster than the voltage across the contacts at current zero, the arc fails to restrike and the current is interrupted.

The following methods can be used to rapidly raise the dielectric strength of the medium toward current zero:

  • Inducing the recombination of ionised particles in the space between contacts into neutral molecules.
  • Ionised particles are swept away and replaced by un-ionised particles.

The following methods can be used to de-ionize the medium:

  • Gap lengthening: A larger dielectric strength of the medium can be attained by rapidly opening the contacts.
  • High pressure: As the pressure around the arc rises, the density of the particles that make up the discharge rises as well, resulting in a faster rate of de-ionization.
  • Cooling: Allowing ionised particles to cool accelerates the natural combining process.
  • Effect of a blast: By substituting unionised particles for ionised particles, the dielectric strength of the medium can be significantly improved.
  • Voltage of the arc: During the arcing period, it is the voltage that emerges across the circuit breaker contacts.
  • Restriking voltage: During the arcing period, it is the transient voltage that emerges across the contacts at or near current zero.
  • Recovery voltage: After the final arc extinction, it is the standard frequency (50 Hz) RMS voltage that occurs across the contacts of the circuit breaker. It is about equal to the voltage in the system.

Types of Circuit Breaker:

Circuit Breaker classified as two types; they are:

  • AC Circuit Breaker,
  • DC Circuit Breaker.

AC Circuit Breaker:

Based on different categories AC circuit breaker have different types:

1. Based on Voltage:

  • Low Voltage Circuit Breaker,
  • Medium Voltage Circuit Breaker,
  • High Voltage Circuit Breaker,
  • Ultra-High Voltage Circuit Breaker.

2. Based on Location:

  • Indoor Circuit Breaker,
  • Outdoor Circuit Breaker.

3. Based on External Design:

  • Dead Tank Circuit Breaker,
  • Live Tank Circuit Breaker.

4. Based on Interrupting Media:

  • Air Blast Circuit Breaker,
  • Oil Circuit Breaker,
  • Air Circuit Breaker,
  • Vacuum Circuit Breaker,
  • SF6 Circuit Breaker.

DC Circuit Breaker:

Similarly, as AC circuit breaker, DC circuit breaker has a type called

  • HVDC (High Voltage Direct Current) Circuit Breaker.

Advantages of Circuit Breaker:

  • It protects against harm caused by an excessive current flow.
  • It’s more dependable.
  • It can be reset because it is controlled by a switch.
  • In operations, it is quite sensitive.
  • It is protective measurement for both HT and LT line systems.

Disadvantages of Circuit Breaker:

  • When compared to a fuse, the installation and repair costs are higher.
  • When opposed to a circuit breaker, a fuse reacts faster

Circuit Breaker Uses:

  • Circuit Breaker are utilised in industries, buildings, commercial complexes, and hotels to switch loads.
  • Plants, electrical machines, transformers, capacitors, and generators are all protected by circuit breaker.
  • Indian Railways employ circuit breaker for electrification.

Rabert T

As an electrical engineer with 5 years of experience, I focus on transformer and circuit breaker reliability in 110/33-11kV and 33/11kV substations. I am a professional electrical engineer with experience in transformer service and maintenance. I understand electrical principles and have expertise troubleshooting, repairing, and maintaining transformers, circuit breakers, and testing them. Tweet me @Rabert_infohe

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