A circuit breaker is a unique device that serves a vital function in an electrical circuit. It’s a switch which you can operate it either manually or automatically. The main reason for their invention is to protect electrical circuits from power surge. During a power overload in an electrical system, the switch detects it and interrupts the flow of current.
Initially, the fuses were the popular form of protecting devices against power overload in the system. But they do have limitations which could cause many inconveniences. For example, you will need to replace the fuse every time after a power surge. This will cost you time, labor and money. Additionally, the fuse is unable to control a heavy power overload.
But with circuit breakers, they can be turned on automatically after a power surge or you can turn them on manually. Also, there will be no need for hardware replacement, as the breaker remains viable after the power overload.
There are various types of this device under different categories. Some of them include interrupting mechanism, voltage, exterior design or location of installation. Below is the breakdown of each category.
Types by:
The exterior design of the device
They are also classified on the basis of their exterior structural style and it is applicable in two ways:
- Live tank circuit breaker
In live tank circuit breaker, the vessel which houses the insulation medium and interrupters is at a potential that is higher than that of the ground.
- Dead tank circuit breaker
In dead-tank circuit breaker, the switching device is found in a vessel which is at ground potential. Also, the switching device contains insulation medium and interrupters surrounding it.
Location of Installation
- Outdoor circuit breakers
They are specifically designed for outdoor application where there is no roof to shield them from weather elements. This is because they have a strong compact external enclosure design. This is when compared to the indoor circuit breakers. Thus they can withstand harsh conditions.
- Indoor circuit breakers
Indoor circuit breakers are made specifically for application in the building. or where there are installed in a weatherproof enclosure. They operate under medium voltage and they contain metal-clad enclosure switchgear.
Voltage
Under voltage classification, the breakers are categorized according to the amount of electric voltage they can support. Thus, there are two main types on the basis of voltage. They include:
- High voltage circuit breakers
The rating of these breakers makes them applicable for 2kV and above. But that’s not all, these breakers are subdivided further into, Breakers which can support 123kV and above and the medium types which can support 72kV and below.
- Low voltage circuit breakers
With low-voltage circuit breakers, they can support 2kV voltage and below. These types of breakers are common in industries which are small scale.
Mechanism of Interruption
- Oil circuit breakers
The oil circuit breakers rely on oil for the extinction of arc. Within the oil as the contacts open after a fault, the arc occurs, oil is around it gets vaporized into hydrogen gas. The bubbles of hydrogen gas will then remain around the region of the arc.
And as we know the high thermal conductivity of hydrogen gas, it will absorb the heat from the arc and also initiates deionization of the medium. In the process, turbulence gets created in the oil which pushes away the arching products from the arc.
The oil breaker exists in two types,
- The low oil circuit breakers
- The bulk oil circuit breakers
The bulk oil circuit breakers as the name indicates relies on large amount of oil. Also, it is further divided into:
- Plain break circuit-breakers. With this type of breaker, the contact separation occurs in the oil. While the arc control system serves to increase the contact separation. The arc extinction will only occur once the critical gap is achieved between the contacts.
- Arc control circuit-breakers. When there is no arc control in the plain break circuit breakers, the Arc control circuit-breakers overcomes it. The implementation of the arc control occurs in two forms: Forced-blast and Self-blast oil circuit breakers.
Self-blast breakers rely on an insulation pressure chamber which is rigid. In this case, the chamber confines the arching gases and the contacts. The development of high pressure in the small chamber forces the gaseous oil through the arc and in the process extinguishes it.
The self-blast breakers use pressure pots which are available in three types: the cross jet, plain and self-compensation explosion pots.
With the Forced-blast breakers, the pressure generation is by use of piston cylinder. This is unlike the self-pressure generation in the Self-blast oil circuit breakers.
In general, the oil in Bulk oil circuit breakers serves two main roles, which is:
- Serves as the extinguishing medium of arc
- Insulation purposes (act as an insulator between the earth and live circuit). This takes less than 10% with the rest of the oil serving as an insulator.
When it comes to Low-oil circuit breakers, the oil serves mainly in the extinction of the arc. And the insulation is done by paper and porcelain.
Pros
- The cooling property of oil is exceptional
- The oil serves as the insulator
Cons
- Products from arcing remain in the oil
- Oil has the potential to cause a fire hazard since its flammable.
- Air Blast circuit breakers
In Air blast circuit breakers, the pressurized blast of air serves as the medium for extinguishing the arc. When a fault occurs, a blast of air under the control of blast valve will cool the arc and open the contacts.
The products from the arching are blasted into the air; this increases the medium dielectric power rapidly. Due to this, arc reoccurrence is inhibited. The arc extinction occurs and current flow is cut shot.
Under air blast circuit breakers, there are three types which are categorized according to the air-blast direction. They include:
- Cross blast
- Axial blast
- Radial blast
In axial-blast circuit breaker, the blast of air is forced in the same direction as the arc formation. The pressurized air will exert force on the contacts opening them. In the same instance, it will also be driving away arc causing extinction.
When it comes to cross-blast circuit breakers, the blast of sir is moving perpendicularly to the direction of arc formation. And in the radial-blast circuit breakers, the blast of air is forced into a radial pattern.
Pros
- The products from arcing are swept away completely by the blast of air.
- Fire occurrences are out of picture
- There is independence of the air blast with respect to the interrupting current.
- It’s convenient as the arcing energy extinction duration is very small.
- It has a higher speed of operation
- Applicable where there are frequent operations since the arc energy is very small.
- The duration time for arc extinction is the same for all voltage sizes.
Cons
- In voltage restriction, it has variation sensitivity
- Requires maintenance of the air compressor
- The air blast extinguishing property of the arc is inferior.
Air circuit breaker applications
- Applicable in both low and high voltage rating
- It’s applicable in the systems of electrical sharing and also GND, that operates at about 15kV.
- It serves in protecting electrical machines, plants, generators, transformers, and capacitors.
- Vacuum circuit breakers
With the vacuum circuit breakers, the medium for extinguishing the arc is the vacuum. The extinguishing property of vacuum is rated supreme, as it’s incomparable to any other. This is attributed by the insulation strength which is the highest.
When the contacts opening of the vacuum circuit breaker occurs, the arc formation is as a result of the contacts metal vapor ionization. But the formation of the arc is extinguished quickly due to the rapid condensation of the vapor.
Pros
- Requires the least maintenance
- Durable and reliable
- Has the capacity to deal with any size of the charge
- Free from fire hazards
- Free from gasses generation both before and after the extinction of the arc
- Very minimal energy from the arc gets released
- It’s powerful enough to withstand the current from lightning.
- Moulded Case Circuit Breakers (MCCB)
Moulded case circuit breakers have an incredible power rating of 1000A. As part of their features, they have protection from earth fault. Additionally, they also contain protection from the current. The good thing about MCCB’s, is they allow you to adjust the trip settings to your convenience. And the process is simple.
- Miniature Circuit Breaker (MCB)
The miniature circuit breakers have a relatively low current rating when compared to MCCB. The rating is at 100A and lower. When it comes to in-built current protection, it has only one. Last but not least, this device has only one trip setting with no adjustments applicable.
- Double Pole circuit breaker
This device supports a current of 220V. From the name, they have a couple of wires that are hot. Moreover, both the poles need interruption during a fault.
- Single Pole circuit breaker
This device supports a current-voltage of 110V. From the name, it has a single hot wire, also included is a neutral wire that gets operated at the same current-voltage of 110V. When a fault strikes, the interruption only applies on the hot wire.
- Arc Fault Circuit Interrupter (AFCI)
The Arc Fault Circuit Interrupter comes in handy during hostile conditions like a great overload of arcing. Through this device, fire you get to avoid fire breakout. But when there is an occurrence of the ordinary arcing, the device remains unresponsive. This means it will not cause arc extinction.
- Ground Fault Circuit Interrupter (GFCI)
Ground Fault Circuit Interrupter is a safety switch that only gets initiated when there is a ground overload current. The GFCI device works by creating an interruption on electrical circuits in case of a fault. This is after detecting any slight variation between the neutral wires and the phase.
- Sulfur-hexafluoride (SF6) Circuit Breakers
The medium here is sulfur-hexafluoride which serves as the arc extinguisher. The gas from sulfur-hexafluoride is electronegative. Thus it draws in free electrons. As the arc hits, pressurized sulfur-hexafluoride gas blasts into the chamber as the contacts get opened.
The free ions that result from the arc are drawn in by sulfur-hexafluoride gas, leaving behind negative ions which are immobile. As the free ions are drawn in by the gas, the arc loses its electrons for conduction. In the process, the surrounding medium insulation power increases and the arc gets extinguished completely.
Types of sulfur-hexafluoride circuit breakers
The types of sulfur-hexafluoride are based on the number of interrupters they have and the voltage they support. Below are the three main types:
- Four interrupter sulfur-hexafluoride which supports to a limit of 725V
- Two interrupter sulfur-hexafluoride which supports to a limit of 400V
- Single interrupter sulfur-hexafluoride which supports to a limit of 220V
Pros
- Works best in hostile or hazardous environmental conditions since it’s sealed.
- The process is free of noise.
- It requires minimum maintenance and pieces of equipment
- There is no atmosphere exhaust.
- Has a supreme property of arc extinguishing.
- The process is free from moisture as the chamber full of gas maintains it dry.
- Has the capacity for larger voltage, since sulfur-hexafluoride dielectric strength is closer to three times more than that of air.
Cons
- Reconditioning of sulfur-hexafluoride is a must after completion of each process
- It’s an expensive process considering the cost of sulfur-hexafluoride
- Air Magnetic Circuit Breakers
Air magnetic circuit breakers, also known as Arch Chute circuit breaker comprises of several plates arranged between the contacts. The plates are either materials with insulation or metallic. During an arc occurrence, the contacts come into contact with the plates in a series arrangement. Due to this, the major occurs is spread across the plates forming smaller arcs and in the process there is a voltage drop. Its plates are metallic in most cases.
In the modern world, having circuit breakers as part of your electrical system is vital. It’s a form of safety mechanisms that shield you and your house from the rage of excess power. Whenever there is a power surge in the building, this device cuts off the electric current flow. Through this, you can call for a technician to rectify the problem. If it’s a minor issue, you can manually turn on the switch of the breaker and get your power on.
Lacking such devices as part of your electrical lines in your household is like a disaster waiting to occur. A power surge has the potential to create a fire hazard in your building. Apart from that, it can destroy all the electronic equipment connected directly to it causing you a devastating loss of property.
What is a circuit breaker?
It is a mechanical device that works like a switch, i.e. it can be turned on and off. But unlike switches, they serve to regulate the power flow limit in a circuit. In case of an overload of power, they cut off the current flow automatically. At times it may not be a power surge, but instead on overconsumption of power than what is provided by the circuit. Also, this switch can be controlled manually by either switching it on or off as the users prefer.
For a device to be a circuit breaker, it has to have the following features:
- It has to have the capacity to complete the circuit or break the circuit under fault conditions.
- It can break the circuit automatically under fault conditions.
- After a fault, it can automatically or manually complete the circuit.
Why do you need a circuit breaker?
The power plant delivers electricity to the houses and building through a power distribution grid. The power at this point is usually at a consistent voltage of 120 or 240 volts depending on the country. In the buildings, the power gets directly into a large circuit which is further distributed into small circuits.
The power delivery cables are two, one being the hot wire from the plant and the other neutral wire which is directed to the ground. The difference in power between the two wires creates a voltage. The charge will move when you close the circuit.
In the building or houses, the neutral and hot wires never come into contact. This is because the charges that flow through the circuit system passes through appliances that serve as resistors. Through this, the size of charge flowing through a circuit gets limited by the appliance resistance. And this keeps the charges at low levels for safety reasons. Excess charges flowing through a circuit, however, have a potential to overheat the wires of the appliances and may cause a fire break out in the building.
Thus, the resistance of the appliance keeps the flowing system of electricity to run smoothly. But at times something may go wrong and the hot wires get linked to neutral or a conductor leading to the ground. It might be overheating of a fun motor fusing the neutral and hot wires. Or accidental puncturing of the power lines with a nail into the wall.
After fusion of hot and neutral wires, there is lower resistance in between. Thus there is a huge charge push by the voltage when this continues, the wires overheat and melt starting a fire. And this is where circuit breakers come in handy. They serve to cut off the power supply whenever it’s on an unsafe level. But how does this happen? Below you are provided with the details of how a circuit breaker works.
Basics of electricity
To understand the workings of this device, you need to go through the below terms. They form the basic foundation of the operation of the breaker. They include:
- Current
- Voltage
- Resistance
- Amps
- watts
Current: this refers to the flow of charges in a given circuit. It may also be defined as the measure of the rate of movement of charges at a given point in the circuit.
Voltage: this is the amount of pressure that causes a chain of electrons in a conductor to move. Majority of the current in the household is under 120V pressure. Though there are electrical appliances that demand higher pressure. This can be in the range of 220V to 240V.
Resistance: the flow of electrons takes place in the conductor due to the pressure exerted on them. The conductor on the other end has properties that work against the current flow, also referred to as resistance. The size of the resistance to electron flow relies on the size and composition make up of the conductor.
Amps: is the measure of electrons that were forced to move under pressure past a given distance in a second.
Watts: is a measurement unit of electrical power. It shows the number of electrons that were pushed into an electrical device to make it functional. And this is what you get billed for by the electrical company at the end of every month.
Voltage, resistance, and current all have an interrelationship. The change on one affects the size of the other. Their relationship is simplified into one equation, where voltage equals to current multiply by resistance (written as V= I × R). Other alternative forms of the equation are (I = V/R, or R = V/I). When you look at this equation, you will understand their linked relation. For example when there is a decrease in pressure and an increase in resistance, the number of charges flowing through the conductor reduces. But if you reduce the conductor resistance and increase more pressure, the amount of electrons flowing through the conductor raises.
The principles of operation
The main role of these devices is to detect the abnormalities and cut off the flow of current. Apart from that, they are also applicable in ordinary operations like turning on and off the current by the operator. The principle of operating theses devices is quite simple.
A typical form of this device comprises of two contacts, one is mobile and the other is fixed. They are also known as electrodes. The two contacts are usually closed in normal conditions of circuit operation.
During a fault in the circuit system, this device will automatically detect the abnormality in the current flow. The response will be to cut off the flow of current by opening the electrode contacts until the issue is resolved.
The automatic turning off of the device is normally under a mechanism which is simple. During the fault, the contacts get pulled away from each other by the trip coil. This occurs after the trip coil gets energized due to the fault.
Some have automatic switches which are remote under remote control, while others are switched “ON” manually. Also, the contacts can get opened manually for other reasons rather than a fault in the system. This is done by turning off the switch of the device.
During the contacts opening as a result of a fault, an important process occurs, which is known as the “Arc phenomenon”. During the automatic contact separation, an arc striking occurs between them. This arc maintains the current flow until it gets distinguished.
Apart from maintaining the current flow after the fault, the arc also creates significant heat energy. This energy has the capacity to damage the circuit system as a whole including the device itself. Hence the arc extinction is one of the main challenges that circuit breakers face.
Formation of the arc during separation of contact
Despite being a cause of devastating damages if not taken care of, the arc serves a vital role in CB. It is often a process which is very simple and occurs between the contacts during their separation.
During electrodes separation as a result of the abnormality in the current flow, the arc formation starts due to intricate processes. The processes happen during the actual electrode contact separation and immediately after parting.
At normal CB functioning, when the electrode contacts are closed, there is a constant mechanical pressure applied to them. This keeps the resistance of the contacts low. But during the opening of the contacts, the mechanical force is withdrawn. Subsequently, this result in an increase in resistance in the contacts, and the coils acts to pull open the contacts.
Just before contacts movement, there is an increase in ohmic heating. And as the electrode contacts begin to move prior to separation, there is further increase in contact resistance. This continues as the contact area reduces until that moment when the contact area is smallest and the resistance at its peak, causing the electrode contacts to melt as a result of ohmic heating.
The rise in temperature as a result of electrode contact heating and also the voltage formation in the gap during the separation has the capacity to generate emission of electrons from the electrode contacts which causes ionization of the contact medium.
This creates a continuous path for the overload current to flow despite the separation of the contacts. The path is known as the arc which is self-sustaining and maintains the link between the contacts even after separation. This is very fatal to the CB and appliances connected to the general circuit.
The properties of the arc rely on the type of medium which exists between the contacts. Some of the common medium used in CB includes sulfur-hexafluoride, air, vacuum, oil, etc. this also depends on the CB type.
Extinction of the arc
For a complete current interruption, the arc extinction must be done successfully. The extinction of the ark relies on its properties which also rely on the design of the interrupter. However, for the extinction to be successful, it has to be done against the gap recovery voltage.
This voltage comprises of 50Hz voltage normal frequency power, and it is together with a transient decaying voltage which is circuit-dependent and it’s generated by energy discharge that was stored in capacitance which is leaking.
For a successful interruption of the current to be complete, it involves the recovery of the arc to a state of an insulator. And at the same time, have a capacity to tolerate the voltage between the electrode contacts which is almost causing a breakdown of the gap. In simple terms, this is a competition between the increasing gap dielectric strength and the increasing gap voltage after the interruption.
Therefore, if the gap post interruption voltage between the electrode contacts is less than the dielectric strength of the contacts gap voltage, then there will be a successful interruption. But if the dielectric strength voltage of the gap is less than the post interruption voltage of the gap, then a gap re-strike is bound to occur.
Recovering from a re-strike is more intricate than a simple competition analogy. But when it occurs, it can fall under the following categories: thermal re-ignition and dielectric breakdown.
- Thermal re-ignition
Exactly after the interruption, the contact gap still harbors high heat energy from the arc remnants. Also, some free electrons are still present. And due to this, when a re-strike voltage imposes across the gap of recovery, this may result in mild post-arc flow of current. The conductivity, in this case, is finite and as a result, there will more induction of thermal energy in the gap.
But if the cooling process is powerful enough to get rid of the heat, and in the process, the post-arc flow of current gets extinguished, then interruption of current will be considered successful. But if it’s not powerful enough, the gap thermal contents will build up gradually and a re-strike will eventually occur through thermal re-ignition. The occurrence of thermal re-ignition takes a few microseconds after the arc extinction.
- Dielectric breakdown
The mechanism of breakdown is essentially a type of spark breakdown. And this is whereby; there is an application of a voltage across the gap of insulation. Also, there is no presence of free charges until a given point when a free electron gets into the gap, and as a result initiates a bit of ionization which will start off the dielectric breakdown.
Breakdown of such kinds tends to take longer periods (hundreds of microseconds) in order to occur after the arc extinction. It happens when the voltage level across the gap has increased relatively high. And it’s then capable of accelerating the free-electron available by giving it sufficient power to start ionization through collision with atoms and molecules available between the contact gap as the medium.