Selecting the Right Circuit Breaker and Its Type
A circuit breaker is a power system protection device that can make or break a circuit.
A circuit breaker operates under fault conditions and isolates the faulty part of the circuit from the rest of it by breaking the circuit. This operation is performed automatically by employing a relay along with the circuit breaker.
It should be noted that circuit breakers may also be manually operated and can also be operated under normal conditions. Hence circuit breakers are also useful switching devices that are used to make or break a circuit in normal conditions.
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Working Mechanism:
In a general sense, a circuit breaker consists of two electrodes or contacts that under normal conditions remain in contact with each other allowing current to flow. But in case of a fault, the contacts open or become disconnected hence breaking the circuit and preventing the fault current from passing through.
The opening of contacts is achieved by energizing the tripping coil of the circuit breaker which causes the contacts to move as shown in the figure. It is also important to know that the tripping coil is energized by the relay, so basically it is the relay that signals to the circuit breaker to operate.
These contacts can also be opened manually, for example during maintenance or switching.
We had previously written an article about How to Select the Right Fuse for Power Systems Protection. If you haven’t checked it out, please click it and grasp the information available in this blog.
Arc Phenomena:
Whenever a short circuit fault occurs, an extremely high current passes through the contacts of the circuit breakers. When these contacts start to open, the area of contact decreases and the current intensity increases rapidly. This causes the surrounding material to heat up rapidly and ionize. This ionized medium thus acts as a path for current to flow delaying the breaking of the circuit path.
This can result in damage to the system and the heat produced can damage the circuit breaker itself. The potential difference between the contacts is quite small but enough to maintain the arc.
Arc Extinction Methods:
This arc needs to be eliminated for the successful isolation and breaking of the circuit. Therefore, it is a major factor in determining the type and size of the circuit breaker to be used in different applications. For this purpose, we have two methods for arc extinction.
1. High Resistance Method:
In this method, the resistance of the arc is increased with time and is increased until the current value drops insufficient to maintain the arc. The disadvantage is the huge loss of energy and heat dissipated in the arc.
2. Low Resistance or Current Zero Method:
This method is used for A.C systems and is most widely used. All sinusoidal current and voltages pass through the zero crossings at every half cycle. The resistance is kept low until the zero crossings where the arc extinguishes naturally, after the zero crossings, the arc is prevented from reoccurring by the quenching medium.
The fastest possible circuit breaker to date can extinguish the arc in 2 cycles while the most common mediums used for quenching the arc are air, oil, Sulphur Hexafluoride SF6, and Vacuum.
Categories of Circuit Breakers:
Circuit breakers can be categorized according to the corresponding voltage level of the system. Therefore, they can be divided into LV, MV and HV breakers.
LV circuit breakers are used for voltages up to 600 V and are further classified into 3 types i.e. Model Case (MCCBS), Power CB and Insulated Case (ICCBS).
MV breakers are used for 600 V to 69 KV systems, while HV breakers are applied to systems having voltages greater than 69KV. The type of medium that exists inside these circuit breakers is used to classify them. They are classified into oil, air, SF6 and vacuum circuit breakers.
Click here to learn types of circuit breakers in more detail.
Important Parameters in the Selection of Circuit Breakers:
- Interrupting capacity /KA: it is the maximum current in which the breaker is designed to interrupt safely at a certain voltage.
- Instantaneous pickup: The settings at which the circuit breaker operates immediately without any intentional delay. All MCCBS and ICBs possess instantaneous trip settings, while for PCBS it is optional.
- Short Time settings: It is the characteristic of a circuit breaker to remain closed for a time interval under a range of high fault currents. It is an important factor in achieving selective coordination among circuit breakers.
- Long Time settings: It is the setting of the circuit breaker to determine the time duration to allow a certain overload current to flow before tripping. (for current magnitudes lesser than short time or instantaneous pickup).
- Continuous Amps: It is the current which the device will carry without tripping or overheating.
- Frame size: Frame size indicates the physical size of the breaker as well as the maximum continuous current it can tolerate.
- Rated KV: It indicates the highest system voltage which the circuit breaker can sustain.
- Rated KVA or MVA: An important characteristic of a circuit breaker is its breaking or rupturing capacity. It is the maximum current that a circuit breaker is capable of breaking at a given voltage and under specific conditions e.g. power factor.
Selection of Circuit Breaker According to Its Applications/Tripping Device:
The selection of circuit breakers in a system usually depends on the intended application, the required design standards, and specifications.
An engineer must consider the parameters discussed here such as short time rating, interrupting capacity, frame size, etc. to determine whether the device is suitable to provide protection, as well as coordination and selectivity.
Conclusively, we can agree that circuit breakers are an essential part of an electrical power system and their proper application is highly important. Along with the basics and working principles of circuit breakers, an engineer must also know the appropriate selection of circuit breakers according to the utilization.
References:
Principles of power system — VK Mehta
Practical power system protection — Mark brown, L.G Hewitson
Let us know if you have any queries regarding this topic and do provide us with your feedback in the comments.
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About The Author
Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.
Abdur Rehman is the CEO and co-founder of allumiax.com and creator of GeneralPAC by AllumiaX. He has been actively involved in various roles in the IEEE Seattle Section, IEEE PES Seattle, IEEE Region 6, and IEEE MGA.
