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.
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.
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.
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.
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.
These breakers are used for voltages up to 600 V and are further classified into 3 types.
They are used for currents of around 20 Amps to 2500 Amps and are often used to switch ON or OFF a circuit. They are placed in a sealed case hence they are not maintainable and are usually applied in switchboards and panel boards.
It should be noted that MCCBS should be tested in compliance with UL489 and NEMA AB-1 standards.
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 as follows:
These Circuit breakers use a high-pressure air blast as the arc quenching medium.
The air blast cools down the arc and pushes away the arc products into the atmosphere consequently extinguishing the arc.
Air circuit breakers have now mostly replaced oil circuit breakers because there no fire hazards involved. They are also compact and have lesser arcing time. Most of the high voltage systems above 110 KV employ Air 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.
It is given by the following formula:
Where IF =rated breaking current in Amperes.
These are a type of MCCB designed to protect motors. They contain an adjustable magnetic trip which can be configured to isolate the motor in a fault condition. This configuration can be done according to the motor type. It should be noted that inrush current is not treated as a fault and is allowed to pass.
These tripping devices are equipped with power electronics and programmable software. They are much faster and more reliable than traditional circuit breakers and require relatively lesser servicing.
They also have additional trip features such as long time, short time, instantaneous tripping and ground fall tripping
These trip units consist of a bi-metallic thermal strip that controls the operation of the circuit breaker. The overheating caused by high short circuit current will result in bi-metal strip to trip the circuit breaker, the delay will depend on the magnitude of fault current. These are mainly used to protect our system from overloads.
These trip units are available in both MCCBs and PCBs and provide instantaneous tripping while 30 cycle short time settings can also be achieved for PCBs by using thermal only units.
Overall all circuit breakers offer protection against overcurrent. The selection of molded case circuit breakers, insulated case circuit breakers, and power circuit breakers in a system usually depends on the intended application, the required design standards, and specifications.
An engineer must consider the parameters discussed above 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.
MCCBs and ICCBs have the highest interrupting capacity along with instantaneous tripping so our system does not need to bear high currents for any time delay. While PCBs have high interrupting capacities, optional instantaneous pickup settings but highest short time rating models.
Operation requirements such as a draw out mounting will require PCBs while fixed mountings will involve MCCBS or ICCBS. Economic advantages are always important so the best compromise between ratings, frame size and cost are selected. MCCBS and ICCBS are relatively less expensive than PCBs.
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.
Principles of power systemVK Mehta
Practical power system protectionMark brown, L.G Hewitson