• Electrical India
  • May 5, 2015

Electrical Circuit Breakers

Many unwanted disturbances or fluctuations in voltage or current of electric circuits take place due to many known or controlled and unknown or uncontrolled reasons – like sparking, high voltage, moisture, leakage, over loading etc. Thus, use of appropriate switchgears protect us from harmful damages... 

-Dr S S Verma

Electricity has become a part of life for present day civilisation. Day and night, we are making use of electricity in one form or the other and many costly electronic appliances are put to use based on electricity. The safety and security of the users as well as a long life of the electronic devices are
desired by every one of us. Many unwanted disturbances or fluctuations in voltage or current of electric circuits take place due to many known or controlled and unknown or uncontrolled reasons – like sparking, high voltage, moisture, leakage, over loading or short circuitt. Under these circumstances, it is mostly desired that the electric circuit should automatically break (on its own) on the occurrence of such unwanted incidents – so that nothing wrong happens to electronic devices as well as to their users.
The circuit breaker is an absolutely essential device in the modern world, and one of the most important safety mechanisms in places where electricity is being used. Whenever electrical wiring in a building has too much current flowing through it, these simple machines cut the power until somebody fixes the problem. Without circuit breakers (or the alternative, fuses), flow of electricity would be impractical because of the potential for fires and other problems resulting from simple wiring problems and equipment failures. Electric breakers play a great role in this direction.
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt current flow. Electrical circuit breaker is a switching device that can be operated manually as well as automatically for controlling and protection of electrical power system respectively.
As the modern power system deals with huge currents, special attention should be given during designing of circuit breaker to control interruption of arc produced during the operation of the circuit breaker. Now, I shall discuss the electronics behind the electric breakers, their working, types and advantages.
The modern power system deals with huge power network and huge numbers of associated electrical equipment. During short circuit fault or any other types of electrical fault, these equipment as well as the power network suffer a high stress of fault current in them – which may damage the equipment and networks permanently. For saving these equipment and the power networks, the fault current should be cleared from the system as quickly as possible.
Again after the fault is cleared, the system must come to its normal working condition as soon as possible for supplying reliable quality power to the receiving ends. In addition to that for proper controlling of power system, different switching operations are required to be performed.
So, for timely disconnecting and reconnecting different parts of power system network for protection and control, there must be some special type of switching devices, which can be operated safely under huge current carrying conditions. During interruption of huge current, there would be large arcing in between switching contacts, so care should be taken to quench these arcs in circuit breaker in safe manner. The circuit breaker is the special device that does all the required switching operations during current carrying condition.

Science of current flow

It is essential to find out how circuit breakers monitor electrical current and how they cut off the power when current levels get too high. The circuit breaker is an incredibly simple solution to a potentially deadly problem. To understand circuit breakers, it helps to know how household electricity works. Electricity is defined by three major attributes: voltage, current and resistance. Voltage is the 'pressure' that makes an electric charge move. Current is the charge's 'flow' – the rate at which the charge moves through the conductor, measured at any particular point. The conductor offers a certain amount of resistance to this flow, which varies depending on the conductor's composition and size. Voltage, current and resistance are all inter-related – we can't change one without changing another. Current is equal to voltage divided by resistance (commonly written as I = V / R). This makes intuitive sense: If we increase the pressure working on electric charge or decrease the resistance, more charges will flow. If we decrease pressure or increase resistance, less charges will flow. The power distribution grid delivers electricity from a power plant to utilising place. Inside the place, the electric charge moves in a large circuit, composed of many smaller circuits. One end of the circuit, the hot wire, leads to the power plant. The other end, called the neutral wire, leads to ground. Because the hot wire connects to a high energy source, and the neutral wire connects to an electrically neutral source (the earth), there is a voltage across the circuit – charge moves whenever the circuit is closed. The current is said to be alternating current, because it rapidly changes direction.
The power distribution grid delivers electricity at a consistent voltage (240 Volts), but resistance (and therefore current) varies in a house. All of the different light bulbs and electrical appliances offer a certain amount of resistance, also described as the load. This resistance is what makes the appliance work. A light bulb, for example, has a filament inside that is very resistant to flowing charge. The charge has to work hard to move along, which heats up the filament, causing it to glow. In building wiring, the hot wire and the neutral wire never touch directly. The charge running through the circuit always passes through an appliance, which acts as a resistor. In this way, the electrical resistance in appliances limits how much charge can flow through a circuit (with a constant voltage and a constant resistance, the current must also be constant). Appliances are designed to keep current at a relatively low level for safety purposes. Too much charge flowing through a circuit at a particular time would heat the appliance's wires and the building's wiring to unsafe levels, possibly causing a fire. This keeps the electrical system running smoothly most of the time. But occasionally, something will connect the hot wire directly to the neutral wire or something else leading to ground. When the hot wire is connected directly to ground, there is minimal resistance in the circuit, so the voltage pushes a huge amount of charge through the wire. If this continues, the wires can overheat and start a fire. The circuit breaker's job is to cut off the circuit whenever the current jumps above a safe level.

Working principle

The circuit breaker mainly consists of fixed contacts and moving contacts. In normal 'on' condition of circuit breaker, these two contacts are physically connected to each other due to applied mechanical pressure on the moving contacts. There is an arrangement stored potential energy in the operating mechanism of circuit breaker, which is realised if switching signal given to the breaker. The potential energy can be stored in the circuit breaker by different ways like by deforming metal spring, by compressed air, or by hydrolic pressure. But whatever be the source of potential energy, it must be released during operation. Release of potential energy makes sliding of the moving contact at extremely fast manner. All circuit breakers have operating coils (tripping coils and close coils), whenever these coils are energised by switching pulse, the plunger inside them is displaced. This operating coil plunger is typically attached to the operating mechanism of the circuit breaker, as a result the mechanically stored potential energy in the breaker mechanism is released in forms of kinetic energy, which makes the moving contact to move – as these moving contacts mechanically attached through a gear lever arrangement with the operating mechanism. After a cycle of operation of circuit breaker, the total stored energy is released and hence the potential energy again stored in the operating mechanism of circuit breaker by means of spring charging motor or air compressor or by any other means. There are electrical characteristics of a circuit breaker, which also should be considered for the operation of the circuit breaker.

Electrical principle

The circuit breaker has to carry large rated or fault power. Due to this large power there is always dangerously high arcing between moving contacts and fixed contact during operation of a circuit breaker. The arc in a circuit breaker can be quenched safely if the dielectric strength between the current carrying contacts of circuit breaker increases rapidly – during every current zero crossing of the alternating current. The dielectric strength of the media in between contacts can be increased in a number of ways, like by compressing the ionized arcing media since compressing accelerates the deionization process of the media, by cooling the arcing media since cooling increase the resistance of arcing path or by replacing the ionized arcing media by fresh gasses. Hence a number of arc quenching processes should be involved in operation of circuit breakers.

Schematic diagram of the function of a circuit breaker...

The simplest circuit protection device is the fuse. When a circuit is closed, all charge flows through the fuse wire – the fuse experiences the same current as any other point along the circuit. The fuse is designed to disintegrate when it heats up above a certain level – if the current climbs too high, it burns up the wire. Destroying the fuse opens the circuit before the excess current can damage the building wiring. The problem with fuses is that they work only once. A circuit breaker does the same thing as a fuse – it opens a circuit as soon as the current climbs to unsafe levels – but we can use it over and over again. The basic circuit breaker consists of a simple switch, connected to either a bimetallic strip or an electromagnet. The hot wire in the circuit connects to the two ends of the switch. When the switch is flipped to the on position, electricity can flow from the bottom terminal, through the electromagnet, up to the moving contact, across to the stationary contact and out to the upper terminal. The electricity magnetises the electromagnet. Increasing current boosts the electromagnet's magnetic force, and vice versa. When the current jumps to unsafe levels, the electromagnet is strong enough to pull down a metal lever connected to the switch linkage. The entire linkage shifts, tilting the moving contact away from the stationary contact to break the circuit. Thus, the electricity shuts off.

Types of circuit breaker

Depending on different criteria, the circuit breakers are named differently. As per their arc quenching media, they are classfied as vacuum circuit breaker, oil circuit breaker, air circuit breaker and dielectric circuit breaker. Also, as per services, they are divided into outdoor circuit breaker and indoor circuit breaker. Depending on the operating mechanism, they can further be divided as spring operated circuit breaker, pneumatic circuit breaker and hydraulic circuit breaker. Based on voltage level, the circuit breakers are named high voltage, medium voltage and low voltage circuit breakers.

Advanced designs 

A bimetallic strip design works on the same principle, except that instead of energising an electromagnet, the high current bends a thin strip to move the linkage.
Some circuit breakers use an explosive charge to throw the switch. When current rises above a certain level, it ignites explosive material, which drives a piston to open the switch. More advanced circuit breakers use electronic components (semiconductor devices) to monitor current levels rather than simple electrical devices.
These elements are a lot more precise, and they shut down the circuit more quickly, but they are also a lot more expensive. For this reason, most houses still use conventional electric circuit breakers.
One of the newer circuit breaker devices is the Ground Fault Circuit Interrupter (GFCI). These sophisticated breakers are designed to protect people from electrical shock, rather than prevent damage to a building's wiring. The GFCI constantly monitors the current in a circuit's neutral wire and hot wire. When everything is working correctly, the current in both wires should be exactly the same. As soon as the hot wire connects directly to ground (if somebody accidentally touches the hot wire, for example), the current level surges in the hot wire, but not in the neutral wire. The GFCI breaks the circuit as soon as this happens, preventing electrocution.
Since it doesn't have to wait for current to climb to unsafe levels, the GFCI reacts much more quickly than a conventional breaker. All the wiring in a house runs through a central circuit breaker panel (or fuse box panel), usually in the basement or a closet.
A typical central panel includes about a dozen circuit breaker switches leading to various circuits in the house.
One circuit may include all the outlets in the living room, and another may include all the downstairs lighting. Larger appliances, such as a central air conditioning system or a refrigerator, are typically on their own circuit breakers.

The Author is a Professor at Sant Longowal Institute of Engineering.