How Circuit Breakers Work?

Circuit Breaker- Need and Definition

Electricity coming to our house or any other places from the power distribution grids forms a large circuit with the lines connecting to the power plant forming one end called the hot wire and the lines connecting to ground forming another end. The electrical charge flows between these two lines and potential are developed between them. The connection of loads(appliances) offering resistance to this flow of charge completes the full circuit and the whole electric system inside the house works smoothly as long as the appliances have sufficient resistance and do not cause any over current. Short-circuiting or too much charge flowing through the circuit or sudden connection of the hot end wire to the ground wire would heat the wires, causing a fire. To prevent such situations circuit protection is used which simply cuts off the remaining circuit in such conditions.

Generally, there are two ways of solving this above problem:

Fuse. : It consists of a thin wire enclosed inside a casing. In the case of excessive current, the fuse wire simply burns up or disintegrates causing the circuit to break. However, they are not reliable and the fuse wire has to be manually changed once it burns. Thus they are mostly not preferred.

Switches: Another way of circuit protection is by ensuring the current flow is stopped or the voltage supply is stopped to the line, in case of over current. This is done by automatic operation of the switch which trips on sensing of overcurrent or any fault, thus isolating the fault line from the whole circuit and again can be switched on to restore the operation.  It is more advantageous as it allows quick identification of the fault zone and quick restoration. It is also electrically safe compared to a fuse.

Electronic Fuse

Before we go into details about the Electronic Circuit breaker, let us have a look at an Electronic fuse.

The voltage rating of a relay should be equal to the applied voltage and the capacitor of 100uF should be used and the current passing through the circuit can be adjusted using a 100K potentiometer. If a fuse is used, the R2 value should be lowered. While SW1 is made on that brings L2 to circuit hence the current across the resistor R2 increases causing a higher voltage drop across R2.

Resettable Electronic Fuse – Circuit Diagram:

Through the preset 100K and R1, this voltage triggers the SCR U1 that operates the relay RL1. This disconnects the supply to the load and at the same time removing the supply to the SCR. The overload should be removed and sw2 should be switched off and switched again to reset. Any SCR can be used to meet the voltage and gate triggering requirements.

Need for Electronic Circuit Breaker

A traditional Miniature Circuit breaker consists of a bimetallic strip to protect against load current and an electromagnet to protect against short circuit current. In case of overloading, the bimetallic strip bends causing the release of the spring with the movement of latch point and eventually opening of the MCB contacts. The electromagnetic coil develops a magnetomotive force across it when a large current flow through it, which causes the latch point to be displaced and this again opens the MCB contacts. Thus in case of overload and short circuit, the MCB trips to off position.

However, there are several disadvantages to this conventional Miniature Circuit breaker:

• They are quite expensive and more is the short circuit current, more is the cost of the MCB.
• The bimetallic strip tends to be deformed easily due to heat or increase in temperature from the surrounding causing a reduction in the breaker’s current capacity.
• Because of mechanical components being used, they are more prone to wear and tear.
• Tripping time is slower.

To overcome all these problems, the most convenient solution is using an Electronic Circuit Breaker or a circuit breaker involving an electronically controlled automatic switch. It does not involve any electromagnetic coil or any thermal strip or any mechanical component.

Defining an Electronic Circuit Breaker

An electronic circuit breaker consists of the automatically operated switch controlled by the feedback from the load. It is based on the fact that at the time of current being too much drawn by the loads or being too much flowing in the line, the switch is automatically closed for a time and the switch is then automatically turned on after that certain amount of time. The switch can be a power electronic switch like an SCR or an electromechanical switch like a relay, which is controlled by any current sensing element like a resistor. This ultra-fast circuit breaking device uses a series resistor to sense the current and while it exceeds the set value, the corresponding voltage drop (across the series resistance) also increases. This voltage is sensed, rectified to DC, and then compared with a preset voltage by a comparator to generate an output that drives a relay through a MOSFET to trip the load instantaneously. The tripping mechanism is very fast as it is based on current sensing principles rather than thermal-based trip mechanisms like MCB. A microcontroller can be used for getting a display on an LCD upon the status of the circuit breaker.

Thus, by using this device, ultra-fast circuit breaking can be achieved to save expensive equipment from possible damage. Using this unique concept a prototype can be developed as project work for electrical engineering students.

An electronic circuit breaker works on the principle of the current sensing mechanism. It provides both overload and short circuit protection as in any case the current through the line is monitored and the switch gets tripped in case of overcurrent being flowing.

Working Example of a simple Electronic Circuit breaker

A current sensing element or a resistor can be used to sense the amount of current flowing through the load.  The voltage drop from the resistor is given to the noninverting input of the comparator and a fixed voltage is given to the inverting terminal of the comparator. In the case of normal operation, (current flowing with an adequate number of loads), the voltage drop across the resistor is less than the fixed voltage and the comparator input is low enough to cause the MOSFET in off condition. The common contact of the relay is connected to the normally closed contact and the circuit is completed with the load getting current supply from the mains.

However when any extra load is connected, the current through the current sensing element increases, which in turn increases the voltage drop across the resistor. At some point in time, this voltage drop is more than the fixed voltage, i.e. input at the noninverting terminal is more than the input at the inverting terminal of the comparator. This causes a high logic output at the comparator, with a voltage enough to trigger the MOSFET to on condition. As the MOSFET conducts, the relay coil gets energized and the common contact is now connected to normally open contact. This causes a hindrance to the flow of current as the circuit is now broken and the loads get switched due to lack of power supply.

Advantages of Electronic Circuit Breaker

• Electronic circuit breakers can be designed to trip at small overloads and they do not react to inrush currents.
• They have faster response time as the response characteristics only depend upon the time taken for the current passing through the conductive semiconductor junction to be zero.
• They do not suffer from the conventional systems’ problems of wear and tear as the components used are electronic.
• They are less expensive as the components used are lighter and less costly and easy to maintain.

Practical Electronic Circuit Breakers

Electronic Protection Switch by Phoneix

It works with a supply of 24 V DC and comes with a monitoring and remote signaling concept. It consists of a remote-controlled reset. It is used to protect relays, programmable controllers, motors, sensors, actuators, valves, etc.

HFDE308032 

It comes with 15-80 A adjustable current features and consists of adjustable long time setting, short time setting and instantaneous setting with status signal and alarm integrated to it.

Photo Credit:

• Electric Fuse by Pragati sales
• NC100H Circuit Breaker by CIRTG