Circuit Breaker is an automatically operated electrical switch designed to allow the buses and components to be energized and de-energized as needed during normal switching operation. And as a protection device that receive a trip signal from a protection relay to clear a fault and isolate the associated loads to protect the overall system.

Circuit breakers would be classified with the below three items:

A. Voltage Level

B. Arc Quenching Medium

All circuit breakers have contacts, and when the contacts separate during an opening operation they draw an arc, assuming that power is flowing through the device. The electric arc forms between the contacts and is drawn out in length as the contacts open. Therefore, circuit breakers require a device to control or remove the arc; the 4 common mediums used to extinguish an arc during breaker contact separation are Oil, Air, Gas, and opening in a Vacuum.

• Air circuit breaker:

ACB contacts open in air and use a combination of the characteristic of the arc, air, and a magnetic field to extinguish the arc. The breaker closed current flowing through the contacts. The main contacts open arc contacts are closed. arcing contacts separate and arc strikes. The arc transfers to the runners and is drawn out horizontally simultaneously, the blowout coil provides a magnetic field to draw the arc into the arc chutes. The arc is transferred to splitter plates to form a number of short series arcs. Small arcs are stretched into loops where it is cooled. Arc is extinguished.

• Air Blast circuit breaker:

Generally, these breakers are used in high voltage applications. They also are used in high voltage DC applications. As the name implies, the contacts of an air blast breaker separate in air. Whereas the medium voltage ACB’s have a mechanical device which displaces air into the arc during opening operation, the air blast breaker uses a stream of compressed air directed toward the separable contacts of the breaker to interrupt the arc formed when the breaker is opened.

• Oil circuit breaker:

Oil circuit breaker contacts are immersed in oil. Current interruption takes place in the oil, which cools the arc developed, and quenches the arc. All three phases of small oil circuit breakers can be placed in one oil tank; however, large high voltage circuit breakers have each phase in a separate oil tank. The oil tanks in oil circuit breakers are normally sealed and the electrical connections between the contacts and external circuits are made through porcelain bushings similar to oil transformers.

• Vacuum circuit breaker:

The vacuum circuit breaker interrupts the current in a vacuum. The contacts of a vacuum breaker are enclosed in a ceramic envelope or "bottle” that is evacuated to an extremely low atmospheric pressure, approximately 0.0023 in. Hg absolute. Although the physics of interruption are quite complex, the vacuum interrupter works because the arc requires a conducting path to sustain it, but within the vacuum bottle there are no gasses to ionize; therefore, for all practical purposes, there can be no conducting path and the arc cannot be sustained.

• SF6 Gas circuit breaker:

Gas breakers are commonly called SF6 breakers for the Sulphur hexafluoride gas used. The arc generated during opening is extinguished in a chamber filled with pressurized sulfur hexafluoride gas. One advantage SF6 over vacuum breakers is that even if the gas bottle loses pressure the residual SF6 gas is normally enough to allow the breaker to open safely under a normal load. There are environmental concerns with the SF6 gas if it leaks.

C. Operating Mechanism

• Spring type mechanism:

where the circuit breaker is equipped with 2 springs, one for close [Red Color] and one is for open [Green Color]. The Closing spring is charged by a DC or AC motor, and it is bigger than the opening spring as the inertia of it shall be enough to close the CB mechanism as well as charging the Opening spring.

The opening spring is charged during the CB closing operation in order to be ready for tripping in case of closing onto fault.

• Hydraulic type mechanism:

Where a pump motor is used to pressurize the oil stored in a low-pressure tank to charge a Spring (Disc) that would be used for closing and opening operations of the CB.

Rated Voltage [Ur]

rated voltage, rms value, kV. It is the maximum rms value of voltage that the equipment can withstand permanently.

Rated Power Frequency Withstand Voltage [Ud]

rated power frequency withstand voltage, rms value, kV during 1 min. It defines the level of rms over-voltages that the equipment may withstand for 1 minute.

Rated Lightning Impulse Withstand Voltage [Up]

rated lightning impulse withstand voltage, peak value, kV. It defines the level of lightning over-voltages that the equipment may withstand.

Rated Short Circuit Breaking Current [I_SC]

The rated short-circuit-breaking-current of a circuit-breaker is the highest value of short circuit current which a circuit-breaker can break under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kA RMS at contact separation.

Rated Short Circuit Making Current [I_MC]

The rated short-circuit making current of a circuit-breaker is the peak value of first current loop of short-circuit current which the circuit-breaker is capable of making at its rated voltage.

Operating Sequence [Duty Cycle]

O – 0.3Sec – CO – 3min - CO

O for Open operation and C for Close operation

0.3Sec and 3min are Dead Times which are The minimum time kept for the Breaker after an open operation in order to be ready for a successful CO operation (Close operation and Instantaneous Open in case of Switching on to fault)

The Delay times is to allow

1. Cooling down for CB Poles to avoid the deformation for the Poles contacts with high temperatures due to the ARC if it closed again which would lead to bad contact resistance.

2. The Insulation medium to recover its strength after the ARC Quenching.

Tests done on the Circuit Breaker

a. Timing. [Using EGIL or TM1720 Breaker Analyzer Test Kit]

• Closing Time.

• Opening Time.

• Close-Open Time.

• Open-Close-Open Time.

b. Contact Resistance. [Using MOM or DLRO Test Kits]

c. Insulation Resistance Test. [Using Megger Test Kit]

A. Timing. [Using EGIL or TM1720 Breaker Analyzer Test Kit]

B. Contact Resistance Test

Contact resistance measurement and its application domain are fairly extensive. At the electric, the connection of circuit has various ways and means, such as connected by welding, by pressing, by plugin and blot tightly, and so on. If you want to know the quality of a connector and its conduction characteristic, you just need to measure its contact resistance.

Measurement methods for different applications are not the same. For example, in the case of measuring the contact resistance of high-power switches and relays, a high current should be used, (approximately 100A). The high current can melt the oxide film and dust between pairs of contact, things just like the condition that is actually happening in working status. In the case of a dry circuit connector, the test current should be low to prevent the joint to be melted by heat, (the current less than 100mA).

There are 4 kinds of reason to induces the measurement uncertainty:

• The definition of contact resistance is not perfect. The voltage drop across the contact includes the voltage drop of the bulk, inevitably. So, the voltage-measuring terminals should be connected as closely as possible to the contact area under test. This will help to prevent the voltage drop, which is caused by the test leads and bulk resistance, from being added into the measurement.

• The oxide film and dust between pairs of contact have been changing with heat. The conductivity of contact has been changing with the temperature uncertainly.

• The stress on the contact could influence the area of contact, and then influence the value of contact resistance. The stress changed with temperature, air pressure humidity and time along.

• Some measurement uncertainties of contact resistance have been also caused by measurement instrument.

The measurements shall be made with D.C by measuring the voltage drop or resistance across the terminals of each pole. Special consideration shall be given to enclosed switchgear and control gear.

The current during the test shall have any convenient value between 50 A and the rated normal current.

Note

• Experience shows that an increase of the main circuit resistance cannot alone be considered as reliable evidence of bad contacts or connections. In such a case, the test should be repeated with a higher current as close as possible to the rated normal current.

• The measurement of the D.C voltage drop or the resistance shall be made before the temperature- rise test, with the switchgear and control gear at the ambient air temperature and after the temperature rise test when the switchgear and control gear has cooled to a temperature equal to the ambient air temperature. The measured resistances in these two tests shall not differ by more than 20%.

• The measured value of the D.C voltage drop or the resistance shall be given in the type-test report, as well as the general conditions during the test (current, ambient air temperature, points of measurements, etc.).

C. Insulation Resistance Test

• It is used to measure the insulation resistance of electrical equipment when applying a DC voltage to the measured part to judge whether there is a dielectric breakdown in the insulation materials.

• The test is to make sure that the insulation can withstand constant stress for a specified length of time without breaking down.

The table shows the recommended test voltages according to the operating voltages of installations and equipment (taken from the IEEE 43-2000 Guide).

In addition, these values are defined for electrical appliances in a wide variety of local and international standards (IEC 60204, IEC 60439, IEC 60598, etc.).