Why circuit breaker testing is needed?
The circuit breaker is a device that can be used to break a circuit. This device can break a circuit manually or automatically. The power system operation requires to turn off or on different circuits and this will be done in transmission lines, generating stations, and also in the distribution section. There is a need to turn ON or OFF the circuit in case of fault conditions or during normal conditions. So in order to do this, we can use a circuit breaker, this device can turn ON and OFF various devices in a power system.
This is a switching device that would protect the circuit in case of an overload. The selected circuit breaker must be able to make and break the circuit during faulty and normal conditions. By testing the circuit breaker we could also determine if the selected CB is suitable for the application or not. So by testing a circuit breaker, we can check its operational characteristics, constructional features, and also we can calculate its electrical parameters. The performance of the circuit breaker can be determined during the normal and abnormal conditions by performing certain tests. The major purpose of testing a circuit breaker is to determine if the circuit breaker is capable to work on certain voltage and current ratings.
How to select a circuit breaker and what are the requirements for a circuit breaker?
- The CB should be capable to carry the rated current without exceeding the temperature limits
- It should interrupt the circuit in case of any fault current because of the short circuit
- CB must be able to carry the short circuit current
- Closing and opening at the required speed
- It should be able to overcome the repulsive forces while closing on fault
- It should close the faulted circuits
- It must switch the capacitive current
- The CB must do the switching of magnetizing and also the small inductive circuit currents
- The CB must act in case of any short line faults, kilometric faults, and also in case if the line current is out of phase conditions
What is the purpose of the circuit breaker in a power system?
- Its major purpose is to reduce the losses at the closed position
- The circuit should be opened or closed according to the requirement
- The circuit should be opened for the planned maintenance
- During the system fault, the circuit should be opened
- It should be capable to do the isolation between poles, open contact, and earth
What are the factors that must be considered before testing a circuit breaker?
- CB’s breaking minimum must be considered
- CB design should be considered
- Design of the contact system
- The operational mechanism of the circuit breaker
What is the test equipment that is used to do the circuit breaker testing?
- Micro-ohmmeter
- Breaker analyzer
- Power supplies
- Vacuum tester
- High current source
What are the different types of tests in a circuit breaker?
Classification of circuit breaker test
- Type test
- Routine test
Type test
This type of test is done to check the quality of the circuit breaker. The quality of the circuit breaker can be determined by this test.
Routine test
This test is done from the factory to make sure that there is no damage to the product. The routine test is done to determine the material quality of the circuit breaker.
What are the different types of type tests?
- Mechanical endurance test
- Thermal tests
- Dielectric test
- Contact resistance measurement
- Short circuit test
Mechanical endurance test
This test is conducted to determine if the breaker is capable to open and close several times. Some of the operations of the relay are by energizing the relay while the rest is done by closing the trip circuit. The mechanical tests are done in a circuit breaker without any current or voltage in the main circuit. While conducting this test there shouldn’t be any mechanical adjustments but lubricants can be used. The mechanical test would give us an idea of the circuit breaker and how it can perform during fault conditions. The response speed of the breaker can be determined by this test. After this test, we would get an idea about how long the circuit breaker could last.
Thermal test
These tests can be done to determine the maximum normal current that a circuit breaker can handle without exceeding the maximum temperature range of the circuit breaker. This test is conducted by allowing the normal current with normal frequency to pass through the breaker. The current will be passed till a steady temperature is attained. When the CB reaches the steady temperature range then the maximum temperature rise of each part must be less than the permissible limit. The rise in temperature can be measured with the help of a thermometer.
Minimum pick up test
This test is done to determine the lowest voltage required to trip or close the coil. By doing this test we can determine if the breaker would operate at the lowest voltage levels so that in case of a power outage the breaker can operate at the station battery voltage level. In order to do this test, apply the lowest specified voltage level to the CB.
Dielectric test
This test is conducted to determine the dielectric conductivity of the breaker, so the insulation testing is done. In this type of test the power frequency testing is done, the test voltage at a certain frequency will be applied to the circuit breaker. The voltage will be applied to the poles while the CB is closed, it will be applied between poles and earth while the circuit breaker is open, and also across terminals with circuit breaker open. So the voltage will be increased during the test and would be kept at the test value for one minute.
Short circuit test
During this test the rated short-time withstand current will be applied to the CB. The ratings of the circuit breaker can be checked by doing this test. This test is done in short circuit testing stations, such as field type testing stations and also the laboratory type testing stations. The short circuit test is classified into two types and they are direct testing and indirect testing. In the direct testing method, the power supply system is used as the source
By doing this test we could determine the ability of the CB to safely interrupt the fault current, making and breaking capacity of the CB at different load currents.
Contact resistance measurement
This test is done to determine the contact condition of the circuit breaker. There is a chance for contact damage while making and breaking contacts at high speed and pressure. This test is carried out by injecting a high current and small voltage so that the contact resistance can be measured.
During this test, the resistance across the closed contact before and after the temperature rise test and the short circuit test value will be used for the testing and also for routine production. The contacts of the circuit breaker should be able to carry the required current without any over-heating. It should be capable to carry a short circuit current for a certain period of time without deterioration or arcing.
Breaker timing test
This test is conducted to determine how fast the breaker can operate. The closing time of the breaker is the time interval for energizing the breaker coils while the contacts are open and to the instant where the contacts come into touch. The opening time of the breaker is the exact opposite of the closing time, it is the time taken by the breaker to separate the contacts from each other during the energizing of the trip coil
ROUTINE TEST
No-load timing measurement
This test is conducted to determine the no-load opening time, closing time, contact-travel characteristics such as velocity would be determined too. This test is done to determine the contact conditions and also the operating mechanism of the circuit breaker.
Contact travel
This is an important type of routine test, by doing this test we can determine the condition of the operating components of a circuit breaker. So we could determine the conditions of the mechanical links and also the interrupter contacts. So by doing this routine measurement we could get a better picture of the overall status of the circuit breaker.
What are the major applications of a circuit breaker?
• Generator CB
• Transformer CB
• High voltage circuit breaker – Capacitor bank CB, Reactor CB, and high voltage DC CB
• Distribution circuit breakers – Traction CB, Industrial CB, and switch dis-connector
