Maintenance and Testing of Electrical Machine

Week No11 Lecture No: 35

Topic Name: Testing of Circuit Breakers

Introduction
Protective relay and circuit breaker are two main basic protection element. A protective relay is a
device that identifies faults and triggers the circuit breaker to isolate the faulty component from the
rest of the system. Relays detect abnormal conditions in electrical circuits by continuously
monitoring electrical quantities like voltage, current, frequency, and phase angle. When a fault
occurs, these quantities change, indicating the presence, type, and location of the fault to the
protective relays. Once a fault is detected, the relay activates the trip circuit of the breaker, causing
the breaker to open and disconnect the faulty circuit. Hence Protective relay and Circuit breaker
perform a vital role in protecting expensive equipment from damage through fault that is
connecting and disconnecting the electrical power in a reliable way that why this requires proving
their reliability with on field tests during installation and with regular maintenance tests during its
lifetimes to prevent costly failures and problems that could even compromise the safety of the
substation.
A circuit breaker is switching device which can be operated manually as well as an
automatically to protect an electrical circuit from damage caused by an overload or short circuit
that might harm an electrical circuit. The three major duty of circuit breaker is defined in term of
its capacity –
(1) Breaking Capacity-It must be capable of opening the faulty circuit and breaking the fault
current. This ensures the safe disconnection of the electrical circuit under high fault conditions.
(2) Making Capacity: - It must be capable of being closed on to a fault.
(3) Short Circuit Capacity: Must be capable of carrying fault current for a short time while
another breaker is clearing the fault.
These capacities ensure that a circuit breaker can effectively manage and protect an electrical
system under various fault conditions.
Basic Construction of Circuit Breaker-

 A circuit breaker consists of two contacts: one is fixed, and the other is movable.
  The contacts are placed in a closed chamber containing insulating fluid, which serves two
function. (1st) it extinguishes the arc drawn between the contacts when the C.B opens.
(second ) It provides the adequate insulation between the contacts and from each contact to
earth.

 The contacts remain connected under normal operating conditions and do not open
automatically unless there is a fault or current surge in the line.

 When the line experiences an overcurrent, the relay sends a signal that open the circuit
breaker contact.

First figure shows the arc formation when the contact is separated, which is extinguishes by
insulating medium and second fig show the symbol of circuit breaker.
Types of circuit Breakers-Circuit breakers can be classified in different category. It can be
classified on basis of rated voltages. Circuit breakers below rated voltage of 1000V are known as
the low voltage circuit breakers and above 1000V are called the high voltage circuit breakers.
According to their services it classifies as Outdoor and indoor circuit breaker and according to the
operating mechanism it classifies as Spring operated circuit breaker and hydraulic circuit breaker.
The most general way of the classification of the circuit breaker is on the basis of the medium
of arc quenching. which refers to the substance or method employed to extinguish the electrical arc
that arises when the circuit breaker interrupts the current flow. Such types of circuit breakers are as
follows: -
Oil Circuit Breakers (OCB) -Utilize oil as a means to extinguish the arc. Oil serves as an
additional insulation medium, improving the overall dielectric strength of the circuit breaker and
allowing for higher voltage applications.
Air Circuit Breakers (ACB): Utilize air as the medium for extinguishing arc. Their main
advantages are their small size, high-speed response time, little maintenance they need and the
considerable reduction in the risk of fire.
Vacuum Circuit Breakers (VCB)- utilize a vacuum to extinguish the electrical arc. They are
compact, require minimal maintenance, and provide noiseless operation.
SF6 Circuit Breakers- Utilize sulphur hexafluoride gas to extinguish the arc. The use of SF6 gas
as an insulating medium allows circuit breakers to provide superior performance, reliability, and
safety compared to other types of circuit breakers.SF6 gas has a high dielectric strength, making it
suitable for use in high-voltage applications.
Circuit Breaker testing
Testing of circuit breakers is more difficult as compared to other electrical equipment like
transformer or machine because the short circuit current is very large. Testing of circuit breakers
is essential for ensuring the reliability and safety of electrical power systems. The key outcomes
and benefits of circuit breaker testing include:

1. Enhanced System Reliability: Regular testing ensures that circuit breakers function
correctly, reducing the possibility of unexpected failures and power outages.
2. Increased Safety: Properly functioning circuit breakers protect electrical systems and
personnel from electrical faults, preventing accidents and equipment damage.
3. Preventive Maintenance: Testing Identifying issues early allows for timely
maintenance and repairs, extending the life of the circuit breaker and avoiding costly
emergency interventions.
4. Documentation and Trending: Recording and analyzing test results over time helps
in monitoring the condition of circuit breakers, allowing for predictive maintenance and
informed decision-making.

(Type tests& Routine Test)

Circuit breaker testing method is broadly classified into Type tests and Routine tests.
Type tests- Type tests are a set of standardized testing performed on circuit breakers to ensure that
their design and performance satisfy the necessary requirements and standards. Type test are not
conducted on each circuit breaker. Tests are often performed on a sample unit to confirm the
overall design, assuring that all units manufactured will perform reliably under specified
conditions. Type test are generally carried out at factory or at any other recognized laboratory.
These test are necessary to prove the design parameters of equipment, tolerances etc. These are
very specialized tests and costly. Temperature rise test; Mechanical test, short circuit test is the
example of Type test.
Routine Test- These tests are initially carried out at factory and routinely at site also to confirm
the healthiness of the equipment year after year. It is a series of inspections and tests that are
performed on a regular basis to ensure that the circuit breaker is functioning properly and reliably.
These tests are critical to ensuring the safety and efficiency of electrical systems. Routine testing
assists spot potential problems early, guaranteeing that the circuit breaker will function properly in
the event of a fault, thereby protecting the electrical system and maintaining safety. These tests are
equipment specific. Simple to carry out at site. Less cost and less man power required to carry out
routine test. Example-Testing of Contact resistance of circuit breaker
(Testing of Circuit Breakers)
Circuit breakers can be tested in a variety of ways, depending on the purpose and nature of the
tests. Some of the important test are-
• Dielectric Test-
• Thermal Test -
• Insulation Resistance Test-
• Short circuit Tests-
• Temperature Rise Test-
• Mechanical Test-
• Impulse Tests-

Dielectric Testing Test

Dielectric testing of a circuit breaker is a critical procedure that assesses the integrity and
performance of the insulation material used in the breaker. This type of testing ensures that the
circuit breaker can withstand high-voltage conditions without breaking down. These tests are
normally made on indoor switchgear.
Procedure for Dielectric Test-
• This test is performing by Apply a specified high voltage across the circuit breakers open
contacts and between the contacts and the ground.
• Maintain the voltage for a specified duration, typically one minute.
• With this the breakers must not flashover or puncture.
• In terms of the results of the tests, dielectric tests provide information about the electrical
breakdown voltage of the insulation material.
• The results of dielectric tests are usually expressed as a voltage or kilovolts (kV)
Advantages of Dielectric Testing

1. Verification of Insulation Integrity: Testing Confirms that the insulation is capable of

withstanding high voltages without breaking down, ensuring reliable performance of the circuit
breaker.

2. Identification of Defects: Test helps to identify manufacturing defects, material degradation, or

damage that could compromise the insulation.

3. Quality Assurance: Test: Ensures that the circuit breaker meets industry standards and
specifications, providing confidence in its quality and performance.

4. Maintains Reliability: Test confirms that the breaker can handle voltage stresses during normal
operation and fault conditions.

Impulse Testing-The outdoor circuit-breaker connected in the system is subjected to high voltage
transients due to switching and lighting. The impulse voltage test is a procedure used to ensure that
a circuit breaker can withstand sudden, high-voltage spikes, similar to what might happen during
lightning strikes or switching surges. This test helps verify the reliability and safety of the circuit
breaker under these extreme conditions.

Under Impulse Testing Process an impulse generator is used. This device can create high-voltage
pulses that simulate lightning strikes or other sudden surges. The waveform is typically a 1.2/50
microsecond wave, where 1.2 µs is the time to reach the peak voltage and 50 µs is the time to half
the peak voltage., Observe and record the circuit breaker’s response to the impulse voltage. The
breaker should not flashover for 10 application of the wave. A higher impulse voltage is specified
for non-effectively grounded system then those solidly grounded system.
Advantages of Impulse Testing-
1. Simulation of Real Conditions: Testing simulates real-life high-voltage surges such as
lightning strikes and switching surges.
2. Reliability Assurance: Testing ensures the circuit breaker can withstand high-voltage
transients without failure.
3. Quality Verification: Testing verifies the manufacturing quality and dielectric integrity of
the circuit breaker.
4. Preventive Maintenance: Identifies potential weaknesses before they lead to actual field
failures.
Insulation Resistance Test
The insulation resistance test is an important instrument in the routine maintenance of circuit
breakers. It is an important test to ensure the integrity of the insulation materials inside the breaker.
This test measures the resistance offered by the insulation to the flow of electrical current,
indicating the health of the insulation.
Under that testing
1. Insulation resistance is measured using a Megger. The megger consists of a megohm meter
and a built-in direct current generator.
2. Two megger terminals are connected across insulation, one to the conductor and the other
to the earthed body.
3. The results of insulation tests are expressed in ohms or megohms (MΩ)
(Interpretation of Results -)
 High Resistance (e.g., >100 Megohms): Indicates good insulation integrity, meaning the
insulation is in excellent condition.
 Medium Resistance (e.g., 1-100 Megohms): May indicate some deterioration in the
insulation. Further investigation or more frequent testing may be required.
 Low Resistance (e.g., <1 Megohm): Indicates poor insulation condition, potentially due to
moisture, dirt, or insulation breakdown. Immediate maintenance is required to prevent
failure
(Advantages of Insulation Resistance Test)-
1. Detection of Insulation Deterioration: Environmental conditions, mechanical stress, and
electrical stressors can all cause insulation in circuit breakers to deteriorate over time. The
Insulation Resistance test can help detect such degradation early on.
2. Preventive Maintenance: Early Warning Provides an early warning of potential insulation
failures, allowing for timely maintenance or replacement before a breakdown occurs.
3. Safety Assurance: Test Ensures the safety of operators and maintenance personnel by
verifying that the insulation is intact and can prevent accidental electrical shocks.
4.Cost Savings: Test minimizes costly downtime by identifying and addressing insulation issues
before they lead to complete circuit breaker failure.
Short Circuit Test-
This test ensures that the circuit breaker can safely interrupt the very high current levels that
occur during a short circuit. Test the breaker, to ensure it can safeguard electrical systems and
prevent damage or fires during a short circuit.
Under that short circuit test process
• A high fault current, much higher than normal operating current, is applied to the breaker. This
simulates a real short circuit condition.
• The current is typically applied using a special test setup that can generate and safely handle
high currents.
• The test monitors how quickly and effectively the breaker trips (opens the circuit) when
exposed to the high fault current. After the test, the breaker is inspected for any damage.
• It's important to follow all safety guidelines and manufacturer instructions carefully during the
test to ensure accurate and safe results.
Advantages of Short Circuit Test-
1. Verification of Protection Mechanism- Testing Confirms that the circuit breaker can
detect and interrupt short circuit conditions promptly, thereby protecting the electrical system and
preventing damage. It verifies that the breaker performs within the specifications provided by the
manufacturer, ensuring it can handle expected fault condition.
2. Improved System Reliability-Testing Ensures that the breaker will operate correctly
during fault conditions, reducing the possibility of system outages and improving overall
reliability.

3. Cost Savings- Testing Prevents costly repairs and replacements of electrical equipment by
ensuring the breaker operates correctly during short circuits.

4. Enhanced Understanding and Documentation- Testing Provides valuable data on

breaker performance under fault conditions, which can be used for future reference and
improvement.

Temperature Rise Test:

 The temperature rise test of a circuit breaker is conducted to ensure that the breaker can
handle its rated current without its components overheating.
 Overheating can cause insulation failure, material degradation, and ultimately, malfunction
of the breaker.
Under the Test Procedure –
• Alternating current of rated value and rated frequency is passed through a closed circuit
breaker, continuously till a steady temperature is attained.
• For temperature measurement thermocouples or temperature sensors is attach to key points
on the breaker, such as contact points, terminals, and other critical components.
• Continuously monitor and record the temperature of various conducting, insulating and
structural parts throughout the test duration at an interval of one or half –an hour.
• When the steady temperature is reached, ensure that none of the components exceed their
maximum safe operating temperatures.
• The temperature rises for rated current should not exceed 40°C for current less than 800A
and 50°C for normal value of current 800A and above.
 Limitation of the Temperature Rise Test in Circuit Breakers-
 Controlled Environment: The test is conducted in a controlled setting that may not fully
replicate real-world conditions, such as varying temperatures and load fluctuations.
 Limited Duration: The test is usually run for a short period, which may not reveal long-
term effects of continuous operation or aging of materials.
 Specific Measurements: testing Measures temperature at specific points, which may not
capture all areas where overheating could occur.
 Real-World Unpredictability: Test Does not account for all possible operating
conditions, like fluctuating loads or extreme environmental factors, that the breaker might
face in actual use.
Mechanical tests:
 Mechanical tests of circuit breakers are essential to ensure they work reliably and last a
long time.
 These tests check how well the breaker performs, how durable it is, and how it holds up
under different conditions.
Process
 Mechanical tests involve repeatedly opening and closing the circuit breaker around 1000
times. With no adjustments to the mechanism.
 After the tests, the contacts, linkages and all the other parts should be in good condition and
should not show any permanent deformation or distortions. The dimensions should be
within original limits. If during repeated operations of the circuit-breakers the breaker
parts in the assembly may fail. The circuit breaker is then judged to have failed the
mechanical test.
 Then the tests will subsequently to be repeated when design and manufacturing
improvements have been implemented.
Advantages of Mechanical Tests-

1. Durability Check: Testing Ensures that the circuit breaker can withstand a large number of
operations, verifying its mechanical durability over time.
2. Reliability Verification: Test Confirms that the mechanical components of the breaker operate
correctly, which is essential for reliable performance.

3. Early Detection of Mechanical Issues: Identifies potential mechanical problems, such as wear
and tear, misalignment, or loose components, before they lead to failure.

4. Compliance with Standards: Test Ensures the circuit breaker meets industry standards and
specifications for mechanical performance.

Although, Mechanical tests are crucial for ensuring the durability and reliability of circuit breakers
by validating their mechanical components and performance. However, they have limitations
related to controlled conditions, potential wear from the tests themselves, and a focus that may not
cover all aspects of breaker operation.
Contact Resistance of circuit Breaker Testing-
This test is especially important for contacts that carry large amounts of current (such as circuit
breaker and bus bars) because higher contact resistance can lead to lower current carrying capacity
and higher losses. Measurement of the contact resistance helps in identification of fretting
corrosion of contacts, and allows contact corrosion to be diagnosed and prevented. Increase in
contact resistance can cause a high-voltage drop in the system, and that needs to be controlled.
Testing Process
The two common checks conducted on the contacts of a circuit breaker are the visual inspection
check and the contact resistance checks.
Under the Visual inspection check involves examining the contacts of the circuit breaker for any
pitting marks due to arcing and worn or deformed contacts.
Under the contact resistance measurement. This involves injecting a fixed current, usually around
100A, 200A and 300 A through the contacts and measuring the voltage drop across it. This test is
done with a special contact resistance measuring instrument. Then, using Ohm’s law, the
resistance value is calculated. The resistance value needs to be compared with the value given by
the manufacturer.
Both these tests need to be done together. As there are cases of contacts having good contact
resistance yet being in a damaged condition. Thus, for a contact to be certified healthy, it needs to
have a good contact resistance and should clear the visual inspection test.