Video 5: Circuit breaker timing principles
This video explains the timing principles of the circuit-breaker and when timing is done: High voltage part and control part. Checking the mechanical operating times of circuit breakers is essential. These times are a kind of operating signature.
Not respecting the required timing specifications may have grave consequences on the equipment, including its destruction.
Length: 13mn 53s
Topic : Circuit breaker timing principles
Good day and welcome to this on-line training course.
In this video, we will examine timing principles as applied to circuit breakers.
This course was produced by Zensol Automation in collaboration with Hydro-Québec,
At the conclusion of this video, the participant will be able to recognize the acceptance criteria and
perform all the connections required to do timing.
What is timing?
High voltage part.
What is timing?
Timing circuit breakers is ensuring that the mechanical operation times for closing and opening all
breaker contacts are within the minimum and maximum acceptable limits.
A mechanical operation is defined as any maneuver or cycle of maneuvers that the circuit breaker is
supposed to perform during closing or opening operations.
This operation is performed while the breaker is isolated from the network.
At the factory, timing is performed during typical tests or for final quality control.
Timing is then repeated during the first field installation.
This becomes the measurement reference for all subsequent timing tests.
Checking the mechanical operating times of circuit breakers is essential. These times are a kind of
Not respecting the required timing specifications may have grave consequences on the equipment,
including its destruction.
High voltage part
The high voltage part consists of all the parts of the circuit breaker where the line current flows.
There is the main contact, the pre-insertion contact, and the insertion resistor
What are the closing and opening times of the main and pre-insertion contacts?
The opening time of a main contact starts at the moment current starts flowing in the Open command
coil until the main contact is open.
The opening time of the pre-insertion contact is similarly defined but it is shown as a half-state in the
The closing time of a main contact starts at the moment current starts flowing in the Close command
coil until the main contact is closed.
The closing time of the pre-insertion contact is similarly defined but it is shown as a half-state on the
What is the insertion time?
The insertion time is the time during which the auxiliary resistor is electrically present in the circuit.
For example, in the case of the close operation, it’s the time difference between the closing of the pre-
insertion contact and the closing of the main contact.
What is the overlap time?
To measure the overlap time, it is necessary to perform a detailed test. The detailed test is a test that
needs particular connections which will be explained later in this module.
This test is the only one that gives separate and simultaneous access to both the main and pre-insertion
The overlap time is defined as the time during which both the main and pre-insertion contacts are
closed at the same time.
What is the short-circuit time?
The short-circuit time is defined as the time difference between the longest contact closing time and the
shortest contact opening time.
The illustration shows the case of a circuit breaker with two contacts per phase.
What is the isolation time?
The isolation time is defined as the time difference between the quickest contact opening time and the
slowest contact closing time.
The illustration shows the case of a circuit breaker with two contacts per phase.
What is contact bounce?
Contact bounce in a circuit breaker is a physical phenomenon which occurs when the breaker’s contacts
are opened or closed.
These bounces appear as jitters on the graphic recordings.
It could be a sign of abnormal damper operation.
It is not necessary to report bounces of less than one millisecond.
In the example showing bounces on closing, one must consider the contact as closed at forty-two
In the example showing bounces on opening, one must consider the contact as open at twenty-five
Why do we talk about static and dynamic resistance in a circuit breaker?
Micro-ohmmeters measure the resistance of the main contacts when they are closed.
That is the measurement of the static resistance of the contacts.
A contact in good order gives values of a few tens to a few hundred micro-ohms. This value depends on
the technology used in the manufacturing of the contacts in the breaking chamber of the circuit breaker.
Static micro-ohmmeters cannot measure the arcing contacts which appear only for a brief moment
during the open operation.
To record the changes in the micro-ohm value over time, it is necessary to use special modules directly
connected to the circuit breaker analyzer.
The analysis of the recorded curve represents the dynamic resistance of the contact. It supplies valuable
information on the state and the operating conditions of the arcing contacts. It is easy to deduce the
amount of wear of these contacts by observing the length of time during which the arcing contacts
What are the standard operations that can be performed on a circuit breaker?
The operations performed by circuit breakers are mechanical actions that either break or re-establish
current on high voltage lines.
We basically recognize four operations:
Let’s see these operations in more detail in the following animations.
This operation breaks the electric current.
The contacts take some time to react to the open command.
The response time starts at the moment when the current starts flowing in the breaker’s command coil
until the opening of the first contact.
The time that the breaker takes to open completely is the operating time.
This time starts at the moment when the current starts flowing in the command coil until the last
The difference between the longest operating time and the response time is the time difference.
Right after the open order is received, the circuit breaker opens the circuit.
The short-circuit time is calculated as the difference between the closing of the slowest contact and the
opening of the quickest contact.
Open- Close- Open operation.
This operation is performed when the circuit breaker is subject to a reclose on a fault.
As soon as the first open is performed, the next close command is delayed by 300 milliseconds. This
delay is standard and allows the breaker’s insulating medium to regenerate. It also allows time for
transient faults to clear.
The second open command immediately follows the close operation on a fault. During timing tests, the
last Close-Open operation of this test lets the contacts end their motion.
The 52A contact is in the control cabinet, and mechanically follows the state of the main contact.
As soon as the main contact has completed its travel and is completely closed, the 52A contact changes
This is the point where the recloser opens.
This animation shows the principle of calculating the isolation time
The command part is the part of the circuit breaker where the energy required to move the moving
contact is ensured. This command includes energy storage devices called energy accumulators. Their
purpose is to store the needed energy to guarantee the opening or closing of the breaker at any time.
The most used energy accumulators are springs and pressurized nitrogen canisters.
The type of command may be a mechanical spring, hydraulic or pneumatic.
How to measure the close or open command current?
The close or open command current may be known by measuring the current flowing through the close
or open command coils, thanks to the current transducers.
Circuit breaker analyzers such as Zensol’s CBA-32P integrate two Hall effect current transducers, one of
which is used for measuring the close coil current while the other is used to measure the open coil
Analyzing the current signals helps to precisely determine the moment when the current starts flowing,
allowing the calculation of the closing or opening times.
Without this information, the operating times cannot be calculated.
The second data is the maximum value attained by the close or open current. These maxima may be
between a few hundred milliamps to a few tens of amps. For example, the CBA-32P possesses 4 direct
current scales: one amp, five amps, ten amps and twenty amps. There are also 4 alternating current
What is the schematic of a circuit breaker control?
This illustration shows a classic circuit breaker control.
We see the various command relays, such as the 52 X intermediate relays, as well as the 52 A and 52 B
We also see the open and close command coils as well as the open and close pushbuttons.
It is of prime importance to understand this schematic to properly connect the command cables for
circuit breaker analyzers such as the CBA-32P.
What is a contact displacement curve?
Thanks to a displacement transducer, properly mounted on the breaker, it is easy to record a signal
proportional to the displacement of the breaker’s moving contact.
This very realistic animation shows the closing of a bulk oil circuit breaker.
The specific parameters required are the total travel, the overtravel, the rebound, and the penetration.
They are generally compared to typical values supplied by the breaker manufacturer.
Analyzing the displacement curves give information that tabular reports cannot show. For example, the
operation times may be correct, but the breaker may still show a bouncing problem linked to a defective
damper. The only way to see these problems is to record and then analyze the displacement curves of
the moving contacts.
How to measure a contact displacement curve?
This animation shows that when the displacement curve is recorded, it is easy to calculate and draw the
instantaneous speed curve during the opening of the contacts. It’s just as easy to calculate the average
We must remember that the speed for a bulk oil breaker of 120 kilovolts or more is measured at twenty
centimeters from contact separation.
For breakers of 69 kilovolts or less, it is measured at seven point five centimeters from contact
Question lesson one.
What is the overlap time in the following timing graph?
The overlap time is 20 milliseconds. See slide 8 for details.
Circuit breaker timing
Circuit breaker libraries