This article explains the timing test of circuit breaker. Why it is necessary to perform breaker timing test. What is the standard circuit breaker timing test procedure. Which equipment is used for CB timing test and what are the accepted results.
Introduction to breaker timing test
Circuit breakers are one of the important and critical part of electric power system. Circuit breakers are the backbone of electrical protection system. Because of their key importance, they must be checked periodically. Circuit breaker timing test is among different maintenance tests which should be performed regularly. It measures the mechanical operating time of the circuit breaker. This test is always important because delay in operation of circuit breaker can have disastrous effects on the equipment when fault occurs.
Operation time: Operation time of the circuit breaker is the closing and opening times of contacts. Definition of these operation times is presented by IEC 56.3.105 as follows.
Closing time: The time interval between energizing the coils of the breaker (when CB contacts are in open position) and the instant when the contacts touch each other.
Opening time: It is opposite to that of closing time. It is the time interval between energizing the trip coil (CB contacts are in open position) and the instant when contacts separate from each other.
Importance of timing test of circuit breaker
As it has already been discussed earlier that circuit has a key role to play in the protection system. The correct operation time of circuit breaker is important due to the following reasons;
- Minimum damage during fault. When fault occurs, relay senses the abnormal condition and gives a trip command to the circuit breaker. The circuit breaker after receiving trip signal, energizes the trip coil and open its contacts thus detaches the faulty part of system from rest of system. The circuit breaker takes some time to do the process. The equipment’s installed in the power system are designed such that they withstand high current (other abnormal condition) for very limited time. If the circuit breaker failed to operate in that limited time, the equipment may be damaged or destroyed. Therefore it is very important that circuit breaker must operate in specific minimuk time (operation time) in order to minimize damage. This feature makes it very important to check the operation time of circuit breaker periodically.
- Relay coordination. Circuit breakers are installed at different positions to provide protection to power system. It is recommended that only faulty part of power system should be detached while the other should remain intact. It is only possible that breaker near to the fault should be tripped while breaker away from load and near to source should remain intact. This is called relay coordination. The timing of the relay are set in such a way that breaker near the fault should trip first. However if the breaker fails to operate in its specific time, the breaker away from fault (near to source) will trip hence healthy part of system will also be detached. So correct operation time is necessary for the relay coordination.
Equipment for CB timing test
Different measuring devices can used to perform timing test of circuit breaker. First- generation devices were based on oscillographic mode of record curves and currently are not used. Second generation testing methods based on digital timers with time-pulse conversion e.g. Circuit-breaker timer.
The equipment which is widely used nowadays for performing timing test of circuit breaker is the Circuit breaker (CB) analyzer. The major probes and connection points are shown in the following figure.
Circuit breaker timing test procedure and Connections
To perform breaker timing test, following connections are required.
- Connection to the input & output of each phase of circuit breaker from circuit breaker analyzer.
- DC supply to the tripping coil of the circuit breaker from external source.
- Connection to the tripping circuit of breaker from CBA.
Following figure shows the typical connection diagram.
As it can be seen from the figure that connection to incoming of each phase of breaker is provided from CBA via red, yellow and blue wires while outgoing is connected with black wire. Positive (+) and Negative (-) sign shows the connection of external battery to the triggering circuit.
When the connections are done, note the state of the circuit breaker from breaker state indicator (on or off). Press the ready button, the circuit breaker will be operated and the time will be displayed on the screen. Results of the timing test of circuit breaker can be printed using thermal printer, downloaded directly to USB via USB port and displayed on PC via USB interface. Repeat the process for closing and opening of circuit breaker.
The exact correct operation time of circuit breaker is provided by the manufacturer. It can be mentioned on the nameplate of circuit breaker. Click here to learn how to interpret name plate of circuit breaker.
However as a rule of thumb, operating time of circuit breaker should fall in the following ranges
HT breakers (above 11kV) Opening time = 1-2 cycles (20 – 40 ms)
Closing time = 2-5 cycle ( 40 to 100ms)
LT breakers (11kV or below) opening time = 2-3 cycles (40 to 60ms)
Closing time = 3-6 cycles (60 to 120ms)
These are just estimated values which can be regarded as acceptable operation time.
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If you have any question regarding Circuit Breaker timing test, please ask in comments.