Electrical power systems all over the world use circuit breakers as important and critical components. As they play a key role, engineers periodically test circuit breakers. One of the most important test methods is the timing test, which measures the mechanical operating time of the breaker’s contacts. A timing test averts damage to a circuit breaker, as the incorrect operation of a circuit breaker prevents fatal consequences on connected equipment and substation personnel.
Various measuring devices have evolved for measuring the operation times of a breaker. For instance, although are no longer in use, first-generation devices used the oscillographic mode of recording curves. The present methods of testing, the second generation, are based on digital timers converting pulses to time. The latest concept for testing circuit breakers is through the analysis of signals from mechanical vibration.
Circuit Breaker Operation Times
The technically operational quality of a three-phase circuit breaker is an important parameter of its operation times—characterizing the process of opening and closing of its contacts. The international standards of the International Electrotechnical Commission, the IEC 56.3.105, define these time parameters. The standard quantitatively describes the switching on and off times of the process as:
Time discrepancy between contacts — the diverging interval characterizing the disconnection or connection of the breaker contacts during non-simultaneous switching.
Closing time — the interval from the time of energizing the closing circuit (the circuit breaker being in the open position), to the moment the contacts strike the poles.
Opening time of the breaker — the interval from the time of energizing the opening release (the circuit breaker being in the closed position), to the moment the contacts separate at all the poles.
Worth noting is the time discrepancy between the contacts of the breaker. This should be within specified tolerance limits—usually 5 ms. This is important, as the time difference in closing or opening of all contacts may cause huge voltage spikes with a potential to damage the network and its equipment.
Digital Timer Meter
Digital timer meters are typically built with interconnected functional blocks such as input circuits, the micro-controller, a display, and a keyboard. The micro-controller is the central unit of the meter, while the input circuit eliminates disturbances and protects the unit.
The input circuit usually includes a converter with an output voltage of about 50 V. An opto-isolator ensures the optical isolation between the parts of the device directly connected to the breaker under test, and the other circuits of the meter. This protects components and digital circuits to ensure standardization of the signal for further digital processing.
Principle of Operation
The measuring process employs a principle known as the time-pulse method. This allows counting of pulses of a regular frequency in time intervals. The software in the micro-controller allows counting the pulses initiated by an external signal obtained from the breaker’s drive system.
A signal corresponding to the opening or closing of each contact of the breaker closes the time gates. As soon as the signals for the contact closure appear, the micro-controller copies the contents of the counter into its memory. The number of pulses the micro-controller counts is proportional to the interval elapsed from the moment of triggering to the moment of closing.