SHORT-CIRCUIT CURRENT INTERRUPTION PROCESS BASIC INFORMATION AND TUTORIALS

How power circuit breaker works?

Circuit Breaker Performance During Interruption; Basic Intervals

The Circuit breaker has two basic positions: closed and opened. In the closed position the circuit breaker conducts full current with negligible voltage drop across its contacts. In the open position it conducts negligible current but with full voltage across the contacts.

This defines two main stresses, the current stress and the voltage stress, that are separated in time. However, the main function of the circuit breaker is neither to conduct nor to isolate. It performs its main function in changing from one condition to the other, that is, the switching operation.

If closer attention is paid to the voltage and current stresses during the interrupting test (figure below), three main intervals can be recognized.

1. High-Current Interval

The high-current interval is the time from contact separation to the significant change in arc voltage preceding the interaction and high-voltage intervals.

2. Interaction Interval

The interaction interval is the time from the significant change in arc voltage prior to current zero to the time when the current including the post arc current, if any, ceases to flow through the test breaker.

3. High-Voltage Interval

The high-voltage interval is the time from the moment when the current including the post arc current, if any, ceases to flow through the test breaker to the end of the test.

State of Interrupting Process

The three intervals described in 1 to 3 follow each other immediately, that is, they cover the whole interrupting process without any discontinuities, even though it might be difficult to establish precisely the moment when one interval ends and the other begins. However, this accuracy may not be required.

State of Interrupting Process During Three Basic Intervals

The quantities determining the physics of the interrupting process change considerably during the circuit breaking operation. In fact, the prevailing physical conditions have different importance during the three time intervals.

High-Current Interval

During the high-current interval, short-circuit current is flowing through the circuit breaker with a relatively small voltage drop across the contacts. A large amount of energy is supplied to the arc establishing the state of ionization, temperature, dynamic pressure, etc, important for the switching function.

Interaction Interval

During the interaction interval, the short-circuit current stress changes into high-voltage stress and the breaker performance can significantly influence the currents and voltages in the circuit. As the current decreases to zero, the arc voltage may rise to charge parallel capacitance and distort current passing through the arc.

After the current zero the post arc conductivity may result in additional damping of the transient recovery voltage and thus influence the voltage across the breaker and the energy supplied to the ionized contact gap.

The mutual interaction between the circuit and the circuit breaker immediately before and after current zero (that is, during the interaction interval) is of extreme importance to the switching process.

High-Voltage Interval

During the high-voltage interval, the gap of the breaker is stressed by recovery voltage. The circuit breaker is now a passive element in the circuit.