Know about the phenomena of arc in Circuit Braker

Introduction

Arc is a critical phenomenon in electrical engineering, especially in circuit breakers. It occurs when current is interrupted, potentially impacting equipment efficiency and safety. Understanding arc behavior is key to designing effective and reliable breakers.

A circuit breaker with bright electrical arcs connecting black cables on a dark background. Above, text reads "Arc In Circuit Breaker," conveying energy and tension.

What is an Arc?

Arc is a crucial part of circuit breaker operation when contacts separate. Understanding arc behavior is essential for switchgear design, as the interrupter's effectiveness relies on how well it manages arc extinction.
The core of an AC circuit breaker is a switching element with variable resistance, typically a high-pressure arc in gases like air, oil, or SF₆. In vacuum circuit breakers, the arc forms in electrode vapor. Other devices, like high-voltage DC converters, use low-pressure mercury arcs or solid-state discharges.
The physics of circuit breaker discharges is mostly understood qualitatively due to complex processes and limited data on high-temperature gases. Still, major progress in the last 10–15 years stems from advances in aerodynamics, computing, and experiments.

Though complex, studying arc phenomena is crucial for understanding circuit breaker design and operation. This article explores key physical processes behind electric arcs to aid that understanding.

Role of arc in current interruption

When two current-carrying contacts open, an arc bridges the contact gap and prevents an abrupt interruption of the current. This arc is useful in a way as it provides a low resistance path for the current after contact separation, thereby preventing current chopping and associated abnormal switching over-voltages in the system.
In AC systems, the arc naturally extinguishes at each current zero. Successful interruption depends on preventing the arc from re-igniting between contacts after this point.
The arc is crucial in current interruption. Without it, current would stop instantly, causing a rapid magnetic field collapse and dangerously high voltage spikes that could damage system insulation.
On the other hand, the arc provides a gradual, but quick, transition from the current-carrying to the current-breaking states of the contacts. It thus permits the disconnection to take place at zero current without inducing the potentials of dangerous values. The function of an arc-control device in a circuit breaker is therefore clearly to employ the beneficent action of the arc as efficiently as possible.

Arc interruption Theories

In the 1930s, Slepian introduced the idea of a 'race' between rising dielectric strength and re-striking voltage after arc extinction. Cassie built on this in 1939 with the first energy-balance theory. By 1941, Slepian and Browne suggested gas turbulence helped extinguish arcs more quickly.
At this time, a considerable amount of work, both experimental and theoretical,commenced around the world, on the manner in which the resistance of the arc changed during the current zero period.
In 1939, Cassie proposed an arc model with a cylindrical column of uniform temperature and current density, where the cross-sectional area varied with current. He assumed power dissipation was proportional to this area and described the model with a differential equation suited for air-blast arcs.

`frac{Rd}{dt}left(frac1Rright)=frac1theta{left(frac v{v_0}right)^2-1}`

where R is the arc resistance, V, the arc voltage at any instant, `V_0`, the arc voltage in steady state, and 𝛳 the arc time constant (i.e. the ratio of energy stored per unit volume to the energy loss rate per unit volume).

In the steady state, Cassie's equation leads to a constant voltage characteristic V = `V_0` which is qualitatively typical of the heavy circuit regime of circuit breaker arcs.
In 1943, Mayr introduced an improved arc model assuming a constant arc diameter with variable temperature and conductivity. He considered power loss only from the arc surface and expressed this behavior with a differential equation.

`frac{Rd}{dt}left(frac1Rright)=frac1theta{left(frac {v_i}{w_0}right)^2-1}`

where i is the arc current at any instant and wo is the energy loss from periphery of arc at steady state.

Cassie’s model describes the pre–current zero phase, while Mayr’s model fits the post-arc period. In 1948, Browne combined both into a unified arc model. Though many models followed, most lacked numerical links to the interrupting medium’s physical properties—except for Butler and Whittaker’s 1972 theory, which established such a relationship.
Because the arc column retains energy, conductance doesn’t instantly drop to zero at current zero, allowing a brief post-zero current. If the recovery voltage rises too quickly, ohmic heating may reignite the arc—causing thermal failure. Even if thermal interruption succeeds, a high re-striking voltage can still trigger dielectric breakdown, leading to dielectric failure.
This is known as dielectric failure during the peak re-striking voltage phase. It features a very fast voltage collapse—too quick to appear on an oscillogram. In contrast, thermal failure shows a gradual voltage drop toward arc voltage over several microseconds. Figure 1 compares both modes, showing dielectric failure’s sharp voltage fall versus thermal failure’s slower decline.

Figure 1: The nature of voltage variation with time under two failure modes

Arcing Process

Arcs in AC circuit breakers occur in two main ways. The first happens during contact separation, where arcing can start even if the circuit voltage is below the normal breakdown level. This is due to existing ions neutralizing space charge, allowing current to flow at low voltage.
This arc behavior occurs in both DC and AC breakers, but in AC breakers, the arc naturally extinguishes at each current zero. It can re-strike only if the recovery voltage is high enough to cause breakdown between separating contacts.
The function of an ac circuit breaker is to prevent re-striking of the arc, which depends upon the following important factors:

The nature and pressure of the medium of arc
The external ionising and de-ionising agents present
The voltage across the electrodes and its variation with time
The material and configuration of the electrodes
The nature and configuration of the arcing chamber

To physicists, arcs are complex scientific phenomena, but to switchgear designers, arcs mainly aim to survive by striking unpredictably and re-igniting after interruption, especially if the design has flaws.
Physicists study arc plasma properties like particle density and conductivity, while switchgear designers focus on insulation and dielectric recovery. This knowledge helps effectively interrupt high currents and voltages.

Conclusion

Arcs form in circuit breakers due to the complex process of interrupting current. Understanding and controlling these arcs helps engineers design safer, more efficient, and longer-lasting breakers used in various industries.