Arc and Flashover Analysis Understanding Electrical Discharges Through Field Simulations

Why does an electrical flashover often occur exactly where you least expect it? Sharp edges, field maxima, and minute geometric deviations—often caused merely by manufacturing tolerances—determine when the breakdown strength of air is exceeded and an electric arc is formed.

An electrical flashover occurs when the local electric field strength along a potential discharge path exceeds the dielectric strength of the medium. This threshold depends on the properties of the insulating medium—for air, this typical value is often around 3 kV/mm. The electric field strength, in turn, results from the charge distribution, which is largely determined by material properties and geometry. Sharp edges, points, or tight radii lead to local field maxima—and these effects often shift the breakdown away from the expected narrowest point toward the location of the highest actual field concentration.

The Jacob's ladder (horn arrester)—two current-carrying rails that open upward, forming an air gap—is familiar from physics classes or from high-voltage systems. Horn arresters (also known as horn gap arresters or horn fuses) were formerly used to safely divert surges to ground. When an excessive voltage occurs, an electric arc ignites between the horns, conducting the current to ground. After ignition, the arc heats the air and rises due to the chimney effect. As the distance between the horns increases, the arc encounters an ever-widening gap until it splits and extinguishes.

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Horn arresters are clear examples of how an electric arc forms and of the interaction between electric fields and fluid dynamics. The same mechanisms are at work in connectors, power electronics, and relays, as well as in electrostatic discharges (ESD)—albeit with varying intensities and dynamics.

Analysis of Electric Fields and Hotspots

An electric field builds up between components at different electrical potentials; this field can become highly concentrated due to small gaps, protrusions, or sharp edges. An electrostatic simulation makes it possible to calculate the fields on real 3D geometric models and thereby identify critical areas early in the development process. By analyzing the field lines—which always run parallel to the electric field vector—it is possible to determine the paths along which discharges are most likely to occur and whether local field strengths reach critical levels.

The actual field distribution depends on the geometric dimensions, charge distribution, and material properties. These effects are not intuitively apparent and can only be identified through an electrostatic simulation prior to hardware assembly. The simulation of the Jacob’s ladder example considered here showed that the edges of the metal rods amplify the local field. As a result, the initial breakdown does not occur at the geometrically narrowest point, but rather at the edges—a typical, often overlooked effect.

Discharges are therefore not random occurrences, but direct consequences of the field distribution. Anyone who understands where and why the field becomes too strong can prevent breakdowns—or deliberately induce them—and thereby lay the groundwork for analyzing the actual flash-over ignition.

Simulation of Rollover Ignition

Inception voltage analysis, as integrated into Ansys Maxwell, allows for a semi-analytical estimation of the breakdown voltage along a path with the highest electric field strength in a gas. The method evaluates the local electric field strength along a path and correlates it with the material properties of the medium. This yields a voltage value at which breakdown becomes likely. While a pure electric-field analysis merely shows where a breakdown is most likely to occur, the inception voltage analysis answers the question of at what voltage a breakdown becomes realistic. The inception voltage indicates when it initiates—but not how the breakdown subsequently develops.

An electric arc does not form instantaneously, but develops over time—even if that time is very short. Ansys Charge Plus allows you to model the temporal evolution of an electric arc using coupled electric fields and gas physics, showing when the first free charge carriers are generated and how they intensify along a preferred path to form a conductive plasma channel. The Jacob’s Ladder simulation clearly demonstrates this behavior: When the current is increased, the electron density initially rises locally, spreads along the strongest field line, and eventually forms a continuous channel.

This reveals local effects that are critical to engineering:

• How quickly does the arc ignite—and how does the initial ignition of the arc occur—and what happens during the discharge process?
• Where does the plasma channel actually begin—at the narrowest point or at the point of maximum field strength?
• What current and voltage waveforms occur during ignition—and how much does the voltage drop during the transition to plasma?
• How do gas, pressure, or humidity affect the process—for example, through local dielectric strength?

This not only produces a clear visualization but also provides a robust physical basis for design decisions. It is therefore clear that ignition is not a random event but a deterministic physical process.

Behavior and Dynamics of Electric Arcs

A stable arc is formed when there is sufficient power in the system to maintain the plasma's conductivity over time. This interplay of electric currents, temperature fields, gas motion, and the Lorentz force can be simulated transiently using a Maxwell-Fluent coupling. Once an arc has been ignited, losses occur in the arc channel, leading to increased temperature and, consequently, lower gas density.

At the same time, the electric currents influence the magnetic field, generating Lorentz forces. To ensure that the coupled physics remains realistic, the electromagnetic and fluid dynamics calculations must communicate with each other at every time step. Here, it is important that the time step size be very small at the beginning to accurately capture the effects, and that it be increased during the simulation to keep the total computation time as short as possible. System Coupling allows the time step size to be made dependent on functions of the currently calculated time.

The Jacob’s Ladder impressively demonstrates this interaction: The arc does not rise due to some “mystical property of plasma,” but rather because of the reduced gas density and the resulting thermal buoyancy. The position and shape of the arc change continuously—as do the flow and temperature fields. The same mechanisms are at work in technical switching devices, where an arc must be controlled or displaced under load.

A clear example is the opening of a relay: When the contact opens, an electric arc is formed, which is pushed sideways out of the contact area by the magnetic field of the integrated permanent magnets. The magnetic field interacts with the current in the plasma channel and generates Lorentz forces that “blow out” the arc. A coupled Maxwell and Fluent simulation realistically models this process: Maxwell provides current density, losses, and Lorentz forces, while Fluent calculates temperature fields, gas motion, and the change in conductivity. In the model, the arc behaves exactly as observed in experiments—a building block for reliable design decisions.

Benefits for Engineering and Development

The combination of field analysis, breakdown voltage, plasma formation, and coupled arc simulation makes it possible to identify discharge risks early and control them in a targeted manner. Instead of merely testing whether something “works or doesn’t work,” this approach provides a deep understanding of where a flashover occurs, how it propagates, and under what conditions it remains stable. Based on this, assemblies can be specifically designed to be more robust, prototype iterations can be reduced, and the path to a safe, standards-compliant design can be significantly shortened.

For product designers and development engineers, this means that they not only recognize when a problem arises, but can also clearly understand why it occurs based on the simulated discharge paths, local field maxima, and arc trajectories. This physical transparency makes all the difference—whether through minor adjustments to the geometry, optimized radii at potential hotspots, or the targeted guidance of the arc via magnetic fields. Especially for components exposed to high voltage gradients in real-world operation, deliberate control of the field distribution often determines whether an assembly will reliably withstand 100, 10,000, or 100,000 switching cycles.

*René Fuger, Ph.D. (Eng.), is a computational engineer at CADFEM Austria GmbH.