Introduction
In high voltage systems, one of the key ideas to understand is how electricity can move through air, gas, or vapor in the form of an electrical discharge. These discharges are important because they explain how arcs are created, how insulation can fail, and how devices like circuit breakers control dangerous current flow. There are two main types of electrical discharges: non-self-sustaining and self-sustaining. Understanding the difference between them provides a foundation for learning how circuit breakers and other protective devices operate.
What Is an Electrical Discharge?
An electrical discharge occurs when voltage is applied across a gap and charge carriers, such as electrons and ions, begin to move between electrodes. Under normal conditions, air and gases are good insulators. However, with enough energy, these insulating materials can allow electricity to pass through. The type of discharge depends on whether the process can maintain itself or requires outside help to continue.
Non-Self-Sustaining Discharges
A non-self-sustaining discharge is weak and relies on external factors to continue. When a small voltage is applied across two electrodes, a few naturally occurring charge carriers in the air may create a tiny current. This current remains very small and does not grow unless something else provides more charges. External ionizing sources such as ultraviolet light, heat, or radiation can supply these additional charges.
In this type of discharge, if the external source is removed, the current stops immediately. The discharge is temporary and cannot feed itself. It is similar to a campfire that only smolders when you blow on it; as soon as you stop, the flame dies out. Non-self-sustaining discharges are important to understand because they mark the beginning stage of ionization in gases, but by themselves they are not strong enough to cause major electrical problems.
Self-Sustaining Discharges
A self-sustaining discharge is much more powerful because it no longer requires outside help to continue. When the applied voltage is raised high enough, moving electrons and ions gain significant energy. As they collide with neutral particles, they knock additional electrons loose, which then produce even more collisions. This chain reaction causes the discharge to feed itself and rapidly grow in strength.
Once this stage is reached, the discharge becomes visible, often as a glow, a spark, or an arc. Unlike non-self-sustaining discharges, these do not fade away when the external ionizing source disappears. Instead, the process continues on its own. This can be compared to a campfire that has grown large enough to burn steadily; once it is burning strongly, it no longer needs assistance to keep going.
Why the Difference Matters
The difference between these two types of discharges is more than just academic. Non-self-sustaining discharges are usually safe and indicate that a small amount of ionization is occurring. Self-sustaining discharges, however, are the beginning of arcs and sparks that can damage equipment, trigger faults, and create dangerous conditions for workers. Engineers must understand this transition to design circuit breakers, insulators, and protection systems that can control or extinguish self-sustaining discharges before they cause harm.
key Takeaways
Non-self-sustaining discharges are weak, temporary, and dependent on outside sources, while self-sustaining discharges are strong, independent, and capable of developing into arcs. This simple difference explains why certain discharges are harmless while others pose serious risks to power systems. Mastering this concept provides a foundation for learning more about high-pressure arcs, vacuum arcs, and how circuit breakers interrupt current to keep electrical systems safe.
