What Is a Dielectric Voltage-Withstand Test?

Why It Matters

The human heart is controlled by electrical impulses, and any interruption of these impulses from outside currents can cause ventricular fibrillation and, in extreme cases, death. Because these stakes are so high, ensuring that no current can reach the human body is a primary goal of electrical safety. In electrical products, the flow of current is conducted through wires coated in insulation. This insulating material is designed to block current and therefore keep the electrical energy in the circuit – away from the user. However, under certain conditions, this insulating material has been known to fail.

What Is Dielectric Breakdown

Dielectric breakdown is a complex phenomenon that affects the insulating material of a circuit. In short, dielectric breakdown results in an insulator becoming a conductor. This means that instead of trapping the current inside the metal conductor, the insulation itself becomes the conducting material. 

Electrical current flows by the movement of electrons across a bandgap. In conductors such as copper wire, there are free electrons that move easily. They have very little resistance, and it does not take a large electric field to move the electrons in a conductor. In an insulator, such as rubber or plastic coatings, there are no free electrons, and it takes a very high voltage to push the electrons out of their stable state. When this high voltage, called the breakdown strength, is applied, there is a physical transformation of the conductive values within the insulator. This dielectric breakdown disrupts the current flow within the circuit and can cause electrical shock via arcing (electricity jumping from the product to the user through the air) or direct contact with the insulating material.

Dielectric Voltage-Withstand Test

As stated above, dielectric breakdown occurs following exposure of the insulation material to an exceptionally high voltage. One test required in many UL standards is a Dielectric Voltage-Withstand Test, which aims to create conditions that could cause dielectric breakdown in the test product. If dielectric breakdown does not occur under these conditions, then the product’s insulation and design is resistant to this hazard, even at extremely high voltages.

The Dielectric Voltage-Withstand Test is often conducted in coordination with other requirements, such as a rain test or grease conditioning. During a rain test, a product is exposed to spraying water from above to determine if there is a fault in the product’s insulation that would allow water to act as a foreign conductor. Grease conditioning, as shown in, but not limited to, UL 507, the Standard for Electric Fans, addresses the risk of insulation breakdown from grease buildup on kitchen ventilating hoods to determine whether it affects the product’s electrical properties. Our colleagues at UL Research Institutes developed the following video to demonstrate a Dielectric Voltage-Withstand Test in action.

Testing and Standards

In a standard, tests are required to help address very specific risks and hazards that are present in a product. These hazards are determined, then a specific test or guideline is implemented within a standard to address that hazard. This hazard-based mindset is evident in the design and implementation of the Dielectric Voltage-Withstand Test. 

During dielectric breakdown, fire can be caused when the current in the insulating material bridges the bandgap in the air, causing a spark. If this occurs, the spark can arc to a human body or any surrounding material and cause a fire or electric shock. Electric shock can also be caused by direct contact. If a user holds the product by insulating material that has become conductive, the current would directly pass into the user. By requiring tests that mitigate the risk of dielectric breakdown occurring, UL Standards & Engagement helps to advance our join UL mission of working for a safer world.