Why is dielectric testing important?

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.

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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.

Test to endure electrical insulators work under required conditions

In electrical engineering, a dielectric withstand test (also pressure test, high potential test, hipot test, or insulation test) is an electrical safety test performed on a component or product to determine the effectiveness of its insulation. The test may be between mutually insulated sections of a part, or energized parts and ground. The test is a means to qualify a device's ability to operate safely during rated electrical conditions.[1] If the current through a device under test is less than a specified limit at the required test potential and time duration, the device meets the dielectric withstand requirement. A dielectric withstand test may be done as a factory test on new equipment, or may be done on apparatus already in service as a routine maintenance test.[2]

An insulation test set; in this pattern, a hand-cranked generator provides the high voltage and the scale is directly calibrated in megohms.

Voltage withstand testing is done with a high-voltage source and voltage and current meters. A single instrument called a "pressure test set" or "hipot tester" is often used to perform this test. It applies the necessary voltages to a device and monitors leakage current. The current can trip a fault indicator. The tester has output overload protection. The test voltage may be either direct current or alternating current at power frequency or other frequency, like resonant frequency (30 to 300 Hz determined by load) or VLF (0.01 Hz to 0.1 Hz), when convenient. The maximum voltage is given in the test standard for the particular product. The application rate may also be adjusted to manage leakage currents resulting from inherent capacitive effects of the test object. The duration of the test is dependent on the test requirements of the asset owner but is normally up to 5 minutes. The applied voltage, rate of application and test duration depend on the specification requirements of the equipment. Different test standards apply for consumer electronics, military electrical devices, high-voltage cables, switchgear and other apparatus.[2]

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Typical hipot equipment leakage current trip limit settings range between 0.1 and 20 mA[3] and are set by the user according to test object characteristics and rate of voltage application. The objective is to choose a current setting that will not cause the tester to falsely trip during voltage application, while at the same time, selecting a value that minimizes possible damage to the device under test should an inadvertent discharge or breakdown occur.

Control panel of a portable high voltage ("hipot") tester ; this instrument can test up to 100 kV AC

See also

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References

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1. ^

   MIL-STD-202G, Method 301, Dielectric Withstanding Voltage

2. a b

   Paul Gill (), Electrical Power Equipment Maintenance and Testing, Second Edition, CRC Press, , page 459

3. ^

   Condor Application Note 5/00, pg. 2

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