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Sunday, February 22, 2015

What is Dielectric withstand Test?

Dielectric withstanding test is used to evaluate the voltage at which insulation between two electrodes gets completely damaged allowing conduction. 

Dielectric testing is a simple, nondestructive method of verifying the adequacy of electrical insulation to withstand transient (surge) events. Transient voltage spikes on power lines are generally the result of nearby lightning strikes, but spikes can also occur for other reasons. In general, such transient spikes have a very short duration--the spike lasts for <20 microseconds. 

A dielectric test can verify the performance headroom of the insulation, ensuring that the insulation will not fail because of insulation degradation from aging, moisture, wear due to vibration, or other causes. 

The voltage level of the dielectric test is generally adjusted based on the environmental conditions to which the end product will be subjected. A higher dielectric test voltage is used for equipment located where environmental conditions are more severe. Passing this more-severe dielectric strength test when the end product is new indicates that the insulation being stressed has enough headroom to provide adequate protection after the end product has been subjected to environmental degradation.

Test Method: In dielectric testing, a high voltage (typically ≥1000 V) is applied between two conductors that are supposed to be electrically insulated from each other. If the two conductors (e.g., an insulated live wire and a metal enclosure) are completely isolated from one another, then the application of a large voltage difference between the two conductors would not allow current to flow between the conductors. In this case, the insulation is said to withstand the application of a large
voltage potential between the two conductors, hence the term dielectric withstand test.

In general, two dielectric test results indicate insulation failure. The first is excessive current flow during the test due to low insulation resistance of the insulating material separating the two conductors. The second is an abrupt dielectric breakdown due to electrical arcing or discharge,
either through the insulation material, over the surface of the insulation
material, or through the air. 

Test Voltage: If the test voltage is too low, the insulation material will not be adequately stressed during the test, allowing inadequate insulation to pass the test. On the other hand, if the test voltage is too high, the test could cause permanent damage to an insulation material that is otherwise adequate for the application. A general rule of thumb used for testing mains wiring that operates at voltages of 120–240 V ac is 1000 V plus two times the operating voltage. Using this rule, 120-V wiring
would be tested using a voltage of 1000 V + (2 x 120 V) = 1240 V ac. 

Test Duration: To adequately stress the insulation, the test voltage is  generally applied for 1 minute. However, many standards allow the test duration to be reduced to 1 second for production line testing to accommodate the large volume. For reduced-duration testing, standards often require the test voltage to be increased by 20% to ensure that 1 second is sufficient to test the insulation adequately.

AC-DC Dielectric Withstanding test- which is advantageous: For instance, An ac test voltage of 1000 V rms will have voltage peaks of 1414 V. Therefore, if a dc test voltage is used, the test voltage must be increased to 1414 V dc to produce the same level of stress to the insulation as would 1000 V ac rms.

The difference in test voltage for dc compared with ac is supported by national testing and standards writing organizations such as Underwriters Laboratories, Factory Mutual Corp., the Institute of
Electrical and Electronic Engineers, and the American National Standards Institute, as well as international organizations such as the International Electrotechnical Commission.

When compared with an ac dielectric test, dc testing offers many advantages. The maximum allowable test current can be set to a much lower level (1 mA is typical). The dc tester shuts down when more than 1 mA of current flows during the test. This highly sensitive test allows
the operator to identify marginal constructions that would have been overlooked by an ac tester.
The lower test-current levels are significantly safer for the operator. At 1mA, the current is enough to shock the operator, but the test current would be automatically shut off when the current flow exceeds 1 mA.

Dc testing offers significant advantages over ac testing. Dc and ac testing provide an equivalent level of breakdown detection due to total insulation failure. However, the heightened accuracy of dc leakagecurrent detection allows marginal insulation systems to be detected. Dc dielectric testing is superior for ensuring operator safety. Neglecting to consider dc testing as an alternative to ac testing potentially jeopardizes both the test operator (with shock hazards during testing) and the
consumer (with marginal insulation).

Courtesy: Compliance engineering; www.ce-mag.com