Dielectric strength is the maximum voltage an insulating material can withstand without failing. Insulation failures cause electrical accidents and damage. Dielectric strength testing exposes insulators to high voltage to test for leaks or faults. Insulation testers range in size and output ratings. Testing of insulating oil is important due to environmental variables. Regular tests should be performed to establish the dielectric strength of switchgear and transformer oil. Solid insulators should also be tested regularly.
Dielectric strength is a technical term that refers to the maximum voltage an insulating material can withstand before breaking or failing. This is a definitive characteristic of insulation materials and is used to classify new types or test the integrity of existing installations. Dielectric strength testing typically consists of exposing insulators to high voltage and short duration values while scanning for insulation leaks or faults. The voltages used to test the dielectric strength of insulators usually range from 5,000 to 400,000 volts (5-400 kV). Oils used as insulators in transformers and switchgear are checked by touching a small sample and testing it in a specialized test rig.
Insulation failures are a leading cause of electrical failures and accidents resulting in millions of dollars in damage and many injuries, often fatal, each year. Establishing the integrity or dielectric strength of insulating elements is an important part of the maintenance regimen of any electrical installation. It is also a key part of the development of new insulation materials. The dielectric strength of any insulator is the maximum voltage it can withstand without fail. This is achieved by exposing the insulating material to very high voltages in an environment where insulation losses or breakdowns are accurately measured.
Also known as hipot testing, dielectric strength testing is performed by a large selection of tools with different capabilities. These range in size and output ratings from small benchtop units capable of generating test voltages of 5 kV to large installation testers that can have outputs of 400 kV or more. Insulation testers can be equipped with alternating current (AC) or direct current (DC) outputs with current ratings ranging from approximately 10 mA to 400 OVA. Specialized testers are also available that can test the failure values of fluid samples taken from the oil baths of transformers and switchgear.
Testing of insulating oil is especially important because the dielectric strength of these fluids is subject to many environmental variables such as humidity and carbon contamination. Moisture in insulating oil is a serious problem caused by condensation of moisture in the air inside the transformer enclosure. Only small amounts of moisture are needed in the oil bath of a large transformer to cause catastrophic failure. For this reason, regular tests should be performed to establish the dielectric strength of the switchgear and transformer oil. Solid insulators such as the PTFE sheaths on high voltage cables can also degrade over time; exposure to ultraviolet radiation, chemicals and lubricants, or excessive heat and should be tested regularly to ensure integrity.
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