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Dielectric strength is an engineering term that refers to the maximum voltage an insulating material can withstand before breaking down or failing. This is a definitive characteristic of insulation materials and is used to grade new types or test the integrity of existing installations. Dielectric strength tests typically consist of exposing insulators to short duration, high voltage values while scanning for leakage or insulation breakdown. 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 tapping a small sample and testing it in a specialized test rig.
Insulation breakdowns are a major cause of electrical failures and accidents which cause millions of dollars of damage and many, often fatal, injuries every year. Establishing the integrity or dielectric strength of insulating elements is an important part of any electrical installation's maintenance regimen. It is also a critical part of the development of new insulating materials. The dielectric strength of any insulator is the maximum voltage it can withstand without failing. This is established by exposing the insulating material to very high voltages in an environment where leakages or breakdowns of the insulation are accurately measured.
Also known as hipot testing, dielectric strength testing is carried out by a wide selection instruments with differing capacities. These range in size and output values from small bench top units capable of generating test voltages of 5 kV to large installation testers which can have outputs of 400 kV or more. Insulation testers may feature alternating current (AC) or direct current (DC) outputs with current values ranging from approximately 10 mA to 400 OVA. Specialized testers are also available which can test the breakdown values of fluid samples taken from transformer and switchgear oil baths.
The testing of insulating oil is particularly important because the dielectric strength of these fluids are subject to many environmental variables such as moisture and carbon contamination. Moisture in insulating oil is a major problem caused by the condensation of airborne moisture inside the transformer casing. Only small amounts of moisture are necessary in a large transformer's oil bath to cause a catastrophic failure. For this reason, regular tests should be carried out to establish the dielectric strength of switchgear and transformer oil. Solid insulators such as the PTFE sheaths on high tension cables can also degrade with time; exposure to ultraviolet radiation, chemicals, and lubricants or excessive heat and should be tested regularly to ensure their integrity.