Insulation leakage

The test instrument must be capable of detecting insulation leakage between live conductors and between live conductors and earth. To do this and comply with lET Regulation 612.3 the test instrument must be capable of producing a test voltage of 250, 500 or 1,000 V and delivering an output current of not less than 1 mA at its normal voltage. 

A phenomenon known as silver migration has limited the use of silver plating in miniature circuit boards under a positive d-c potential within a damp environment silver can migrate across insulation. On drying, silver is found in the insulation or media creating a leakage path. Silver plating is forbidden in many military specifications for circuit boards. 

Explosion-proof enclosures are characterized by strong metal enclosures with special close-fitting access covers and breathers that contain an ignition to the inside of the enclosure. Field wiring in the hazardous environment is enclosed in a metal conduit of the mineral-insulated-cable type. All conduit and cable connections or cable terminations are threaded and explosion-proof. Conduit seals are put into the conduit or cable system at locations defined by the National Electric Code (Article 501) to prevent gas and vapor leakage and to prevent flames from passing from one part of the conduit system to the other. 

Low-temperatiire liquefied gases are shipped in special ships with insulation between the niiU ana an inner tank. Poisonous materials are shipped in separate tanks built into the ship. This prevents tank leakage from contaminating harbors. Separate tanks are also used to transport pressurized gases. 

Irrespective of the class of an insulation system and its quality, it will have some leakage current through its dielectric circuit on application of a high voltage. For all practical piiiposes. therefore, we can consider an insulation system as an imperfect capacitor. 

Figure 9.6 Representation of leakage current in an HT insulation system...

Verification of insulation resistance or measurement of the leakage current 14/435... 

Insulation resistance or measurement of leakage current, both before and after the dielectric test... 

The leakage current is measured as in lEC 60298. This method is generally applicable to an HT system by applying the full rated voltage between the insulating surface, say, between a phase and the ground. The leakage current thus measured should not exceed 0.5 mA. 

Referring to Table 14.7, the recommended insulation value will ensure the following leakage current for an LT system ... 

In this case also verification of insulation resistance or the measurement of leakage current will be carried out before and after the HV test. 

Prevention from ground leakage as such is important, as it causes corrosion through electrolysis and may damage the insulation of wires due to ageing. 

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