In any electrical installation, some current will flow through the protective ground conductor to ground. This is usually called leakage current. Leakage current most commonly flows in the insulation surrounding conductors and in the filters protecting electronic equipment around the home or office. So what’s the problem? On circuits protected by GFCIs (Ground Fault Current Interrupters), leakage current can cause unnecessary and intermittent tripping. In extreme cases, it can cause a rise in voltage on accessible conductive parts.
Insulation has both electrical resistance and capacitance—and it conducts current through both paths. Given the high resistance of insulation, very little current should actually leak. But, if the insulation is old or damaged, the resistance is lower and substantial current may flow. Additionally, longer conductors have a higher capacitance, causing more leakage current. That’s why GFCI breaker manufacturers recommend one-way feeder length to be limited to 250 feet (76.2 m), maximum.
Electronic equipment, meanwhile, contains filters designed to protect against voltage surges and other disruptions. These filters typically have capacitors on the input, which adds to the overall capacitance of the wiring system and the overall level of leakage current.
Find out more about leakage current basics, how to minimize the effects of leakage current, and the measurement of leakage current to ground in our application note "Leakage current measurement basics."
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To provide a margin of safety for the consumer, regulatory agencies usually require that a product exhibit a line voltage leakage current of less than 0.5mA. With some products equipped with 3-prong plugs and warning stickers, the permissible leakage current may be as high as 0.75mA, but the typical limit is 0.5mA. Since hipot tests are usually required for 100% of the units in a production line, and since hipot tests are more stringent, line voltage leakage tests are normally specified as design or type tests and not as production line tests. Line voltage leakage tests are typically required on all medical products as a production test.
Line voltage leakage test are conducted with a circuit similar to that shown in Figure 17, measuring the leakage current under various fault conditions such as “no ground” or with line and neutral connections reversed. Voltage is applied first with normal line and neutral connections, followed by a test with the connections reversed, and then with no ground.
The measurement of leakage current is a requirement for type testing of any mains powered product. A compliance laboratory or National Recognized Test Lab (NRTL) normally performs the type testing on a sample of products during the design phase. Once the type testing is complete generally no further leakage testing is required on a production basis with the exception of medial products. Leakage current measurements are routinely performed on the production line for medical products for safety reasons.
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