Conformity - November 2008 - (Page 46) voltage and live parts of low voltage. The test voltage between live parts and dead metal parts is 1000VAC plus twice the rated voltage, which is typically 1250VAC for North America. The test voltage maybe reduced to 1000VAC for motors of less than ½ hp (373 Watts), and input voltage of not more than 250VAC. No breakdown shall occur during the test time. Breakdown is defined as a rapid and uncontrolled current flow. Arcing is acceptable as long as the arcing stops when the voltage is reduced. The hipot tester used should have a 500VA or higher transformer, or less than 500VA if the high voltage potential can be maintained during the test. Most digital hipot testers have load regulation and meet the latter requirement. Production testing for dielectric withstand is to be performed on every furnace. The requirements are the same as outlined above with the following exceptions. If the furnace uses a low voltage circuit, the circuit shall be connected to chassis or dead metal parts during hipot testing between mains and dead metal parts. The test time can be reduced from 60 seconds to one second if the test voltage is increased by 120%. The equipment for Dielectric Withstand testing shall have a visual indication that high voltage is being applied, visual and/or audible indication of failure, and a manual reset switch in the event of failure. Leakage Current testing measures the current available from exposed conductive surfaces and ground. This test is commonly referred to as earth line leakage for products with a three-prong power cord, and enclosure leakage for products with a two-prong power cord. All products that use an AC line power source have some associated leakage current when the device is turned on and operating. This leakage current normally flows from the AC line source through the ground path in the product and back to earth ground through the ground blade on the power cord. On products without a ground blade (or when the ground on the product has a fault), a potential can develop on metal surfaces of the product. If an individual then comes in contact with the exposed metal surface, he or she then becomes the ground path for the product. Under this condition a certain amount of leakage current flows through the individual exposed to the metal surface. If the leakage current is extremely low (typically less than 0.5mA), the individual should not notice that they are the path for the current flow. At levels higher than this, the individual can experience a startled reaction or worse. During Leakage Current testing, the product is powered up and the leakage current flowing though the ground wire and/or any exposed metal on the product is measured using a multimeter or dedicated leakage tester. The standard requires a 1500Ω resistor and 0.015uF capacitor in the current measurement circuit to simulate the impedance of the human body. This provides a more realistic idea as to how much current would flow through a typical individual. The multimeter or leakage tester needs to have a frequency response from DC to 100 kHz and measurement error of not more than 5%. The Leakage Current testing discussed here is different from the measurement of leakage current during a hipot test. During a hipot test, high voltage (generally greater than 1000V) is applied between the hot and neutral lines and the ground of the device under test (DUT), and the leakage current is then measured. In Leakage Current testing, the product is powered on and operated via standard line voltage (e.g., 120VAC), and the leakage current is then measured using a circuit to simulate the impedance of the human body. The configuration in Figure 4 is called out in the standard. The switches shown on the mains side are to switch between normal and reverse mains conditions. The maximum leakage current is 0.75mA for a blower rated 250VAC or less. Figure 3: Production Test Dielectric Withstand Conformity november 2008
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.