As you may or may not already be aware, the new Plug Discharge Test has appeared, or will appear in the next edition of IEC 61010, IEC 60065, IEC 60335 and IEC 60950. The purpose of the test is to make sure that the energy contained in any primary capacitance is discharged before it becomes a shock hazard to the user when the device under test (DUT) is unplugged from a mains power.
May 30, 2013
At first glance, this test may not appear to be complicated or time-consuming, but we’ve recently being hearing a buzz in the field that the opposite is true.
Depending on the standard, the allowable time for the voltage to decay to safe levels varies, and the voltage that is considered “safe” also varies. The test is conducted slightly differently by each standard, but each standard includes a unique method to determine when the voltage present on the primary mains of the plug cap has decayed to that safe value, as shown here.
STANDARD | PASSING CRITERION | COMMENTS |
IEC 61010 | 33V RMS: 10 sec | None |
IEC 60065 | 35V RMS: 2 sec | Repeat 10 times to determine most unfavorable result |
IEC 60950 | 37% of mains voltage: 1 sec | Calculated OK, measured if not possible to calculate |
IEC 60335-1 | 34V RMS: 1 sec | Repeat 10 times |
Some labs are currently approaching the plug discharge test very cautiously. To alleviate possible questions regarding completeness when conducting this test, some have found it best to conduct the test starting at the positive and negative peak of the input waveform. We’re also hearing that in order to prove that the worst case has been captured, some labs are having to conduct the test multiple times (because while the point of plug disconnection would not appear to modify the energy stored in a capacitor contained within the DUT, if tests are conducted at the positive and negative peaks, there can be no question of whether the worst case was contemplated and tested). Further, the wording of IEC 60065 states that the test may be repeated 10 times, which typically means that a conservative stance would be to conduct the test 10 times for every DUT.
Many labs are indicating that this test takes much longer to conduct than planned, especially if that conservative approach is taken. For each test, the oscilloscope must be connected and fired, and if the approach of testing at the mains wave peaks is followed, it will probably take multiple tests to capture that moment. Further, 10 repetitions are mandated by IEC 60335-1 and suggested by IEC 60065.
Field reports have noted that some test sequences covering the Plug Discharge Test can absorb an entire day of lab time. Your results may vary, but these reports tell us that conducting this test is more time consuming than it may appear on the surface. Until experience suggests otherwise, the Plug Discharge Test should be scheduled conservatively to make sure your report is delivered on time!
Jeff Lind is an expert in compliance, regulatory and safety testing, with more than thirty years of industry experience. He is President and CEO of Compliance West USA, a leading provider and manufacturer of hipot and ground continuity test equipment for production line and laboratory product safety testing.
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