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DRL 6-27-97

1.0 When testing fully assembled units precautions must be taken to insure against damage to internal components. Additionally, incorrect techniques may cause a voltage imbalance to occur, which could lead to a condition whereby a good unit could appear to fail dielectric testing.

1.1 In order to avoid the preceding situations the following information is presented, so that intelligent choices may be made regarding dielectric test selection.

2.0 The input Y capacitors are rated for 2000VAC or 2800VDC max during a routine production dielectric test, higher voltages could cause a subtle deterioration in the dielectric media. Capacitor manufacturers recommend DC, since it reduces the corona effect in the capacitor.

3.0 Voltage imbalance occurs when a direct input to output dielectric test is run (see figure 1). The voltage division which takes place between the input Y capacitor(s) and the output to ground capacitor will be proportional to the reactance ratio's of these components during an AC dielectric test. If a DC dielectric test is run the voltage division ratio's will be predicated upon the leakage current ratio's of the respective capacitors, since this is an unknown quantity, determining the voltage division ratio for series DC capacitors is not meaningful.

4.0 If the voltage stress on the output to ground capacitor exceeds the capability of the capacitor or the PCB trace spacing premature breakdown could occur. This breakdown may trip a dielectric test apparatus. Often excessive voltage stress may build up on the output to ground capacitor and associated PCB conductors.

4.1 However in actual operation with the ground point terminated in an ordinary fashion, an output to ground voltage surge is not germane to the question of safety isolation. The input to ground (basic) and double/reinforced insulation barriers are the fundamental concerns.

5.0 Since all double/reinforced insulation components are pretested for dielectric breakdown before assembly onto the PCB, the prime concern then for the final assembly testing is the input to ground and output to ground testing. The test procedure shown in figures 2 & 3 address those concerns and meet the requirements of the following safety specifications:

6.0 IEC950, IEC601-1, IEC1010-1, EN60950, EN61010-1, UL1950, UL2601-1, UL3101-1, CSA 22.2#950, CSA 22.2#234, CSA 22.2#601-1, CSA 22.2#1010-1, UL544, CSA 22.2#125 This following procedure should meet any other worldwide safety specification also.


Basic Insulation: Insulation which insulates the input AC line voltages from the Earth Ground.

Reinforced Insulation: Insulation which is suitable to insulate the input AC line voltages from the output circuit, when the output is floating freely from ground.

Double Insulation: Similar to Reinforced Insulation, the terms can be interchanged for the most part, minor difference not germane to this Application Note.

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