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From the Q&A

With TJ Byers

HiPot Testing


I would like to know if a circuit is readily available for testing the insulation strength of a transformer (HiPot test) at about 1,500 volts according to the equation:

High Pot test voltage = 1000V + 2(mains supply voltage)
                                  = 1000V + 2 x 120VAC = 1,240VAC

B. Bill


The configuration is a simple matter of stacking an AC voltage on top of a DC voltage, as shown below. This is a test that is described in detail in MIL-STD-202. And the document stresses the same thing I do: You are working with lethal voltages at times. Always keep that in mind before you throw the power switch!

What you didn’t specify is the duration of the applied voltages (typical times range from 10 sec to 1 min) or the environmental conditions. These are variables, too. Higher temperatures, for example, take a toll on insulation. And the longer the voltages are applied at these elevated temperatures, the more likely the insulation is to break down. You need to look at your application. Is this a power transformer operating 24/7, or a furnace control transformer with a lower duty cycle? Are we talking Bangladesh (high humidity)? Iraq (high temperature)? Each test must take into consideration the conditions under which the equipment is to be operating. The HiPot equation – DCV + 2ACV – is just one part of a bigger picture.