HIPOT testing and Y capacitors
The real risk is an impulse breakdown at elevated voltage. It is tested for 60 s or raised 10% for 1 s.







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The rationale is that every product must pass the dielectric HIPOT test to ensure the leakage does not exceed 250uA at the rated voltage. The default test duration is 1 minute , which may be reduced to 1 second by a further 10% increase in the voltage.  It is not possible for the grid to sustain these Voltage levels which may occur due to an inductive machine transient, or an induced from nearby lightning. The real event duration would be short, but this test is to stress the insulation in case ionization breakdown leads to failures. 

The Y caps however induce a steady current at line voltages in order to shunt SMPS conducted noise which would cause HIPOT failure by this steady HIPOT test. 

The HIPOT test is a dielectric breakdown threshold test and not intended to test line filter Y cap current at elevated steady-state voltages.  It is intended to stress manufacturing process breakdown (pun intended) due to insufficient clearance gaps or lack of silicone added insulation in the process.  

I once had qualified a PowerOne OEM supply that was later transferred to Mexico production. I also startd testing all HIPOT with DC secondary earth grounded to reflect application stresses.  When this is grounded, the isolation transformer which has a small coupling capacitance but at switched rates is now a much lower impedance than the stray capacitance of primary gaps at line frequency.  This raises the voltage across those gaps  and stresses the primary gaps with more voltage with secondary DC grounded. It resulted in 30% failures in HIPOT with more than 6 weak spots and supplier was disqualified until corrected.

NB to all those Test Engineers.