How does hipot testing work? How does the device under test not experience horrible failure when high voltage Is applied to the unit.

 

The only things that are hi-pot tested are those that are designed to withstand high voltage. The test is to ensure that they work correctly by measuring the hi-pot current. If they fail, they can self destruct but the current used is very limited.
NOTE: cables that have been hi-pot tested should be discharged before being used. The high dielectric of the insulating material in the cables can hold a high voltage charge for a very long time which could cause damage to equipment when connected.

 

I'm probably wrong but it's always been my understanding that the voltages on one product from a manufacturing run are destroyed. To learn what the item can withstand. Then from that point on the Hipot limit is set lower (slightly) for further testing. And that limit is then published for those doing the further testing.

 

They only test insulation. I never had anything I tested fail.

 

The hipot test voltage is applied been Live & Neutral (tied together) to Earth/Ground.

The applied voltage is testing the insulation between the primary circuit and earth/ground – normally such insulation is subject to 1500Vac.

You may ask yourself why the insulation is subject to such a high voltage, when the mains supply voltage is limited to 240Vac – the answer to that question is because the mains supply can have transient voltages of over 2000V that could appear across the insulation L/N to Earth/Ground.

 

Having helped with documentation to complete our UL cert, they’re looking to me for answers about the dielectric testing. First I’ve warned people about some of the possible dangers, that we will need a dedicated test station with a proper ground. I’ve asked for the UL documentation which should have the voltage/testing requirements.

I would like to know the procedure. So what I’m getting is that the device under test (DUT) has its main switch off and the test is between HOT and GROUND. The voltage is ramped up to the test voltage which may be 1000 - 1500 volts (AC? which means peak of 1400 x 2100 volts?). The idea is that dielectric will allow for some current because of capacitance but will not breakdown. Would AC leave residual charge on the dielectric?

This is a new test for me. Thanks.

 

The production hipot test requirements will be set out within the UL report (supplied to you by UL) covering the product. The actual hipot voltage will be dependent on the product safety standard applied to the product and the electrical rating of the product. As a guide, products rated to 120Vac will be tested at 1000Vac and those rated to 240Vac tested at 1500Vac.

Type test of the product will involve the application of the hipot voltage for 60 seconds, but to avoid stressing the insulation in production, the test time is limited to between 1 – 4 seconds.

The equipment mains switch should be in the on position – otherwise you will only be applying the test voltage on components/wiring up to the switch. Having the switch on should not result in equipment damage as the hipot voltage is only being applied between primary circuits and earth/ground.

You may like to consider mains circuits within the product that are energised by the operation of a relay. On one of the products I was involved with, an internal relay was swapped out for a ‘Run’ relay (which had the switching contacts shorted) to ensure all the mains circuits were subject to the hipot test. The ‘Run’ relay was brightly coloured to ensure that it was not left in the product by mistake.

After test, there may be a residual charge on any Y safety capacitors (between L/N to Earth/Ground), but as these are normally limited to a few nF, the stored energy will not be at a hazardous level.

UL will require you to keep records of the hipot tests conducted – normally this will include a go/no go operational test of the hipot tester at the start of each shift, together with a log of the serial numbers of the products tested by date. You should also ensure that you have procedures in place instructing the test operator what to do in the event of a failed production sample.