Should I use a hi-pot or a megohmmeter?
When should you use a hi-pot and when should you use a megohmmeter? This seemingly simple question doesn’t have such a simple answer since the difference lies not with the function of each instrument, but the intended result.
Let’s first examine what each device does:
A hi-pot (a shorted way to say high potential or high voltage) is a term used for electrical safety testing instruments used to verify electrical insulation in finished appliances, cables or other wired assemblies, printed circuit boards, electric motors, and transformers.
A megohmmeter is a special type of ohmmeter used to measure the electrical resistance of insulators.
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Essentially, both the hi-pot and the megohmmeter do the same thing, but with vastly different outcomes. Both instruments apply a comparatively high voltage across insulation and draw leakage current through the insulation, commensurate with its insulating properties and condition. But, a megohmmeter gives the operator a measurement, while a hi-pot takes action.
When to use a Megohmmeter
A megohmmeter measures extremely small currents, typically in nano-amps, that make it through imperfections and degradation of the insulating material. Per Ohm’s law, it converts this to a resistance reading. These are typically in megohms. Anything less than one MΩ is generally considered to be out of service.
Once the operator gets the reading with the megohmmeter, he/she has to decide if the equipment under test should remain in service, get cleaned, get repaired, or be scrapped. Because it works with only a few milli-Amps of test current, the megohmmeter has limited power and does not damage the insulation. This is important because the most effective use of a megohmmeter involves repetitive, routine testing in order to establish a lifecycle trend and perform preventive maintenance.
When to use a High-pot tester
In contrast, a hi-pot uses higher voltages and currents, and while it can provide a measurement, its primary function is to identify and kill the weak; like taking marginal equipment out of service by breaking down weak insulation before a costly malfunction occurs on the job. Testing can be performed in DC or AC voltage.
Hi-pots are also known as ‘dielectric withstand testers’. The item under test is supposed to withstand the applied voltage; however, a compromise can be made in the ‘tip-up’ test. The tester has a voltage and a current meter, and the two are expected to rise in sync so long as the insulation is ‘withstanding’. When the current starts to rise faster than the voltage, the test is abruptly stopped before increased damage can occur. The item under test then needs to be repaired.
Choosing the right test equipment
Choosing between a hi-pot or megohmmeter to perform testing really boils down to what you are testing, what voltage and current is needed to test, and a little bit of personal preference.
If you’re looking for a measurement to track lifecycle trends in your equipment, then a megohmmeter is your tool.
However, if you are looking to truly stress your equipment under test to determine if it’s still meeting standards and operating safely, a hi-pot test will help you take more decisive action.