Test voltage specifications
All Megger insulation testers are provided with an accuracy specification on the test voltage. As an example, the MIT400 Series is specified as: Terminal voltage: -0% +20% ±1 V. This means that the test voltage will never go below the selected voltage against an acceptable megohm load, but it may rise by as much as 20%. On newer models (MIT400 an example), the actual test voltage is displayed.
Designing a tester to output precisely the selected test voltage would be useless and impractical overkill in a field-test model. Look to the laboratory/R&D market for that, and get your wallet out. Such a model would be comparatively expensive, larger and more difficult to handle and operate, have the safety ratings affected, and not be practical in the field.
What is the significance of -0 +X%?
Test voltage should not go below selected. It’s about like getting 11 in a bag of a dozen. The amount of leakage current pulled through imperfections/deteriorations in insulating material is dependent on applied test voltage. The test result should indicate the ability of the insulating material to stand up to the selected voltage; e. g., 500 Vdc, not 485. Loading up the voltage slightly does no harm to the integrity of the test result while permitting design of a meter that is field practical.
What is meant by an acceptable megohm load?
It is generally accepted (by standards organizations like IEEE) that insulation that is going to operate at building-wiring voltage (120, 240, 480) is not serviceable at less than 1 MΩ of resistance at voltages commensurate with operating voltage. Hence, a 1 kV megohmmeter should exhibit a rapid voltage rise up to the level of 1 MΩ, beyond which it should maintain selected test voltage. Only when the tested insulation is in a deteriorated state and should not be in service does the tester then reduce output voltage. This is accomplished by limiting output current to a few mA. If insulating material can pass more than a few mA, it is no longer fit to act as insulation. By limiting current, the tester cannot damage the test item (is not acting as a “high-pot”).