Testing Isulation Resistance

A DC tester uses direct current to test IR and an AC tester uses alternating current to test IR.

Do you see any difference in the two types of current? Look at them on a scope.That may be a clue.

if AC is used then the capacitance introduced by the insulation will be part of the impedance measured and then the resistance is required to be calculated . . . DC is used if resistance alone is required to be measured

You have not told us the type of equipment nor the operating voltage. If it is ordinary "stuff" like house wiring it won't make a difference. If it is medium or high voltage power equipment then the tests provide different clues as to their health. Also some tests are inappropriate/less sensitive/more destructive if used indiscriminately or on the wrong type of equipment. Google on the title of your question for more info.

Not only for IR for HV test(Hipot) too AC & DC are used,google and see.

The square root of 2 is one difference.

On a high speed production line, where products come into contact with a high voltage DC hipot.......there is no difference because making contact with the probes sets up an impulse which by definition in terms of LaPlace Transforms contains lots of high frequency components. A continuous AC tester will most likely do the same. Sometimes, it is not the device but the method that makes a difference.

An AC tester that uses a zero crossover start and stop circuit however will generally just supply the fundamental frequency thereby setting up a condition by which the impedance (including capacitance and resistance) can be carefully measured.

A DC tester that ramps up at some rate will still have some AC component. You have to decide what you really are trying to test and then check your method.

i guess the question, by Selimo, is very simply relating to insulation resistance measurement using a simple instrument commonly known as a Megger . . . Selimo, please clarify so that all of us chase the same goose . . . if i have guessed wrong, i will be happy to be corrected . . .

What was actually happening Soebfatehi is that, we had a 11kv bus bar fault where we took off all the bars pieces and spouds to be cleaned and bars to be re insulated with reychem insulation, after that we wanted to test(inject about 11 to 22kv in to the bars) to make sure all is right, yes i understand i have to know my country standards, which is Lesotho standards.

i stand corrected

Get one of these.

http://www.fluke.com/Fluke/usen/Insulation-Testers/Fluke-1555-1550C.htm?PID=69212

Generally DC testers are used in a DC environment, AC in an AC environment.
Unless you're testing high voltage traction motors, etc. A DC meter is probably not needed.

Use DC for IR Test