When testing high voltage insulation for rotating machinery, an over-potential test is carried out to determine the dielectric strength of the insulation. This is done by applying a high AC voltage across the insulation for a period of time. If a DC test voltage is to be used instead of AC, IEEE Std. 95 recommends using 1.7 times the AC test value. I would have expected a DC value of 1.41 times the AC test voltage to be recommended(i.e. the RMS value). After doing some reading, I found that the 1.7 value is more accurate when the insulation is very close to the ground potential (like a stator bar inside a core slot in a generator).

This got me thinking... How is the scenario where the insulation is close to the ground differ from a scenario where the insulation is in open air? I am trying to understand the physics behind how DC voltage affects insulation differently than AC. Any input is greatly appreciated.

 

After doing some reading, I found that the 1.7 value is more accurate when the insulation is very close to the ground potential (like a stator bar inside a core slot in a generator).

This is stumping me. Can you say more about why 1.7x is, "more accurate"? Who said so? What is it more accurate at doing?
Meanwhile, I'm thinking that nearby turns will be at a different voltage from each other when AC is applied and the insulation will become the dielectric in a distributed capacitor (with AC). Current through that capacitance will be active continuously with AC but only once with DC.

 

Is this a three-phase machine (or a test designed to be used with such a machine)?

Note that √3 = 1.732

With all the √3 running around in three-phase calculations, I wouldn't be surprised if that's were it originates. You would have to look at the maximum voltage that appears across the insulation at any point in time.