Fluke 1507 Insulation Tester Readings

[Saturn_Europa]

I am using a Fluke 1507 Insulation Tester for the first time. I am using it to test a motor that has a short to ground. The switch board shows a short whenever the motor is running. I am going to replace the motor.

With the Fluke 1507 set to 1000 volts it is showing a reading of around 0.4 M ohms on each leg to ground. The voltage on the display of the meter will only go to around 514 volts.

I am suspecting the 1507 wont put out the full 1000v because it is sensing a ground?

Whats the rule of thumb on good/bad megger readings on motors?

 

[Saturn_Europa]

Do most people stick by this Rule of Thumb from a Megger publication:

"""Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm. For example, a motor rated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms. In practice, megohm readings normally are considerably above this minimum value in new equipment or when insulation is in good condition."""

Ive been using a Megger MIT 410/2 for years and it gives a very clear pass-fail indictor.

 

[synchro]

With the Fluke 1507 set to 1000 volts it is showing a reading of around 0.4 M ohms on each leg to ground. The voltage on the display of the meter will only go to around 514 volts.

I am suspecting the 1507 wont put out the full 1000v because it is sensing a ground?
...

In the specs for the Fluke 1507, under the Insulation Resistance category it says short circuit current is 1 mA nominal. 514 / 0.4M = 1.3 mA. And so the meter is probably only reaching 514 volts because it's going into a current limiting region of its operation. And so the insulation is in a poor enough condition that the meter can't test it at 1000V, which should be a red flag in itself.
Does the meter still read about 0.4M for a 500V test voltage?

 

[ptonsparky]

I failed those that wouldn't come up to test voltage, but we knew already, because we were called.

Testing underground was where we might pass it, if the test voltage would come up above what the operating voltage was. Meaning we had found and repaired one fault but there was another or maybe several. It could delay immediate repairs or replacement until a more convenient time.

 

[garbo]

Used a fluke model 1587 combination VOM/ megger . I noticed the voltage would be lower on a grounded or low resistance to ground reading but was only concerned with the resistance display on drives & motors that I PM'ed every week.For 480 volt drives liked to see at least 10 megohms to ground as to not get ground fault trips. Was amazed on a lot of motors over ten years old I often measured 2.2 gigiohms on them. The highest megger reading while using 1,000 volt test .