Theory:Ok there is a motor /starter with the T leads /3 phase motor,I believe is connected across the M coil.Now here is a 500 volt megger...which was supposed to from ground to each motor lead,basically to determine the health of the windings:with the power shut off the readings were taken fr.T1,2,3 without removing the leads ..is this ok or not 2 thoughts i just going to ground would not complete a path within the circuit hence no damage to other components within the ckt & 2 damage could occur due to voltage form the megger ect. was I clear here? ...need i good thoery person here....
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If I follow your description correctly--You are connecting the megger leads to T1 and ground, Then T2 and ground, then T3 and ground. Taking 3 different readings.
This is done of course with power off as you cannot megger cables when live.
The T lead connections are isolated from the remainder of the circuit with the exception of the motor windings themselves. So there is no danger involved.
I worked on industrial sites for 25 years during their 6 month outages. That was the only time for maintenance work as the plants operated 24hr day 7 days a week.
Same with power plants.
We performed 150-200 motor and cable megger readings every outage, just that way. Never disconnect the motor leads from the motor UNLESS you get a bad reading. Then you have to isolate whether it is the cable or the motor.
On 480 and 600 volt systems, use the 1000 volt setting and use 1 megohm as the absolute minimum. and all should be good.
Let me know if more clarification is needed or I have missed anything you were looking for on the question.