High Voltage Testing courtesy of Siemens Lab
IEEE Std 95-1977 is entitled IEEE Recommended Practice for Insulation Testing of Large AC Rotating Machinery with High Direct Voltage. It speaks of recommended practice during and for of testing insulation with direct voltages higher than 5 kV of large ac rotating machines rated at 10000 kVA or greater. 

The purpose of this recommended practice is:
1)To provide uniform procedures for performing high direct voltage acceptance tests and routine maintenance
tests on the main ground insulation of windings of large ac machines

2)To provide uniform procedures for analyzing the variations in measured current so that any possible relationship of the components of these variations to the condition of the insulation can be more effectively studied

3)To define terms which have a specific meaning as used in this document

Below is the recommended safety precaution during high direct voltage testing of insulation of Large AC Rotating Machines.

Personnel should be advised before the test that after application of high direct voltage there will be a residual charge in the winding which is dangerous and that de-energizing the test source will not immediately de energize the machine winding under test.

Windings which have been tested must be solidly grounded before being approached by personnel. There is a possibility that after a test, if a ground is removed before minimum grounding time, there will be a voltage buildup to a level that will be dangerous to personnel or equipment.

Ground should be kept in place until the winding is discharged. This may require several hours, depending upon the size of the machine winding. 

Objects close to the machine under test should be grounded. 

Upon completion of the high direct voltage test, the test voltage control should be turned to zero.

After the voltage has decayed to half value, the winding should be discharged through the special discharge resistor ordinarily provided with the test set.

The winding may be solidly grounded as soon as the voltage has been reduced to zero.

If a high alternating voltage test is to follow a high direct voltage test, it is advisable to double the minimum grounding time to ensure that the absorbed charge does not contribute to puncture when the alternating voltage test is applied.

Otherwise, the absorbed charge, superimposed upon the peak alternating voltage dielectric stress, may exceed the electric strength of the winding.

A machine should not be placed in service after a high direct voltage test until the winding has been grounded. 

Dissipation of the absorbed charge cannot be accelerated by the application of alternating potential or by the application of direct voltage with reversed polarity. Severe insulation voltage gradients will be introduced in the winding if this is attempted.

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