Question 158Electricity & Electronics - Assistant Engineer

The correct answer is D) When the machine has been shut down and grounded for a period of 15 minutes. This is the correct answer because a megohmmeter measures the insulation resistance of electrical equipment, and the most accurate readings are obtained when the equipment has been de-energized and allowed to discharge any static electricity buildup. After 15 minutes of being shut down and grounded, the machine will have stabilized and the megohmmeter will provide the most reliable and precise insulation resistance measurements. The other options are incorrect because A) taking measurements while the machine is in operation could be affected by electromagnetic interference, B) measurements immediately prior to restarting may still have residual static electricity, and C) measurements during static discharge would not provide an accurate representation of the insulation resistance.