Should i disconnect equipment, When i am doing megger test? how to choose voltage of a megger.?

 .Yes you should disconnect the equipment, when you are doing megger test

The voltage of megger is chosen according to the rating voltage of equipment .

A MEGOHMMETER (MEGGER) is used to measure very large resistances, such as the insulation of wiring. To use a megger, isolate the resistance being measured from other circuits, connect the meter leads, turn the hand crank, and note the meter indication. Normal insulation will indicate infinity.

The following SAFETY PRECAUTIONS should be observed when using a megger.

·         Disconnect the item (equipment) being checked from other circuitry, if possible, before using megger.

·         De-energize and discharge the circuit completely before connecting a megger.

·         Use meggers for high-resistance measurements only. Never touch the test leads while the handle is being cranked.

How to choose voltage of a megger?

You must know, what voltage of your Installation or cable is. E.g. if it’s used for 230 v. Then you can use 250V Megger or even 500V megger. For 440V application or cables use 500V or 1000v megger. For HT cables 3.3kv and above you can use 5kv megger. Ohms value will come according to the reading. If reading is in Mega-ohms or Giga-ohms it will appear on the scale or change tapping accordingly if any.