Why is it sometimes necessary to connect the G terminal to a megohmmeter?
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When measuring insulation resistance, only the “L” and “E” terminals are generally used. However, when measuring the insulation resistance of a cable with respect to ground or when the leakage current of the device under test is high, the “G” terminal must be used, with this terminal connected to the shielding layer or casing. This causes the current flowing over the insulated surface to bypass the measuring coil of the galvanometer and flow directly through terminal G to form a circuit; as a result, the measured insulation resistance is merely the volume resistance of the cable’s insulation. I have some doubts about this passage: since there is current flowing across the insulating surface, that proves there is a leakage issue and the insulation is defective; how can this current not pass through the ammeter (so as to avoid measurement)?
When measuring the insulation resistance between the cable core and its surface, the shield wire is connected to the G terminal in order to eliminate the effect of surface leakage current. When the air humidity is high or the surface of the cable insulation is not clean, the leakage current on its surface becomes significant. To prevent the leakage current from affecting the measurement of the internal insulation of the device being tested, the shielding metal of the cable is generally connected to the G terminal of the megohmmeter.