Can we use an insulation tester for Ground Testing?

No. This is a common error. Field operators are often issued a MEGGER ® instrument from stores,
without its being checked to determine whether it’s an insulation or ground tester. Insulation testers are
designed to measure at the opposite end of the resistance spectrum from a ground tester.

No one wants grounds that measure in megohms Insulation testers use high test voltages in the kilovolt range. Ground testers are limited, for operator safety, to low voltages. Insulation testers do commonly have low-voltage, low-resistance continuity functions and these are frequently misused to make jiffy ground tests.

However, a continuity test can only make an arbitrary measurement between an installed electrode and a
reference ground, which is assumed to have negligible resistance. This does not afford a reliable
measurement of the resistance the earth offers to a ground fault current. Even this arbitrary measurement
may not be reliable, since a dc continuity test can be influenced by soil transients, the electrical noise that
is generated by utility ground currents trying to get back to the transformer, as well as other sources.