One common
method of measuring resistances ranging 0–1000 MΩ is by using meggers or
insulation-resistance testers. This is the usual ohmmeter with a battery used for
voltage source.
Resistance
is directly displayed on the front panel digital display. A range of voltages can
be selected while testing.
This
instrument is used to measure very high resistances, such as those found in
cable insulations, between motor windings, in transformer windings, etc.
Normal
multimeters do not provide accurate indications above 10 MΩ because of the low
voltage used in the ohmmeter circuit.
Meggers can
apply a high voltage to a circuit under test and this voltage causes a current
if any electrical leakage exists. This makes it useful as an insulation tester.
Some
laboratory test meters have a built-in high-voltage source. The high voltage permits
accurate high-resistance measurement, but such meters are usually not portable.
The megger
is essentially a portable ohmmeter with a built-in high-voltage source. The
built-in high-voltage source may be derived from a magnet-type DC generator or battery.
In a DC generator-type
megger, a hand crank is used to turn the armature to produce voltages up to
500, 1000, and 2500 (depending on the model used).
An
electronic battery-operated type of instrument is popular because it is light, compact,
and can be held and operated in one hand, i.e., there is no generator to turn.
High-testing
voltage is produced by an electronic circuit, which uses an internal battery as
an energy source.
No comments:
Post a Comment