Amprobe MEGATEST-5000 Megohmmeter - Manual (Page 8)

When the meter is in its "TIMER" mode, set a duration of 10’ to determine the efficiency of
an insulator at a certain voltage. During the test, insulation resistance may diminish,
remain steady or increase. Diminishing resistance means insulation is not good. On the
contrary, the more the resistance increases the better the insulation.
This instrument can also be used in a laboratory, and without regard for legal regulations,
in order to establish the efficiency of electronic equipment, installations or boards. This
kind of use, however, requires further technical skills so as not to damage the device being
tested.
2.4.1 Other possible fields of employment
In the field of automobiles, this instrument can be used to check the efficiency of the
battery ignition, spark plug cables or spark plugs without having to dismantle parts in
difficult locations.
Sparking promptness measurement in heating plants: a spark is generated in order to start
combustion in gas and Diesel fuel systems. As time passes, the spark generator - just like
the spark plug in a car - loses its efficiency and spark generation becomes increasingly
difficult.
A diagnosis can be performed by measuring the insulation of the electrode that launches
the spark. If insulation is poor, then it will be necessary to replace the electrode. If the
insulation is good, then it will be necessary to check the generator creating it.
Measuring insulation efficiency for manufacturers of transformers or electric motors.
The quality of the insulation can be monitored in order to take countermeasures in case of
deterioration, before the quality goes below the acceptable level.
2.5 Use of the guard
In certain cases, surface currents may falsify the measurement.
Application of a voltage to an insulator that you want to measure can create two different
currents: one flowing inside the specimen, the other moving across its surface. They are
parallel. If you want to measure only the resistance due to the current flowing inside the
specimen (which is the meaningful resistance), you will have to use another wire to absorb
the surface current (see figure).
(+) (-)
GUARD
Measurement current
Surface leak current
Since the voltage between the GUARD and (-) is very low, only those currents indicated
above will circulate.
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