“A Stitch in Time...”

FACTORS AFFECTING INSULATION RESISTANCE READINGS
Remember that the measured resistance (of the insulation) will be
determined by the voltage applied and the resultant current (R = E/I).
There are a number of things that affect current, including temperature
of the insulation and humidity, as mentioned in the previous section.
Right now, let’s just consider the nature of current through insulation
and the effect of how long voltage is applied.
Current through and along insulation is made up partly of a relatively
steady current in leakage paths over the insulation surface. Electricity
also flows through the volume of the insulation. Actually, as shown in
Fig. 5, our total current comprises three components:
1. Capacitance Charging Current
Current that starts out high and drops after the insulation has been
charged to full voltage (much like water flow in a garden hose when
you first turn on the spigot).
2. Absorption Current
Also an initially high current which then drops (for reasons discussed
under the section Time-Resistance Method).
3. Conduction or Leakage Current
A small essentially steady current both through and over the
insulation.
As shown in Fig. 5, the total current is the sum of the three components
and it is this current that can be measured directly by a microammeter,
or in terms of megohms at a particular voltage by means of a Megger
instrument (ohmmeter). Because the total current depends upon the
time that the voltage is applied, you can see now why Ohm’s Law R = E/I
only holds, theoretically, at an infinite time (that is, you’d have to wait
forever before taking a reading).
In practice, as you will see in the test methods described below, you
read a value that is the apparent resistance – a useful value to diagnose
troubles, which is what you want to do.
A STITCH IN TIME