Amprobe MEGATEST-5000 Megohmmeter - Manual (Page 9)

2.6 Notes on insulation measurements
Measurement of such a high resistance as insulation resistance is very critical, since the
currents involved are minute, even lower than those circulating inside our nervous system.
When performing measurements, in order to avoid making mistakes take the following
precautions:
• Keep the measuring cables suspended in the air, or at most resting on one point only.
• The area surrounding the measuring equipment and the equipment being tested must
be free of any object.
• Nobody must circulate near the cables or the resistance being measured.
• The operator must not move and, especially, must not move his/her arms.
These precautions are especially important when measuring resistances higher than
50G
Ω.
Such precautions are required because any kind of movement leads to capacity variations
that introduce themselves parallel to the resistance being measured. Each capacity
variation implies a variation of the electrical charge; if the capacities are very small, then
the relevant currents will interfere with the equally small ones circulating in the test
specimen.
We advise you always to use the Guard terminal.
Even the presence of air currents can affect the results of the measurement.
The diagram below shows the stray resistance due to air conduction and the stray
capacitance, Sc, present between the operator and the measuring circuit.
Sc
INSTRUMENT
RESISTANCE
BEING MEASURED
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