Insulation Resistance Testing - QuadTech 1865 Instruction Manual

Section 3 : Applications
3.1

Insulation Resistance Testing

Insulation resistance of materials is one of several parameters that may indicate the
condition of insulation. An insulation test is to measure the resistance offered by the
insulating members of a component part to an impressed direct voltage tending to
produce a leakage of current through or on the surface of these members. There are times
when knowledge of insulation resistance can be very important, for example; when
resistance is high it may be the limiting factor in design of a high-impedance circuit,
when resistance is low it can disturb the operation of circuits intended to be isolated.
Insulation resistance measurements should not be considered the equivalent of a voltage
breakdown test. Material with high insulation resistance could possess a mechanical
fault that might fail during a voltage test and conversely material with low insulation
resistance might not breakdown during a voltage test.
Factors that affect insulation resistance measurements include such things as temperature,
humidity, previous conditioning, test voltage, charging current and duration of the test
voltage (electrification time). It is characteristic of certain components (for example,
capacitors or capacitive components or materials) for the current to fall from an
instantaneous high value to a steady lower value, consequently the measured insulation
resistance will increase from an appreciable time as test voltage is applied. Because of
this it may take minutes to approach maximum insulation resistance readings, thus
specifications usually require that readings be taken after a specified time, again
electrification time. A routine test that has been widely adopted for insulation testing
calls for the measurement of the apparent leakage resistance after a test voltage has been
applied for 1 to 2 minutes.
For discussion on insulation resistance measurement methods and procedures refer to
MIL-STD-202 and ASTM (American Society for Testing and Materials) Standard D257.
Applications Page 73 of 87