Burster RESISTOMAT 2408, Manual, Page: 75 / 90

Model 2408
RESISTOMAT
®
Page 75
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.