UEi Test Instruments DMEG3 - Manual (Page 5)

Note: High precision resistance (continuity) readings can be improved
by using the null adjust feature to offset the resistance value of the test
leads. To offset this resistance value, follow the instructions through
step 4, then adjust the null adjustment knob to indicate zero on the
dial or digital display. Your resistance or continuity readings will now
display only the resistance in the circuit, not the combined resistance
of the leads and the circuit.
Measuring Insulation Resistance
Insulation resistance testing (IRT) is the process of evaluating an
insulating material’s integrity. Any time you use IRT procedures; you
will be applying a relatively high DC voltage (250, 500 or 1000 Volts)
to two separate conducting paths. For example, the “conducting paths”
might be the hot and neutral wires in a cable set. With this voltage
applied, your meter will measure the extremely small amount of current
that flows between the two paths. Using the principles of Ohm’s law,
the resistance value is displayed.
What needs to be tested: All insulating materials begin
deteriorating from the day they are made. Knowing this, a wide
variety of insulation specifications have been created to accommodate
different environments in which they will be used. Some of those
ratings refer to environmental conditions such as indoor use, outdoor
use, chemical resistant, high or low temperature, maximum voltage or
current ratings and many more. When insulating materials are subjected
to conditions other than what they are rated for, and as time takes its
toll, deterioration accelerates.
Frequently the life span of an insulating material is known to
be less than the life span of the appliance it is being used in. Motor
windings are a good example. Industrial manufacturing equipment,
commercial refrigeration systems and other processes require
predictive and preventive maintenance (PM) procedures to ensure
uninterrupted operation. Insulation resistance testing should be part of
that maintenance process.
IRT procedures are recommended, and often documented by numerous
engineering and maintenance organizations such as IEEE, NETA and IEC.
You can access these organizations and the documents they have
produced related to insulation resistance testing through the internet.
Some documents are fee-based.
You will commonly find IRT procedures performed in these tasks:
• Predictive/preventive maintenance on installed motor
windings - recommended for all motors 750 watts (1 HP)
or greater
• Predictive/preventive maintenance on commercial
HVAC compressors
• Testing integrity of insulation on buried cables powering
well-pumps
• Verifying safety standards for appliances and
biomedical equipment
• New-installation verification
• Troubleshooting electrical faults
• Fire and flood damaged building non-destructive inspections
• Electrical product manufacturing quality control
Insulation Resistance Testing Methods: Insulation resistance testing
can be performed using a variety of methods. The proper method will be
determined by the circuit or cable’s rated voltage ca p a c i ty, by the purpose
of your test (troubleshooting, preventive maintenance, etc.) and by the
function of the circuit you are testing.
Each motor, cable, appliance or other circuit being tested will have its
own unique characteristics resulting from its type of insulation, where it’s
installed and other criteria. Accordingly, specific test values cannot be
documented in this manual. Instead, the general behaviors-under-test and
some of the generally accepted “rules of thumb” will be provided to assist
you in establishing your own test and maintenance pra c t i c e s .
Cable or Conductor Rated Voltage
Whenever possible, use the equipment manufacturer’s recommended
test voltage. Most conductors (shielded, paired, etc.) have a voltage limit
printed on the outer insulation that can be used in the absence of a
manufacturer’s recommendation. When using the rating printed on the
outer jacket, use two times the value, up to the 1000 volts maximum.
Commonly used voltages.
Humidity and Dew Point
In oder to make an accurate assessment of a motor’s life expectancy,
conditions must be similar each time it’s tested. If the equipment you are
testing is at or below the dew point temperature, water-condensation
may collect around the windings and connections. Condensation ca n
make a motor appear to be failing rapidly when it actually has years of
s e rviceable-life left.
Humidity also affects readings at the same time it affects insulation
resistance when the motor is running. Seasonal deviations in insulation
resistance may be noted, but should not be neglected. The same motor
may perform fine in winter when humidity is low and begin to fail in
summer when humidity increases.
Humidity and dew point information is available for some areas
by phone (see your local listing). UEi also makes special purpose
instruments to give you accurate and instant humidity, dew point and
temperature information.
DMEG3/IRT3-MAN P. 4
Specified Cable/Equipment
Voltage Rating
50-100
100-440
440 and above
Megohmeter DC Voltage
Level
250
500
1000