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Results
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In the ANSI/NEC world, the standard ANSI/NETA ATS-2009 [2]
provides test procedures and acceptance levels for most types of
electrical equipment.
Table 100.1 provides representative acceptance values for IR
test measurements, which should be used in the absence of any other
guidance (from the
manufacturer or other standards):
Nominal EquipmentVoltage (Vac)
Min TestVoltage (Vdc)
MinInsulationResistance
(M)
250 500 25
600 1,000 100
1,000 1,000 100
2,500 1,000 500
5,000 2,500 1,000
8,000 2,500 2,000
15,000 2,500 5,000
25,000 5,000 20,000
34,500 and above 15,000 100,000
NFPA 70B [3] also provides some guidance on insulation
resistance testing for different types of equipment.
Factors Affecting Test Results
There are two main factors that will affect IR test results:
Temperature
Electrical resistance has an inverse exponential relationship
with temperature, i.e. as temperature increases, resistance will
decrease and vice versa.
Since the minimum acceptable IR test values are based on a fixed
reference temperature (usually 20oC), the measured IR test values
must be corrected
to the reference temperature in order to make sense of them.
As a rule of thumb, the resistance halves for every 10oC
increase in temperature (and vice versa). So if the measured IR
test value was 2M at 20oC,
then it would be 1M at 30oC or 4M at 10
oC.
ANSI/NETA ATS-2009 Table 100.14 provides correction factors for
IR test measurements taken at temperatures other than 20o
C or 40o
C, which werein turn based on the correction factors in the
freely available Megger book "A stitch in time..." [4].
Humidity
The presence (or lack) of moisture can also affect the IR test
measurements, the higher the moisture content in the air, the lower
the IR test reading. If
possible, IR tests should not be carried out in very humid
atmospheres (below the dew point). While there are no standard
correction factors or guidance
for humid conditions, it is good practice to record the relative
humidity of each IR test so that they can be used for baseline
comparisons in future tests.
For example, having past data on the IR test values for dry and
humid days will give you a foundation for evaluating future test
values.
Related Tests
For equipment maintenance, the Dielectric Absorption Test is
normally performed in conjunction with the IR test using the same
testing equipment. The
results are either in the form of a Dielectric Absorption Ratio
(DAR) or a Polarisation Index (PI). Refer to the Dielectric
Absorption Test article for more
details.
References
1. IEC 60364-6, "Low voltage electrical installations - Part 6:
Verification", 2006
2. ANSI/NETA ATS, "Standard for Acceptance Testing
Specifications for Electrical Power Equipment and Systems",
2009
3. NFPA 70B, "Recommended Practice for Electrical Equipment
Maintenance", 2010
4. Megger, "A stitch in time - The Complete Guide to Electrical
Insulation Testing"
(http://www.biddlemegger.com/biddle/Stitch-new.pdf) , a
free book which is an excellent resource on IR testing
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