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Insulation Resistance Test Values

The insulation resistance values on this page are based off of representative values suggested by the NETA Standards Review Council. Use these values in the absence of consensus standards dealing with insulation-resistance tests.

Test results are dependent on the temperature of the insulating material and the humidity of the surrounding environment at the time of the test.

Electrical Apparatus and systems other than Transformer and Motor Windings (20�C)

Nominal Equipment Rating (Volts) Minimum DC Test Voltage Recommended Minimum Insulation Resistance (Megohms)
250 500 25
600 1,000 100
1,000 1,000 100
2,500 1,000 500
5,000 2,500 1,500
8,000 2,500 2,500
15,000 2,500 5,000
25,000 5,000 10,000
34,500 5,000 100,000
46,000 and up 5,000 100,000

Reference: ANSI/NETA-ATS 2017 & ANSI/NETA-MTS 2015 Table 100.1 - Insulation Resistance Test Values Electrical Apparatus and Systems Other Than Rotating Machinery


Transformer Insulation Resistance Test Values

Transformer Coil Voltage Rating Recomended Test Voltage (DC) Recommended Minimum Insulation Resistance in Megohms
Liquid Filled Dry Type
0 - 600 1000 100 500
601 - 5000 2500 1000 5000
> 5000 5000 5000 25000

Reference: ANSI/NETA-ATS 2017 & ANSI/NETA-MTS 2015 Table 100.5 - Transformer Insulation Resistance Testing


Insulation Resistance Acceptance Test Values for Motor Windings (1 Minute @ 40°C)

Winding Rated Voltagea Recomended DC Test Voltage Recomended Minimum Insulation Resistance in Megohms: Windings Before 1970, Field Windings, Others Listed in NETA ATS Table 100.11b Recomended Minimum Insulation Resistance in Megohms: DC Armature, AC Windings, (form-wound coils) Recomended Minimum Insulation Resistance in Megohms: Random-Wound Stator Coils, Form-Wound Coils below 1kV
< 1,000 500 kV + 1 100 5
1,000 - 2,500 500 - 1,000 kV + 1 100 -
2,501 - 5,000 1,000 - 2,500 kV + 1 100 -
5,001 - 12,000 2,500 - 5,000 kV + 1 100 -
> 12,000 5,000 - 10,000 kV + 1 100 -

aRated line-to-line voltage for three-phase ac machines, line-to-ground voltage for single-phase machines, and rated direct voltage for dc machines or field windings.

bkV is the rated machine terminal to terminal voltage.

Reference: ANSI/NETA ATS-2017 Table 100.11 - Insulation Resistance Test Values Rotating Machinery for One Minute at 40° C


Insulation Resistance Maintenance Test Values for Motor Windings (1 Minute @ 40°C)

Winding Voltagea Recomended Minimum DC Test Voltage Recomended Minimum Insulation Resistance in Megohms: Windings Before 1970, Field Windings, Others Listed in NETA MTS Table 100.11b Recomended Minimum Insulation Resistance in Megohms: DC Armature, AC Windings, (form-wound coils) Recomended Minimum Insulation Resistance in Megohms: Random-Wound Stator Coils, Form-Wound Coils below 1kV
< 1,000 500 kV + 1 100 5
1,000 - 2,500 500 - 1,000 kV + 1 100 --
2,501 - 5,000 1,000 - 2,500 kV + 1 100 --
5,001 - 12,000 2,500 - 5,000 kV + 1 100 --
> 12,000 5,000 - 10,000 kV + 1 100 --

aRated line-to-line voltage for three-phase ac machines, line-to-ground voltage for single-phase machines, and rated direct voltage for dc machines or field windings.

bkV is the rated machine terminal to terminal voltage.

Reference: ANSI/NETA MTS-2015 Table 100.11 - Insulation Resistance Test Values Rotating Machinery for One Minute at 40° C. Values based on IEEE Std 43-2013.

Tags: acceptance testing, cable and busway, circuit breakers, codes and standards, ieee, insulation testing, maintenance, megger, motors, neta, overpotential testing, reference guides, rotating machinery, switchgear, test procedures, transformers

Source: https://testguy.net/content/240-Insulation-Resistance-Test-Values

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