Printed from Kezuriba (kezuriba.net/en/maintenance/insulation/)
Insulation resistance
Judges pass or fail against the low-voltage circuit standard values (0.1/0.2/0.4 MΩ) from the working voltage and the measured value. How to choose the test voltage and how to measure.
When comparing,
apply the voltage
and use the 1-minute value!
÷Insulation resistance pass/fail
The standard values are those for low-voltage circuits in Article 58 of the Ministerial Ordinance on Technical Standards for Electrical Equipment (Japan). The guide to test voltages follows an insulation tester maker's explanation (usage examples from JIS C 1302). Before measuring, always check that the circuit can take that voltage.
Insulation resistance standard values for low-voltage circuits (Technical Standards for Electrical Equipment, Article 58, Japan)
| Voltage | Value |
|---|---|
| 300 V or less, with voltage to ground of 150 V or less | 0.1 MΩ or more |
| 300 V or less, other than the above | 0.2 MΩ or more |
| Over 300 V | 0.4 MΩ or more |
Values for low-voltage circuits at the place of use, between conductors (line to line) and between the circuit and earth (to ground). They must be met for each circuit that can be isolated by a switch or overcurrent breaker. Voltage to ground is the voltage between a conductor and earth in a grounded circuit, and the voltage between conductors in an ungrounded circuit.[1][2]
Application examples
| case | Value |
|---|---|
| Single-phase 100 V (100 V to ground) | 0.1 MΩ |
| Three-phase 200 V (voltage to ground over 150 V) | 0.2 MΩ |
| Three-phase 400 V | 0.4 MΩ |
Examples of fitting common circuits into the categories of Article 58. Voltage to ground depends on the grounding system, so check it on the actual circuit.(formula derived by Kezuriba)[1](1 source, for reference)
When insulation resistance cannot be measured
If measuring insulation resistance is difficult, it is acceptable if the leakage current with the working voltage applied is 1 mA or less (Article 14 of the Interpretation of Technical Standards for Electrical Equipment, Japan).[3](1 source, for reference)
Choosing the insulation tester test voltage
| Voltage | Application |
|---|---|
| 25 V/50 V | Telephone-line equipment and circuits |
| 100 V/125 V | 100 V class low-voltage distribution circuits and equipment, control equipment |
| 250 V | 200 V class low-voltage circuits and equipment |
| 500 V | Low-voltage distribution circuits and equipment up to 600 V; inspection at completion |
| 1000 V | Circuits and equipment over 600 V; high-voltage installations |
Usage examples that Kyoritsu Electrical Instruments took from its commentary on JIS C 1302:2018 (insulation testers). Choose the test voltage after checking the voltage that may be applied. Hioki explains that in maintenance tests, a voltage higher than the operating voltage is applied to confirm that the value is at or above the minimum.[2][4]
How to measure
On an insulation tester, the earth terminal is + and the line terminal is -. Line-to-line measurement often involves connected indicator lamps and transformers, and is mostly done only at completion. Measurement to ground is done to check for leakage and electric shock hazard.[2](1 source, for reference)
The 1-minute value and changes in the value
When voltage is applied, the value changes over time because of the charging of the insulation (absorption current), and it can take hours to days to settle. When comparing, using the value 60 seconds after applying the voltage (the 1-minute value) is the general practice (Hioki). For cable insulation diagnosis, PI (10-minute value / 1-minute value) and DAR (1-minute value / 15-second value) are also used (Kyoritsu Electrical Instruments). The value also changes with temperature and humidity.[4][2]
Circuits that include electronics, inverters or servo amplifiers
Cautions for measuring circuits that include inverters, servo amplifiers or control boards with an insulation tester (disconnect before measuring, short the main circuit terminals together, do not apply voltage to control circuits, etc.) are normally given in the equipment maker's instruction manual, but this could not be confirmed from published materials this time. Kyoritsu Electrical Instruments' table gives 100 V/125 V for measuring control equipment. Always follow the equipment's instruction manual.[2](could not be confirmed in published sources)
📚Sources
Items without a mark are those on which two or more materials from different publishers agree. For product values, each manufacturer's documentation takes precedence. The text is summarized in Kezuriba's own words.
- Ministry of Economy, Trade and Industry (METI), Japan, "Ministerial Ordinance on Technical Standards for Electrical Equipment (Ministry of International Trade and Industry Ordinance No. 52 of 1997), Article 58 (e-Gov Law Search)"
- Kyoritsu Electrical Instruments, "Insulation Resistance Measurement Guide (Basic Knowledge of Electrical Measurement)"
- Ministry of Economy, Trade and Industry (METI), Japan, "Interpretation of Technical Standards for Electrical Equipment, Article 14 (Insulation performance of low-voltage circuits)"
- Hioki, "How to measure insulation resistance (Measurement knowledge)"