Insulation resistance describes the ability of an insulating material to prevent current flow between electrical conductors or between conductor and earth. It is measured with DC voltage and expressed in megohms (MΩ). The most important limit values: for electrical installations up to 500 V nominal voltage, DIN VDE 0100-600 requires at least 1 MΩ at 500 V test voltage, for portable equipment of protection class I DIN VDE 0701-0702 requires at least 1 MΩ, for protection class II at least 2 MΩ. GOBA supplies insulation materials with defined insulation resistance values for motors, transformers and industrial applications. All limit values, the measurement procedure and the typical error sources can be found in the tables on this page.
Why insulation resistance matters
A high resistance value indicates that the insulation is intact and only a negligible leakage current flows. If the value drops, this points to moisture, contamination, ageing or mechanical damage of the insulation. Sound insulation protects people against leakage currents, electrical devices from damage and installations from failures due to insulation faults.
Insulation resistance that is too low has four typical consequences:
- Short circuits and fire risk
- Accelerated ageing of equipment
- Personal injury from electric shock
- Increased losses through leakage currents across contaminated or damp surfaces
Which standards and limit values apply to insulation resistance?
Limit values for electrical installations according to DIN VDE 0100-600
For the initial verification of electrical installations, DIN VDE 0100-600 (table 6.1) defines the test voltage and the minimum value depending on the nominal voltage of the circuit:
| Circuit nominal voltage | Test voltage (DC) | Minimum insulation resistance |
|---|---|---|
| SELV and PELV (protective extra-low voltage) | 250 V | 0.5 MΩ |
| up to and including 500 V, including FELV | 500 V | 1.0 MΩ |
| above 500 V | 1,000 V | 1.0 MΩ |
If surge protective devices (SPDs) are present in the circuit, the test voltage may be reduced to 250 V DC, the required minimum value then remains 1 MΩ. The same minimum values apply to the periodic verification according to DIN VDE 0105-100.
Limit values for equipment according to DIN VDE 0701-0702
For portable electrical equipment, these minimum values apply to testing after repair and to periodic testing:
| Equipment type | Minimum insulation resistance |
|---|---|
| Protection class I | 1 MΩ |
| Protection class II | 2 MΩ |
| Protection class III (extra-low voltage) | 0.25 MΩ |
| Equipment with switched-on heating elements | 0.3 MΩ |
The test voltage is 500 V DC, for equipment with surge arresters or SELV/PELV supply 250 V DC. If a protection class I device with heating elements above 3.5 kW does not reach the minimum value, it is still considered safe if the protective conductor current remains below 1 mA per kW of heating power.
Guide values for motors and windings
For motor windings, a proven rule of thumb is: minimum insulation resistance in MΩ equals nominal voltage in kV plus 1, measured at a winding temperature of 40 °C. A 400 V motor should therefore reach at least 1.4 MΩ. In practice, healthy low-voltage windings are well above this, values from 5 MΩ upwards are common, and new windings reach several hundred MΩ up to the GΩ range. Falling values over several measurements are an early warning sign of moisture or ageing of the winding insulation.
Typical practical values by application area
- Household appliances: 2 to 10 MΩ
- Industrial installations: 10 to 100 MΩ or more
- High-voltage systems: above 1 GΩ
How does the measurement of insulation resistance work?
Procedure of the insulation measurement
- Disconnect the installation or device and secure it against reconnection.
- Disconnect sensitive loads: electronics, frequency converters, controllers and dimmers can be damaged by the test voltage or distort the result.
- Select the test voltage according to the standard, usually 500 V DC (see tables above).
- Measure between the live conductors and the protective conductor, applying the test voltage until the reading has stabilised, a measuring time of about one minute is common.
- Compare the reading with the limit value and document it.
Influence of test voltage and DC voltage
Measurement is carried out with DC voltage because AC voltage would drive an additional reactive current through the capacitance of the insulation, distorting the result. The test voltage must match the nominal voltage: too low a voltage does not reveal weak points, too high a voltage can damage sensitive components.
Standards for measurement
- DIN VDE 0100-600: initial verification of electrical installations
- DIN VDE 0105-100: periodic verification of electrical installations
- DIN VDE 0701-0702: testing of portable equipment
- DIN EN 61557-2: requirements for insulation testers
Which test instruments are used for insulation measurement?
- Handheld insulation testers: compact and ideal for on-site measurements
- Stationary test instruments: for comprehensive tests in industrial applications
- Multimeters with insulation function: for basic resistance measurements
Key selection criteria are compliance with DIN EN 61557-2, a test voltage suitable for the application and functions such as automatic discharge of the device under test and measurement data storage.
How is insulation resistance calculated?
Insulation resistance follows Ohm's law, where U is the applied test voltage and I is the measured leakage current:
R = U / I
A calculation example: if a leakage current of 0.5 µA flows at 500 V test voltage, the insulation resistance is 500 V divided by 0.0000005 A, that is 1,000 MΩ or 1 GΩ. The measuring instrument performs this calculation internally and directly displays the resistance value.
Which factors influence insulation resistance?
- Temperature: as a rule of thumb, insulation resistance roughly halves for every 10 K rise in temperature. Readings are therefore only comparable with a temperature reference.
- Moisture: damp insulating materials and surface moisture reduce the resistance drastically, often by several orders of magnitude.
- Contamination: conductive deposits on the surface create creepage paths and reduce the surface resistance.
- Ageing: thermal and electrical stress embrittles insulating materials, the insulation class defines the permissible continuous temperature.
- Measuring time: the value rises during the measurement due to polarisation effects. For windings, the polarisation index is therefore often calculated from the readings after 1 and 10 minutes.
Insulation resistance starts with the insulating material
Meeting limit values is a question of testing, exceeding them reliably is a question of material selection and processing. The achievable insulation resistance of a component depends on the volume resistivity of the insulating material, the material thickness, the creepage distances of the design and the cleanliness of processing. Three points from our manufacturing practice:
- Material selection according to environment: hygroscopic materials such as unimpregnated pressboard lose insulation resistance massively in humid environments. For humid or thermally highly stressed applications, polyester films, polyimide films or aramid papers are the more robust choice.
- Clean cut edges and particle-free processing: conductive particles or frayed edges from cutting can create creepage paths that devalue the best insulating material. In the stamping and cutting of insulating parts, clean tools and burr-free edges are therefore part of quality assurance.
- Plan sufficient creepage distances: the measured insulation resistance of an assembly is often set by the shortest creepage path between the potentials, less often by the material itself.
We manufacture insulating and molded parts from materials with defined insulation properties and support you in material selection as part of our consulting services.
GOBA conclusion: material and cut edge determine the reading
The limit values are quickly stated: at least 1 MΩ at 500 V test voltage to DIN VDE 0100-600, and at least 2 MΩ for portable class II equipment to DIN VDE 0701-0702. Whether an assembly holds these values is decided long before the measurement. Unimpregnated pressboard absorbs moisture and drops measurably in humid environments, while polyester and polyimide films and aramid papers hold their value. A burr on the stamped edge shortens the effective creepage distance. So before sampling, check the environment the part will sit in and choose the material accordingly: with insulation and moulded parts this is the cheapest place to gain insulation resistance. And the temperature belongs in the report, because every additional 10 K roughly halves the value.
