Insulation Resistance

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.

Limit values according to DIN VDE 0100-600 and DIN EN 50678 / 50699 (formerly DIN VDE 0701-0702) as given by Bender, windings and polarisation index according to IEEE 43 (1974, 2000 and 2013 editions, as summarised by Reliability Magazine), temperature factors and assessment according to the Megger guide “A Stitch in Time”. The calculator does not replace a test according to the standard.

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:

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:

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, IEEE 43 has specified minimum values at a winding temperature of 40 °C since its 2000 edition: 5 MΩ for windings below 1 kV and 100 MΩ for most form-wound windings built after 1970. The older rule of thumb of nominal voltage in kV plus 1 MΩ now only applies to windings built before 1970; a 400 V motor would reach 1.4 MΩ with it. In practice, healthy low-voltage windings are well above this, 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

1. Disconnect the installation or device and secure it against reconnection.
2. Disconnect sensitive loads: electronics, frequency converters, controllers and dimmers can be damaged by the test voltage or distort the result.
3. Select the test voltage according to the standard, usually 500 V DC (see tables above).
4. 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.
5. 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. For capacitors the value is also multiplied by the capacitance and stated as a time constant in MΩ·µF, with guide values for each dielectric in the article on capacitor insulation.

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:

1. 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.
2. 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.
3. 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.

FAQ on Insulation Resistance

What is the minimum insulation resistance?

In electrical installations up to 500 V nominal voltage at least 1 MΩ at 500 V test voltage (DIN VDE 0100-600), in SELV and PELV circuits at least 0.5 MΩ at 250 V. For portable equipment, DIN VDE 0701-0702 requires at least 1 MΩ for protection class I and 2 MΩ for protection class II.

What is a good insulation resistance?

A good insulation resistance is well above the minimum values, for example in the range of 10 MΩ to 100 MΩ, depending on the quality of the insulating material and the ambient conditions. New installations and windings often reach values in the GΩ range.

What insulation resistance is required according to VDE 0100 part 600?

According to VDE 0100-600, the minimum value for circuits up to and including 500 V nominal voltage is 1 MΩ at a test voltage of 500 V DC. For SELV and PELV, 0.5 MΩ at 250 V applies, for circuits above 500 V, 1 MΩ at 1,000 V test voltage.

Which insulation resistance applies to a motor?

According to IEEE 43, at a winding temperature of 40 °C, at least 5 MΩ applies to windings below 1 kV, including a 400 V motor, and 100 MΩ to most form-wound windings of 1 kV and above built after 1970. The older rule of thumb of nominal voltage in kV plus 1 MΩ only applies to windings built before 1970. Healthy windings are considerably higher in practice.

What are typical insulation resistance values?

Typical insulation resistance values vary by application:

• Household appliances: 2 to 10 MΩ
• Industrial installations: 10 to 100 MΩ or more
• High-voltage systems: above 1 GΩ

Why does insulation resistance decrease?

The most common causes are moisture, conductive contamination, thermal ageing of the insulating material and mechanical damage. A higher temperature during measurement also reduces the value: roughly by half for every 10 K rise in temperature.

Which problems can a low insulation resistance cause?

Insulation resistance that is too low can cause the following problems:

• Short circuits: risk of electrical fires
• Current leakage: increased energy losses and safety risks
• Equipment damage: shortened service life due to flashovers or malfunctions

At which voltage is insulation resistance measured?

The standard is 500 V DC. In SELV and PELV circuits and for equipment with surge arresters, measurement is carried out at 250 V DC, for circuits above 500 V nominal voltage at 1,000 V DC.