Insulation resistance

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Under insulation resistance is defined as the ohmic resistance component between electrical conductors with one another or against the ground. Since there is no ideal insulator , every insulation also forms an ohmic resistance, the value of which can be very high, but is nevertheless always finite.

Neglecting frequency-dependent factors

Every insulation barrier also has a capacitance which, in the case of alternating voltage, causes an additional current not caused by the insulation resistance. In order to measure the insulation resistance, you must therefore measure with direct voltage . For the level of the leakage current (permissible current in the protective conductor ), on the other hand, all capacities and also interference protection capacitors play a role, which is why leakage current measurements are carried out with the nominal AC voltage. The high-voltage test is also often tested with alternating voltage, u. a. in order to determine the absence of pre-discharge and also because many insulating materials in the high-voltage range are too inhomogeneous to be able to load them with high DC voltages for a longer period of time.

Measurement method

Since the insulation resistance decreases with increasing voltage, it makes no sense to measure it with a common ohmmeter or multimeter at voltages of a few volts. Even if some of these devices are now able to measure resistances in the Giga-Ohm range, they do not provide any reliable information about the dielectric strength of a device or system. The resistance measurement must be carried out with higher voltages.

During the device test, the level of the test voltage is therefore defined in the relevant standards depending on the protection class. The typical value is 250 volts, 500 volts, 1000 volts, 2500 volts or 5000 volts.

For a device of protection class I, measurements are usually carried out with a test voltage of 500 V direct voltage ( VDC ). For devices of protection class II, a test voltage of 1000 V DC generally applies. A precise specification can be found in the relevant standards. A distinction must also be made between the initial test and the repeat test.

In the case of devices of protection class II in particular, it is important to check the insulation of live conductors or parts in relation to operationally voltage-free metal parts. If the insulation is defective, voltage can reach touchable parts, which could lead to injury and possibly even a life-threatening threat. As an alternative to the insulation test with direct voltage, a test with alternating voltage can also be carried out. For a standard-compliant test, however, the voltage forms specified in the standard must be observed. The required test time can also be found in the standards.

When testing capacitive elements or structures, a capacitive charging current can occur at first. In these cases, the test time must be extended by these loading times. In addition, a capacitive charging current should not be interpreted as faulty insulation. To evaluate the insulation resistance, only the ohmic component of the resistance has to be taken into account.

With a 400 VAC network (VAC, English-language abbreviation for alternating voltage ), all live conductors are measured with a direct voltage of 500 V to earth. The outer conductors among each other are measured with a direct voltage of 1000 V.

Change in insulation resistance with increasing system scope

The insulation resistance depends not only on the insulation material but also on the length of the cables. For example, if you look at a two-core cable, the insulation forms a certain resistance value over a certain length unit. The longer the line, the more of these resistances are connected in parallel, so that the total insulation resistance decreases in an inverse proportion with every increase in length. This applies to all components of an electrical system, so it must be assumed that the larger the system, the smaller the insulation resistance.

Change in insulation resistance due to other factors

Insulation resistance can change due to aging processes, moisture, pollution, damage, radiation and chemical or physical influences. In particular, the paper and fabric-insulated cables used in the past were very sensitive to moisture, while z. B. PVC insulated cables have an increased sensitivity to sunlight, heat and chemical atmospheres. In the case of air-insulated systems (e.g. overhead lines and busbars), particular attention must be paid to the sensitivity to dirt.

Danger from insulation resistance values ​​that are too low

Insulation faults can result in uncontrolled fault currents that are sufficiently high to endanger human life, start fires or cause other property damage. In the past, agricultural fires often occurred in which hay or other easily flammable stored goods were ignited by leakage currents - the result of poor insulation values.

Monitoring of the insulation status

In industrial systems that are to be protected from failure, so-called insulation monitors are often used, which enable the insulation resistance to be monitored continuously. If this falls below a certain value, error messages are issued and parts of the system may be switched off. For safety reasons, insulation measurements should also be carried out on other systems .


  • Alfred Hösl, Roland Ayx, Hans Werner Busch: The electrical installation in accordance with regulations, residential construction, commercial industry. 18th edition, Hüthig Verlag, Heidelberg, 2003, ISBN 3-7785-2909-9
  • Hans-Günter Boy, Uwe Dunkhase: Electrical installation technology The master's examination . 12th edition, Vogel Buchverlag, Oldenburg and Würzburg, 2007, ISBN 978-3-8343-3079-6