Insulation Resistance
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Insulation resitance
Fundamentals
Insulation resistance IR is the electrical resistance that exists between current-carrying conductors and live parts (shielding) on the one hand and earth potential on the other. It is used to assess the ability of the insulation material (cable sheathing, electrical equipment housings) to separate electrical potentials from each other and is determined primarily by the insulating material, but not by its thickness. In good insulators, this value is very high, but can be measured with high-sensitivity high-ohm meters.
The insulation resistance of plastics can change during their service life due to unavoidable ageing processes and cracks, moisture (see: standard atmospheres), contamination, radiation and chemical and/or physical influences.
Since the insulation resistance decreases with increasing measurement voltage, the IR cannot usually be determined at low voltage values with conventional ohmmeters or multimeters, even if these can measure resistances in the gigaohm range. For this reason, and also due to practical requirements, resistance measurements to determine insulation resistance must always be carried out at higher voltages (e.g. 500 V).
Schematic setup for determining insulation resistance
To test the insulation resistance, a test specimen made of the insulation material to be tested is placed between two electrodes [1–4]. These can be two geometrically identical, plane-parallel plates that form a capacitor (Fig. 1).
| Fig. 1: | Schematic representation of the measurement of insulation resistance IR on a plane-parallel plate test specimen |
However, the electrodes can also be applied to the test specimen as films or with conductive silver paint (surface resistance). In both cases, a high-impedance meter or electrometer with high sensitivity (approx. 1020 Ω) must be used (Fig. 2) [2, 3].
| Bild 2: | Plate- or comb-shaped electrodes with connected high-ohm meter |
Information content of the insulation resistance parameter
As can be seen from Fig. 1 and Fig. 2, as well as from the comparison with the determination of the volume resistance and the surface resistance, the insulation resistance parameter is not a material parameter, but depends on the volume resistance and the surface resistance due to the measurement setup. However, insulation resistance is suitable for comparing the behaviour of components under electrical voltage.
The insulation resistance of plastics can only be meaningfully specified if the test specimen geometry, the electrode material and its geometry, the application of the electrodes and other measurement conditions are consistent. It is suitable for testing the electrical properties of functional components and for testing the suitability of electrical products [5].
See also
References
| [1] | ASTM D 257 (2014; reapproved 2021): Standard Test Methods for DC Resistance or Conductance of Insulating Materials |
| [2] | DIN IEC 60167 (1993-12): Methods of Test for Insulating Materials for Electrical Purposes – Insulation Resistance of Solid Materials (IEC 60167:1964) (VDE 0303-31:1993-12) (withdrawn; replaced by DIN EN 62631-3-3 (2016-10)) |
| [3] | DIN EN 62631-3-3 (2016-10): Dielectric and Resistive Properties of Insulating Materials – Part 3-3: Determination of Resistive Properties (DC Methods) – Insulation Resistance (IEC 62631-3-3:2015) (VDE 0307-3-3: 2016-10) |
| [4] | DIN IEC 60512-2 (1994-05): Electromechanical Components for Electronic Equipment – Basic Testing Procedures and Measuring Methods – Part 2: General Examination, Electrical Continuity and Contact Resistance Tests, Insulation Tests and Voltage Stress Tests; (identical with IEC 60512-2:1985) (withdrawn) |
| [5] | Schönhals, A.: Electrical and Dielectrical Properties. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022), 3rd Edition, pp. 330–368 (ISBN 978-1-56990-806-8; e-Book ISBN 978-1-56690-807-5); see AMK-Library under A 22) |