Electrical Strength

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Electrical strength, Electrical breakdown strength or Electrical breakdown resistance

General principles and definition

Electrical strength is the electric field strength at which the conductivity of an insulator increases by several orders of magnitude.

A polymeric insulation material cannot withstand the influence of a high voltage indefinitely. At a certain voltage or after a certain time after the application of a high voltage, it irreversibly loses its insulating properties. This is associated with structural damage to the plastic. If the voltage applied to a test specimen is continuously increased, the current flow increases proportionally at first, then non-linearly, and finally increases extremely sharply when a certain voltage is reached.

This voltage is called the breakdown voltage U_d. The conductivity increases by many orders of magnitude, and the plastic loses its insulating properties under spark and light arc formation. This behaviour is called electrical breakdown. An analogous phenomenon is observed when a sufficiently high voltage is applied to the test specimen for a sufficiently long time [1, 2].

Fundamentals of electrical breakdown

The mechanism of electrical breakdown in solid dielectrics is still poorly understood theoretically. Generally, two stages of electrical breakdown are distinguished. The breakdown-prepared stage, in which the plastic loses its electrical strength, and the stage in which the plastic is destroyed and breakdown is completed [3, 4]. Furthermore, depending on the mechanism, three basic forms of electrical breakdown can be distinguished:

the purely electrical breakdown (field breakdown, internal breakdown),
thermal breakdown and
long-term breakdown (partial charge-induced breakdown, electrical ageing).

Determination of the electrical strength

For testing, the insulation material is clamped between two electrodes to which an electrical voltage is applied. During the test, the applied voltage is continuously increased until the electrical strength is reached [5, 6]. During this process, the test specimen heats up only slightly between the electrodes due to the short test time. The rapidly increasing electric field strength changes the structure of the insulator shortly before the breakdown by forming a more or less branched breakdown channel system, resulting in irreversible damage to the material, which enables the breakdown.

Electrical strength is a material-specific property. In addition to the chemical structure of the plastic, it is strongly dependent on the thickness of the test specimen, the type of voltage stress and the ambient conditions.

Electrical strength as a material parameter

The material parameter that characterises the behaviour of a polymeric insulating material with regard to electrical voltage stresses is the electrical breakdown strength E_d

E_{d}\,=\,{\frac {U_{d}}{d}}

,

where d is the smallest distance between the electrodes and is also referred to as the impact distance. The electrical strength is not a material constant. In addition to the chemical structure of the plastic, it is strongly dependent on the thickness of the test specimen, the way in which the stress is applied and the ambient conditions such as temperature and humidity. It represents the electrical field strength at which the conductivity of an insulating material increases by several orders of magnitude.

Characteristic values of electrical strength for plastics

As an example, short-time values of the electrical strength for selected plastics are compiled in the table.

**Table 1**: Short-time electrical strength values for selected plastics [2, 7, 8] at T = 23 °C
Plastic film	Thickness d (μm)	E_d (kV mm^-1)	References
polypropylene (PP)	40	200	[2]
polyester	40	160	[2]
poly (vinyl chloride) (PVC)	40	150	[2]
cellulose – aceto butyrate	40	130	[2]
cellulose triacetate	40	120	[2]
PE	40	110	[2]
PE	1,000	40	[7]
PET semi-cristalline		30	[7]
PET amorphous		250	[7]
PA 6	1000	15	[8]
PBT	1600	13	[8]
PVDF	1000	40	[8]
PEEK	1600	25	[8]
PS	1600	43	[8]
ABS	1000	41	[8]
SAN	1600	32	[8]
PSU	1000	30	[8]
PES	1000	35	[8]
PEI	800	33	[8]
PVC-U	600	40	[8]

Equipment system for measuring characteristic values

The electrical parameters electrical breakdown strength and contact resistance are determined with an electric strength tester or a high-impedance resistance measuring device with connected measuring capacitor (Fig. 1).

Fig. 1:

High-voltage tester RMG 15 AC with electrode arrangement and test cage from Cooper Bussmann, Wuppertal

[1]	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: 978-1-56990-807-5; ePub ISBN: 978-1-56690-808-2; see AMK-Library under A 22)
[2]	Carlowitz, B.: Tabellarische Übersicht über die Prüfung von Kunststoffen. Giesel Verlag für Publizität, Isernhagen (1992) (ISBN 978-3-980-29420-1; see AMK-Library under C 9)
[3]	Ku, C. C., Liepins, R.: Electrical Properties of Polymers. Carl Hanser Munich Vienna (1987) (ISBN 978-0-521-55219-6)
[4]	Whitehead, S.: Dielectric Breakdown of Solids. Clarendon Press, Oxford (1953)
[5]	DIN EN 60243-1 (2014-01): Electric Strength of Isulating Materials – Test Methods – Part 1: Test at Power Frequencies (IEC 60243-1:2013)
[6]	DIN EN 60243-2 (2014-08): Electric Strength of Isulating Materials – Test Methods – Part 2: Additional Requirements for Tests Using Direct Voltage (identical with VDE 0303-22:2014-08)
[7]	Bonten, C.: Kunststofftechnik: Einführung und Grundlagen. Carl Hanser Munich Vienna, 2. updated Edition (2016) (ISBN 978-3-446-44674-8) (see AMK-Library under H 52)

Weblink

[8]	https://www.kern.de/de/richtwerttabelle (access on 20.07.2022)