IEC TR 63434:2023

Low voltage switchgear and controlgear - Partial discharge voltages and PD-level in low voltage switchgear and controlgear

IEC TR 63434:2023 is intended to provide awareness about partial discharge phenomena. This document gives guidance for some conditions when partial discharge can occur in low voltage switchgear and controlgear connected to networks of up to 1 000 V AC. Internal operational voltages can exceed these values. This document gives guidance on the design of conductors and dimensioning of insulation exposed to electrical fields.
This document explains the partial discharge phenomena considering electrical field stress, type of insulation material and other construction parameters, such as the voltage, frequency, temperature, humidity and the distances within the device.

General Information

• Status: Published
• Publication Date: 26-Sep-2023
• ICS: 29.130.20 - Low voltage switchgear and controlgear
• Technical Committee: SC 121A - Low-voltage switchgear and controlgear
• Drafting Committee: WG 1 - TC 121/SC 121A/WG 1
• Current Stage: PPUB - Publication issued
• Start Date: 27-Sep-2023
• Completion Date: 18-Jul-2023

Overview

IEC TR 63434:2023 - Low voltage switchgear and controlgear - provides awareness and guidance on partial discharge (PD) phenomena in low-voltage switchgear and controlgear connected to networks up to 1 000 V AC (internal operational voltages may be higher). It explains how electrical field stress, insulation type, construction geometry and environmental conditions (voltage, frequency, temperature, humidity, clearances and voids) influence PD inception, PD-level and the risk of insulation failure. This Technical Report focuses on design guidance and interpretation, not on prescriptive test procedures.

Key topics

• Partial discharge physics: homogeneous vs. inhomogeneous electric fields, PD inception/extinction behavior and the influence of electrode geometry.
• Field stress and insulation: how field distribution through air gaps, voids and solid insulation affects PD risk and insulation dimensioning.
• Design and dimensioning examples: practical examples and models for clearances, series combinations of air gaps and solid insulation for AC (and discussion for DC).
• Material and environmental effects: temperature and humidity impacts on dielectric strength, and how these change PD inception voltages and operational margin.
• Application rules and mitigation: recommended measures to reduce PD probability through design decisions (conductor layout, distances, insulation thickness) and construction practices.
• Informative annexes: summaries of research on PD in low-voltage devices and voltage factors relevant to PD coordination.

Practical applications

Who uses IEC TR 63434:2023 and how it helps:

• Switchgear and controlgear designers: optimize conductor layouts, clearances and insulation thickness to reduce PD risk and extend service life.
• Insulation/material engineers: understand temperature/humidity influences on dielectric strength for realistic dimensioning.
• Test and certification engineers: interpret PD test results and understand limitations of PD testing at low field strengths (document notes PD testing is unreliable below about 4 kV/mm and references IEC 60270 for PD test procedures).
• Manufacturers and asset managers: apply guidance to reduce premature insulation ageing, improve reliability and set internal PD acceptance practices.
• Standards developers and researchers: reference consolidated background, examples and research summaries in Annex A/B.

How it is used:

• Design reviews, failure-mode assessments, risk mitigation for PD-related insulation aging, and harmonizing PD coordination for products rated up to 1 000 V AC.

Related standards

• IEC 60664-1 (insulation coordination - notes on PD testing thresholds and field strength guidance)
• IEC 60270 (partial discharge measurement techniques - test procedures)
• IEC Electropedia / IEC vocabularies for terms and definitions

Keywords: IEC TR 63434:2023, partial discharge, PD-level, low voltage switchgear, controlgear, insulation dimensioning, electrical field stress, PD inception, PD testing.

Standards Content (Sample)

IEC TR 63434 ®
Edition 1.0 2023-09
TECHNICAL
REPORT
Low voltage switchgear and controlgear – Partial discharge voltages and PD-
level in low voltage switchgear and controlgear

IEC TR TR 63434:2023-09(en)
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IEC TR 63434 ®
Edition 1.0 2023-09
TECHNICAL
REPORT
Low voltage switchgear and controlgear – Partial discharge voltages and PD-

level in low voltage switchgear and controlgear

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.130.20  ISBN 978-2-8322-7573-3

– 2 – IEC TR 63434:2023 © IEC 2023
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Basic information (physics) . 10
4.1 Discharge phenomena . 10
4.1.1 General . 10
4.1.2 Homogeneous electric fields . 10
4.1.3 Inhomogeneous electric fields. 11
4.2 Dimensioning examples . 14
4.2.1 General . 14
4.2.2 Influence of design and temperature on a series connection of
clearances and solid insulation for AC voltage . 17
4.2.3 Series connection of clearances and solid insulation by design for DC
voltage . 24
4.2.4 Solid insulation – dimensioning – material characteristics . 24
5 Application rules . 29
5.1 General . 29
5.2 Partial discharge considerations . 30
5.3 Measures to prevent/reduce the probability of partial discharges . 30
Annex A (informative) Research on partial discharge in low-voltage switchgear and
controlgear . 31
A.1 General . 31
A.2 Investigations on switchgear . 31
A.3 Reference to other products relevant for applications . 35
Annex B (informative) Voltage factors when considering partial discharge effects . 36
Bibliography . 38

Figure 1 – Temperature dependent variation of the breakdown field strength Ê of air
d
per Equation (1), α = 0,8, θ = 20 °C, p = p = 1 013 mbar . 11
Figure 2 – Inception voltage U depending on the electrode radius r, R ≫ r . 12
i,RMS
Figure 3 – Maximum electrical field strength Ê depending on the electrode edge radius r . 13
Figure 4 – Paschen curve Ê = f (p × d) for air . 13
d
Figure 5 – Principle terminal / contact arrangement of a 3-pole device, capacitive
voltage divider . 15
Figure 6 – Field strength in the air gap, inhomogeneous, η = 0,5 , ε = 4, cold state . 19
r2
Figure 7 – Field strength in the solid insulation, inhomogeneous, η = 0,5, ε = 4, cold
r2
state . 19
Figure 8 – Field strength in the air gap, inhomogeneous, ƞ = 0,5, ε = 12 at 130 °C
r2
operational temperature . 20
Figure 9 – Field strength in the solid insulation, inhomogeneous, ƞ = 0,5, ε = 12
r2
operational temperature . 20
Figure 10 – Gaps and voids in a solid and combined solid / gaseous insulation [7] . 21

Figure 11 – Model of a void of thickness t in an insulation wall of defined thickness d [20] . 23
Figure 12 – Principle arrangement of electrodes and insulation walls of a 3-pole device . 24
Figure 13 – Permissible field strength for dimensioning of solid insulation according to
Equation (18) . 25
Figure 14 – Breakdown at high frequency, solid insulation; d = 0,75 mm [23] . 26
Figure 15 – Breakdown at high frequency, solid insulation, influence of humidity;
conditioning at 50 °C; 1: mica-filled phenolic, d = 0,75 mm; 2: glass-silicone laminate, d
= 1,5 mm [24] . 27
Figure 16 – Dielectric strength Ê of different types of thermoplastic insulation material
d
depending on the temperature . 28
Figure 17 – Dielectric strength E of PA6-GF30 in dry and moist condition
d,RMS
(equilibrium moisture content at 23 °C/50 % RH) depending on the temperature θ (°C) . 29
Figure A.1 – Example of phase resolved partial discharge measurement on a MPSD at
room temperature and at elevated operational temperatures . 31
Figure A.2 – PD Testing (690 V, basic insulation, 20°C). 32
Figure A.3 – Inception (U ) and extinction (U ) voltage during partial discharge
i e
measurements on motor protection switching devices (MPSD) at elevated
temperatures . 33

Table 1 – Relationship between electrode radius r and corona inception voltage
U . 12
i,RMS
Table 2 – Ranking of the internal field strength of different gap and void shapes [7] . 22
Table A.1 – Inception U and extinction U voltage depending on the temperature as
i e
per Figure A.1 and Figure A.2 . 32
Table A.2 – Maximum discharge values and number of events observed at the test
voltage as per Figure A.1 and Figure A.2 . 32
Table A.3 – Ratings and design parameters of the investigated motor protection
switching devices (MPSD). 34
Table A.4 – Partial Discharge (PD) acceptance levels in different IEC documents. 35
Table B.1 – Coordination of rated RMS voltage with partial discharge voltage and
extinction voltage . 37

– 4 – IEC TR 63434:2023 © IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
LOW VOLTAGE SWITCHGEAR AND CONTROLGEAR –

PARTIAL DISCHARGE VOLTAGES AND PD-LEVEL IN
LOW VOLTAGE SWITCHGEAR AND CONTROLGEAR

FOREWORD
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IEC TR 63434 has been prepared by subcommittee 121A: Low voltage switchgear and
controlgear, of IEC technical committee 121: Switchgear and controlgear and their assemblies
for low-voltage. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
121A/549/DTR 121A/556A/RVDTR
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Report is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
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The committee has decided that the contents of this document will remain unchanged until the
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specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – IEC TR 63434:2023 © IEC 2023
INTRODUCTION
The application of this document is intended to provide awareness about partial discharge
phenomena. Special emphasis is given to the electrical field stress through solid insulation as
it relates to the risk of insulation failure.
IEC 60664-1[1] is only providing requirements for partial discharge testing of solid insulation
when the peak value of the operational voltage exceeds 700 V and the average field strength
is higher than 1 kV/mm. However, in practice, partial discharge testing gives random results
below 4 kV/mm, mainly because of the intrinsic fluctuation of PD inception voltage in gaps, the
variations of the shape and size of the internal voids in solid materials, and the large influence
of the temperature and humidity on the material characteristics. Therefore, this document is
providing guidance related to the proper design of the insulation and the selection criteria of
the material.
___________
Numbers in square brackets refer to the Bibliography.

LOW VOLTAGE SWITCHGEAR AND CONTROLGEAR –

PARTIAL DISCHARGE VOLTAGES AND PD-LEVEL IN
LOW VOLTAGE SWITCHGEAR AND CONTROLGEAR

1 Scope
This document is intended to provide awareness about partial discharge phenomena. This
document gives guidance for some conditions when partial discharge can occur in low voltage
switchgear and controlgear connected to networks of up to 1 000 V AC. Internal operational
voltages can exceed these values. This document gives guidance on the design of conductors
and dimensioning of insulation exposed to electrical fields.
This document explains the partial discharge phenomena considering electrical field stress,
type of insulation material and other construction parameters, such as the voltage, frequency,
temperature, humidity and the distances within the device.
This document does not cover:
• phenomena associated with semiconductor power switching by effects on equipment placed
downstream of semiconductor power switching systems;
• partial discharge test procedures (see IEC 60270) [2];
• pure DC systems which are under consideration;
• selection of solid insulation material.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
homogeneous electric field
electric field which has an essentially constant voltage gradient between electrodes, such as
that between two spheres where the radius of each sphere is greater than the distance between
them
Note 1 to entry: The homogeneous field condition is referred to as case B in IEC 60664-1[1].
[SOURCE: IEC 60050-442:2014[3], 442-09-02]
3.2
inhomogeneous electric field
electric field which does not have an essentially constant voltage gradient between electrodes

– 8 – IEC TR 63434:2023 © IEC 2023
Note 1 to entry: The inhomogeneous field condition of a point-plane electrode configuration is the worst case with
regard to voltage withstand capability and is referred to as case A in IEC 60664-1[1]. It is represented by a point
electrode having a 30 μm radius and a plane of 1 m × 1 m.
Note 2 to entry: For frequencies exceeding 30 kHz, the field is considered to be inhomogeneous when the radius
of curvature of the conductive parts is less than 20 % of the clearance.
[SOURCE: IEC 60050-442:2014[3], 442-09-03]
3.3
partial discharge
PD
electric discharge that only partially bridges the insulation between conductors
Note 1 to entry: A partial discharge may occur inside the insulation or adjacent to a conductor.
Note 2 to entry: Scintillations of low energy on the surface of insulating materials are often described as partial
discharges but should rather be considered as disruptive discharges of low energy, since they are the result of local
dielectric breakdowns of high ionization density, or small arcs, according to the conventions of physics.
[SOURCE: IEC 60050-212:2010[3], 212-11-39, modified ─ Addition of the synonym “PD”.]
3.4
partial discharge intensity
amount of partial discharge occurring under given conditions
Note 1 to entry: In practice the partial discharge intensity is usually expressed in picocoulombs or in joules.
[SOURCE: IEC 60050-212:2010[3], 212-11-40]
3.5
partial discharge inception voltage
U
i
lowest peak value of the test voltage at which the apparent charge becomes greater than the
specified discharge magnitude when the test voltage is increased above a low value for which
no discharge occurs
Note 1 to entry: For AC tests the RMS value may be used.
[SOURCE: IEC 60050-212:2014[3], 212-11-41]
3.6
partial discharge extinction voltage
U
e
highest voltage at which partial discharges are extinguished when the voltage applied is
gradually decreased from a higher value at which such discharges are observed
[SOURCE: IEC 60050-212:2010[3], 212-11-42, modified – added abbreviation U ]
e
3.7
impulse withstand voltage
U
imp
highest peak impulse voltage, of prescribed form and polarity, which does not cause breakdown
under specified conditions of test
3.8
internal partial discharge
partial discharge inside an insulating material
[SOURCE: IEC 60050-212:2010[3], 212-11-43]

3.9
working voltage
highest RMS value of the AC or DC voltage across any particular insulation which can occur
when the equipment is supplied at rated voltage
Note 1 to entry: Transient overvoltages are disregarded.
Note 2 to entry: Both open-circuit conditions and normal operating conditions are taken into account.
[SOURCE: IEC 60947-1:2020[4], 3.7.52]
3.10
recurring peak voltage
U
rp
maximum peak value of periodic excursions of the voltage waveform resulting from distortions
of an AC voltage or from AC components superimposed on a DC voltage
Note 1 to entry: Random overvoltages, for example due to occasional switching, are not considered to be recurring
peak voltages.
[SOURCE: IEC 60050-442:2014[3], 442-09-15]
3.11
temporary overvoltage
overvoltage at power frequency of relatively long duration
Note 1 to entry: A temporary overvoltage is undamped or weakly damped. In some cases, its frequency may be
several times smaller or greater than power frequency.
[SOURCE: IEC 60947-1:2020[4], 3.7.53]
3.12
transient overvoltage
short duration overvoltage of a few milliseconds or less, oscillatory or non-oscillatory, usually
highly damped
[SOURCE: IEC 60050-614:2016[3], 614-03-14, modified – “overvoltage with a duration” has
been replaced with “short duration overvoltage” and the notes have been deleted.]
3.13
glow discharge
self-maintained gas conduction for which most of the charge carriers are electrons supplied by
secondary-electron emission
[SOURCE: IEC 60050-121:1998[3], 121-13-13]
3.14
electric breakdown
abrupt change of all or part of an insulating medium into a conducting medium resulting in an
electric discharge
[SOURCE: IEC 60050-614:2016[3], 614-03-15]
3.15
electric strength
quotient of the maximum voltage applied without breakdown, by the distance between
conducting parts under prescribed test conditions
[SOURCE: IEC 60050-212:2010[3], 212-11-37]

– 10 – IEC TR 63434:2023 © IEC 2023
3.16
glass transition
physical change in an amorphous material or in amorphous regions of a partially crystalline
material from a viscous or rubbery condition to a hard one, or the reverse
[SOURCE: IEC 60050-212:2010[3], 212-12-28]
3.17
glass transition temperature
T
g
midpoint of a thermodynamic temperature range over which the glass transition takes place
[SOURCE: IEC 60050-212:2010[3], 212-12-29]
4 Basic information (physics)
4.1 Discharge phenomena
4.1.1 General
Partial discharges are well known in the field of high voltage applications and typical for
inhomogeneous electrical field distributions.
Partial discharges in electromechanical switchgear can appear in different ways: through air
directly (external PD), on surfaces of supporting parts (surface PD) and within voids of solid or
liquid insulation components like phase separation walls (internal PD). Surface discharges in
air have generally larger values than those inside of cavities of solid insulation [5].
NOTE In [6] a simulation is shown illustrating the potential locations of PD generation in a Motor Protection
Switching Device (MPSD).
4.1.2 Homogeneous electric fields
The breakdown field strength or dielectric strength of an air gap of 1 mm distance with a
homogeneous electric field – e.g. between parallel plane electrodes – is about 4 kV/mm (see
Figure 4). In technical applications the size of the air gaps between electrodes and insulation
material can be 1 mm or less.
NOTE For breakdown processes in homogeneous electric fields in air in general the Paschen-Law [7], [9] is valid.
The breakdown field strength Ê is depending on the temperature T. The temperature
d
correction is done by using equation (1) with T = 293 K and the worst case temperature
coefficient α = 0,8.[[7], p. 169], see Figure 1.
a


pp/
(1)
E = E

d do
TT/
0
Figure 1 – Temperature dependent variation of the breakdown field strength
of air per Equation (1), α = 0,8, θ = 20 °C, p = p = 1 013 mbar
Ê
d 0
For 130 °C this results in a dielectric strength of a 1 mm air gap with homogeneous field
distribution Ê of 3,1 kV/mm (77,5 % of the 20 °C-value). This is the crest value of the field
d
strength and related to the peak value of the applied voltage. The effective value of a
corresponding sinusoidal voltage is reduced by the factor 1/√2.
4.1.3 Inhomogeneous electric fields
The PD inception voltage is related to inhomogeneous electrode distribution – like point to plane
electrodes – with a degree of homogeneity or utilization factor η lower as the limit of η ≈ 0,2 [9].
This value gives the quotient of average field strength to maximum field strength for an
electrode arrangement [5], [8]. Stable glow discharges in air can principally appear at field
strength values of 2,5 kV/mm (see [9] and IEC 60664-1:2020, Figure A.1[1]). With a low
utilization factor of 0,2 the PD inception peak voltage for an inhomogeneous electrode
arrangement – like a point or edge electrode against a plane electrode – with an air gap of
1 mm is only about 800 V or 566 V according to the tables in [7].
peak RMS.
NOTE 1 Because, in practice, partial discharge testing gives random results below 4 kV/mm, the threshold of
2,5 kV/mm is considered only for initial analysis purposes.
The utilization factor η is dependent on the electrode distance R and the electrode radius r [5],
[8]. A reference model in [8] shows the relation between radius r and inception voltage U ,
i
expressed in RMS, based on real dimensions (see Table 1 and [8]).

– 12 – IEC TR 63434:2023 © IEC 2023
Table 1 – Relationship between electrode radius r and corona inception voltage U
i,RMS
R ≫ r r U
i,RMS
μm
V
10 450
50 1 000
100 1 400
500 3 500
1 000 5 500
5 000 15 000
Key
V – potential
r – electrode radius
R – distance to earth plane
Figure 2 – Inception voltage U depending on the electrode radius r, R ≫ r
i,RMS
The graphical depiction of Table 1 up to a radius of 0,5 mm shows clearly that the inception
voltage decreases more rapidly below r = 100 μm corresponding to an inception voltage of only
450 V with a radius of 10 μm.
The evaluation of FEM-simulations on a particular contact arrangement in Figure 3 also shows
the progressive rise of the field strength once the edge radius is lower than 0,1 mm [10] and
confirm the dependencies in Table 1 and Figure 2.

Figure 3 – Maximum electrical field strength Ê depending on the electrode edge radius r
In addition to the influence of the radius, the breakdown field strength in air (Paschen-curve
Figure 4) needs to be considered.

Figure 4 – Paschen curve Ê = f (p × d) for air
d
For practical applications, in addition to the inception voltage, the extinction voltage is of
relevance. Only if the permanent operational voltage remains below that value, a discharge
initiated e.g. by a disturbance can extinguish. Persisting PD would not only destroy adjacent
materials thermally or by UV radiation but also chemically for example through forming of nitric
acid depending on the materials involved.
Field inhomogeneity also occurs when an electrode is mounted on a solid insulation or when a
layer of solid insulation material is positioned in an air gap between electrodes. For the first
situation the inception voltage in the wedge between electrode and support is roughly inverse
proportional to the relative permittivity ε [7].
r
α

1
(2)
U ∝

i
ε
r
– 14 – IEC TR 63434:2023 © IEC 2023
With typical measured values α = 0,45.0,5.
The degree of relative permittivity variation over temperature is highly dependent on the glass
transition temperature T and moisture uptake of the insulating material. For polyamides, with
g
glass transition temperature close to room temperature in humid conditions the variation can
be as high as factor 5 between 20 °C to 130 °C [6], [11]-[15], [16], [17], [18]. For other polymers
with higher glass transition temperature or lower hygroscopic behaviour the variation can be
comparatively low (e.g. by a factor 2).
Attention is drawn to the dependency of the relative permittivity in regard to temperature,
frequency as well as the state of the material (dry or conditioned). Such data are often not
available. IEC 60664-1[1] states that the insulation behaviour of the solid insulation is directly
affected by its intrinsic material characteristics. Electrical, mechanical and other stresses which
might affect the insulation behaviour over the lifetime of the product needs to be considered.
IEC 60664-1:2020[1] considers discharge phenomena at room temperature only. The following
example shows how a calculation, considering a defined elevated operational temperature,
reduces the electrical strength.
With the dependency as per Equation (2) and a change of the relative permittivity from ε = 3,4
r20
(relative permittivity at 20 °C) to ε = 11 (relative permittivity at 130 °C), also the relation of
r130
the inception voltages at 130 °C and 20 °C in an air gap (wedge between electrode and support)
would change to

0,5
U ε
i130 r20
∝=0,56 (3)


ε
U
i20 r130
Both effects (air temperature see Equation (1) and change of the relative permittivity) would
lead to a relation of

U
i130
 ∝×0,56 0,775 =0,43
(4)
U
i20
This corresponds to dielectric strength in a homogeneous field of a 1 mm air gap of
Ê = 1,72 kV/mm (43 % of the 20 °C-value). For a specific electrode arrangement with
dmin
η = 0,5 [8],[12]), the dielectric strength is further
inhomogeneous field (utilization factor of
reduced to a peak value of 860 V/mm, corresponding to an RMS value of about 600 V. This
means that a device according to this example operated at 690 V would exceed the dielectric
strength of the contact arrangement and partial discharges would occur at the assumed
operational temperature, whereas at room temperature they would not occur.
NOTE 2 The utilization factor can vary due to the design and manufacturing process.
This example shows that the operational temperature can have a significant influence on the
dielectric characteristics of the devices and therefore needs to be considered.
4.2 Dimensioning examples
4.2.1 General
The following examples deal with the series connection of clearances and solid insulation as
designed within the equipment. The dimensioning examples in IEC TR 60664-2-1[19] describe

just one gap and one insulation layer. The example, including the Equations (5), (6) and (7), is
valid for an ideal solid insulation layer (with no air voids). But a typical dimensioning task occurs
at the terminals of multipole devices. In compliance with the example given in IEC TR 60664-
2-1 [19] and [13], the following example is derived for a 3-pole device.
The intention of this example is to point out how the electric stress in the air gaps of an insulation
construction is influenced and how it changes under the influence of operational temperature.
In the example the full distance between the electrodes has been determined by the design.
The objective is to optimize the distribution of this full distance into the thickness of the solid
insulation material and air gaps to avoid partial discharge.
For AC voltage, the impedances of the series connected insulators are determined by their
capacitances. Usually, for rather low frequencies as considered in IEC 60664-1[1], the dielectric
losses are not considered for the calculation of the voltage distribution. However, the dielectric
permittivity εr of the solid insulator has a decisive influence on the voltage distribution especially
with regard to the values changing with temperature and/or humidity condition (see also
Figure 17).
For an easy calculation and consideration of the capacitive voltage distribution, those
capacitances are considered as plate-to-plate capacitors with a homogeneous field distribution
first. This situation is described in Figure 5.

Key
1,2,3 phase poles
U applied AC voltage
C , C capacitance of the clearance
1 3
U , U voltage across the clearance
1 3
C capacitance of the solid insulation
U voltage across the solid insulation
d , d length of air gap
1 3
d length of solid insulation wall
ε permittivity of the solid insulation
r2
Figure 5 – Principle terminal / contact arrangement of a 3-pole device,
capacitive voltage divider
– 16 – IEC TR 63434:2023 © IEC 2023
In order to prevent any deterioration of the solid insulation, the solid insulation is to be designed
to withstand the entire operational voltage and the air gaps are to be designed to prevent
inception of partial discharge under operational conditions.
For the following considerations and examples, it is defined that the overall distance between
the phase poles (d + d + d = 1,5 mm) is kept constant (design rules). This means that, as the
1 2 3
thickness of the solid insulation decreases, the distance of the air gap gets bigger or vice versa.
Between 2 poles of the 3-pole configuration C , C and C form a capacitive voltage divider
1 2 3
according to Figure 5 and the applied AC voltage U is divided according to Equations (5), (6)
and (7), and in the voltages U , U and U . In symmetrical designs U is equal to U .
1 2 3 1 3
 
d /ε
1 r1
UU× (5)
 
d //ε ++dε dε/
 1 r1 2 r2 3 r3 
 
d /ε
2 r2
UU×
  (6)
d //ε ++dε dε/
 1 r1 2 r2 3 r3 
 
d /ε
3 r3
UU× (7)
 
d //ε ++dε dε/
 1 r1 2 r2 3 r3 
For symmetrical air gaps the capacitances C and C are given by Equations (8) and (9):
1,3 2
A
C ε×
(8)
1,3 0
d
1,3
A
C =εε××
(9)
2 0 r
d
where
A is the area of the plate-to-plate capacitors C and C ;
1,3 2
ε is the dielectric constant of the air;
ε is the relative permittivity of the solid insulation.
r
For the voltage division, the capacitance ratio as given in Equation (10) is relevant.
d 1
CC= × ×
(10)
d ε
1r
The breakdown field strength of the clearance (Ê ) can be calculated using the AC breakdown
voltage and the corresponding clearance. For simplification, the examples given in this
application guide are based on homogeneous field conditions first. The breakdown field strength
=
=
=
=
of the solid insulation (Ê ) is specified by the manufacturer of the material ideally with long term
temperature dependency.
A precise calculation or simulation of the voltage distribution is much more complicated to make
and the above equations can only be considered as an approximation, assuming homogeneous
field distribution. For small distances up to around 0,1 mm, this approximation is rather precise.
For rather large distances, it is not directly appropriate but can be improved using a utilization
factor η evaluated as per the method of Schwaiger [7], [12]. The factor depends on the overall
η
shape but mainly on the two design parameters electrode distance d and radius r. A value of
= 0,5 or even 0,2 is an appropriate assumption in many cases [8], [13].
4.2.2 Influence of design and temperature on a series connection of clearances and
solid insulation for AC voltage
4.2.2.1 General
For AC voltage, the voltage distribution is calculated according to the relevant capacitances.
The following examples are based on Figure 5, which shows a simplified common design with
a clearance and a layer of solid insulation in series. The dimensions chosen for the examples
are derived from typical circuit-breakers or contactors in the range of up to 20 kW at 400 V.
4.2.2.2 Example showing the distribution of the electrical field at room temperature
This example shows that in an insulation configuration of a solid wall with two adjacent gaseous
(e.g. air) gaps between two phase poles, a considerable electrical field stress can occur in the
air gaps. Depending on the utilization factor η this can initiate partial discharges even at room
temperature. Considering the following list of parameters
d = 0,2 mm,
1,3
d = 1,1 mm
ε = 4 (at room temperature)
r2
Application of Equation (10) results in: C = 1,375 C
1 2
Application of Equations (5) and (7) results in: U = 0,297 U
1,3 0
Application of Equation (6) results in: U = 0,408 U
2 0
the result is that about 30 % of the full voltage drops across each clearance. Assuming that
U = 690 V and the utilization factor η = 0,5, the peak electrical field strength in the air gaps
0 RMS
is approximately 2,9 kV/mm, which exceeds the threshold for glow discharges. In case of
flashover of the clearances, however, the full voltage is applied to the solid insulation.
4.2.2.3 Example showing the distribution of the electrical field at a high operational
temperature
This example shows how a considerable change in operational temperature (from 20 °C to
130 °C) increases the electrical stress on the gaseous (air) gaps in the vicinity of a solid
insulation wall.
Since the relative permittivity can be strongly temperature dependent the results using a
ε = 15 would change to
r2
Application of Equation (10) results in: C = 0,367 C
1 2
Application of Equations (5) and (7) results in: U = 0,424 U
1,3 0
Application of Equation (6) results in: U = 0,152 U
2 0
– 18 – IEC TR 63434:2023 © IEC 2023
In this case about 40 % of the whole voltage is applied across each clearance. Assuming the
same conditions as for Example 1 (see 4.2.2.2) the electrical field strength in the air gaps then
is even approximately 4,1 kV/mm which is close to the breakdown condition. In case of flashover
of the clearances, however, the full voltage is applied to the solid insulation which has also a
reduced electrical breakdown strength caused by the rise of the relative permittivity.
Temperature dependent values of dielectric strength are not generally available from the
manufacturers or from literature. Same is valid for long term degradation. Usually designers
refer to RTI (Relative Temperature Index) ratings as suitability criteria to select materials for
the intended operational temperatures. These values do include some uncertainties related to
the evaluation methods and they also imply that the relevant property has declined to 50 % of
the initial value. For higher temperatures, this condition is reached in shorter time. As per the
Arrhenius law a change to 10 K higher temperature results in 50 % of their useful lifetime.
4.2.2.4 Description for the distribution of the electrical field for different dimensions
and temperatures
4.2.2.4.1 General
A more general description of the influence of the design parameters air gap and solid insulation
dimension is given in the Figure 6 to Figure 9 and are expressed in field strength Ê [kV/mm] for
the different cases of inhomogeneous field with utilization factor η = 0,5 in cold state (see
Figure 6 and Figure 7) and warm state (and Figure 9). The total distance between two phase
poles is kept constant. As reference lines the limit above which stable glow discharges can
occur in the air gap (2,5 kV/mm) [9] or the field strength value for solid insulation as given in
IEC 60664-1[1] (1 kV/mm) are marked as dashed line.
Field strength in “cold state” Figure 6 shows the field strength Ê in the air gaps depending
1,3
on the size d of the air gap. Furthermore, the characteristics are shown for different
1,3
conventional operational voltages from 400 V up to 1 000 V.
The higher the operational voltage is, the larger the clearance and creepage distances (air gap)
need to be dimensioned, as a trend, to withstand the power frequency voltage to avoid PD
occurrence.
The example illustrated in Figure 6 to Figure 9 with d = 0,1 mm and d = 1,3 mm appears to
1,3 2
be at or above the limits of the dimensioning criteria of IEC 60664-1[1] when operated with
690 V or higher.
Figure 6 – Field strength in the air gap, inhomogeneous, η = 0,5 , ε = 4, cold state
r2
Figure 7 shows the field strength Ê in the solid insulation. The marked 1 kV/mm criterion as
given in IEC 60664-1[1] for solid insulation appears to be uncritical for operational voltages up
to 500 V. Above that value more detailed examinations are required.

Figure 7 – Field strength in the solid insulation, inhomogeneous,
η = 0,5, ε = 4, cold state
r2
The example illustrated in Figure 6 and Figure 7 with d = 0,1 mm and d = 1,3 mm appears
1,3 2
to be at or above the limits of the dimensioning criteria of IEC 60664-1[1] already at room
temperature when operated with 690 V or higher.

– 20 – IEC TR 63434:2023 © IEC 2023
4.2.2.4.2 Field strength at elevated operational temperature
Figure 8 shows the field strength Ê in the air gaps depending on the size d of the air gaps
1,3 1,3
at operational temperature (e.g. 130 °C).
At all voltages of 400 V and higher the limit of 2,5 kV/mm for glow discharges is exceeded for
d < 0,3 mm. This would raise the probability of the occurrence of partial discharges in the air
gaps.
Figure 8 – Field strength in the air gap, inhomogeneous, ƞ = 0,5, ε = 12
r2
at 130 °C operational temperature

Figure 9 – Field strength in the solid insulation, inhomogeneous,
ƞ = 0,5, ε = 12 operational temperature
r2
Figure 9 shows that as higher the operational voltage is, the larger the solid insulation is
dimensioned, as a trend.
Figure 9 shows the field strength Ê in the solid insulation at operational temperature. An
electrical field strength of 1 kV/mm peak can be reached for thicknesses below 1,25 mm for
400 V or below 1,1 mm up to 500 V. For higher voltages more detailed exam
...