General

JERG-2-213A

DESIGN STANDARD
INSULATION

May 10, 2012

Revision A

Japan Aerospace Exploration Agency

JERG-2-213A

This is an English translation of JERG-2-213A. Whenever there is anything ambiguous inthis document, the original document (the Japanese version) shall be used to clarify the intent
of the requirement.

Disclaimer
The information contained herein is for general informational purposes only. JAXA makes
no warranty, express or implied, including as to the accuracy, usefulness or timeliness of any
information herein.
JAXA will not be liable for any losses relating to the use of theinformation.

Published by
Japan Aerospace Exploration Agency
Safety and Mission Assurance Department
2-1-1 Sengen Tsukuba-shi,Ibaraki 305-8505, Japani

JERG-2-213A

Contents
1 Scope of application ................................................................................................ 1
1.1 Purpose............................................................................................................. 1
1.2 Scope of application .......................................................................................... 1
2 References .............................................................................................................. 1
3 Terms ....................................................................................................................... 1
4 General requirements .............................................................................................. 2
5 Design requirements and design methods .............................................................. 3
5.1 Insulation design ............................................................................................... 3
5.1.1 Insulation design using dielectric gas ............................................................. 3
5.1.2 Insulation design using solid dielectrics ......................................................... 5
5.1.3 Creeping insulation design ............................................................................. 9
5.2 Measures with double insulation ....................................................................... 9
5.2.1 Definition of double insulation ........................................................................ 9
5.2.1.1 Space insulation (interconductor insulation) .............................................. 10
5.2.1.2 Solid body insulation .................................................................................. 11
5.2.2 Parts to apply double insulation .................................................................... 11
5.2.3 Implementation example of double insulation .............................................. 12
6. Appendix ............................................................................................................... 13
Appendix I Mechanism of breakdown ....................................................................... 13
I.1 Mechanism of breakdown [1-3] ............................................................................ 13
I.1.1 Conduction of solid body .................................................................................. 13
I.1.2 Breakdown mechanism of solid body ............................................................... 14
I.1.2.1 Electronic breakdown process ....................................................................... 15
I1.2.2 Thermal breakdown process .......................................................................... 19
I1.2.3 Electromechanical breakdown ........................................................................ 22
I1.2.4 Partial discharge ............................................................................................. 24
I1.2.5 Treeing............................................................................................................ 29
I1.2.6 Tracking .......................................................................................................... 29
I1.3 Creeping discharge mechanism ........................................................................ 30
I1.3.1 Creeping discharge in gaseous phase............................................................ 30
I1.3.2 Creeping discharge in vacuum [4,5] ............................................................... 32
I1.3.3 Discharge on surface ...................................................................................... 35
I1.3.4 Influence of charging on breakdown ............................................................... 36
Appendix II Influence of concentrating electric field and electron emission fromelectrode material ..................................................................................................... 39
II1 Relation between electrode shape and electric field [1] ....................................... 39
II2 Electrode material and electron emission ............................................................ 41
II3 Influence of work function in metals ..................................................................... 43
II4 Surface treatment of electrode ............................................................................ 43
Appendix III Influence of neutral molecular on discharging ...................................... 45
III1 Paschen’s law in gas [1,3] .................................................................................. 45
III2 Issues and measures on hot launch ................................................................... 47
Appendix IV Creeping distance and discharge voltage ............................................ 49
IV1 Creeping distance and discharge voltage [1] ..................................................... 49
IV2 Concept of preventing creeping surface discharge ............................................ 52ii

JERG-2-213A

1 Scope of application
1.1 Purpose
The insulation design standard (referred to as “this standard” in this document)
specifies fundamental requirements about power supply system insulation designof spacecraft developed by the Japan Aerospace Exploration Agency (referred toas “JAXA” in this document).
The appendices summarize the explanation of causes for breakdown andinformation required to utilize this standard.
1.2 Scope of application
This standard is applicable as a design standard insulation design for the power
supply system up to a nominal voltage of 100V for spacecraft developed by
JAXA.
The insulation design of printed circuit boards shall be compliant with other
JAXA's printed circuit board design standard and shall not applicable to thisdesign standard.
This standard assumes that project-specific specifications should be defined if
there are parts which have difficulty in conforming to this standard, provided thatthe reasons for the difficulties are clearly presented.

2 References
This standard was created with reference to the following documents:(1) IEC document
IEC-J60950 Safety standard of information technology devices(2) Japanese Industrial Standards
JIS C 0704 Insulation distance, insulation resistance and withstand voltage ofcontrol device
JIS C 2110 Dielectric strength test method of solid electric insulation material

3 Terms(1) Primary power supply
A power source which is generated/controlled by the power system (solar
panel system and power supply system) of an aerospace vehicle, andcommonly distributed to devices loaded in the aerospace vehicle.
1

JERG-2-213A(2) Primary power supply bus line
A bus line which distributes and supplies the primary power supply.
Applicable bounds are those which have the same potential as the primary
power supply input of loaded devices.
(3) Creeping surface discharge
Discharge that occurs on the boundary surface between different dielectrics.
(4) Conditioning
A process in manufacturing electrode structure, which removes projections
on an electrode, adherences and micro particles on an electrode. There aremethods such as current conditioning, glow discharge conditioning, andspark conditioning.
(5) Paschen’s law
Under a constant temperature of a gas, in a uniform electric field, a voltagethat generates sparking (Vs: Spark Voltage) is a function of product pd (gaspressure (p) and gap length (d) ). This is called Paschen’s law, because
Paschen discovered this relationship based on this experiment.

4 General requirements
Various types of insulation materials are used in electronic devices and components
on an aircraft, contributing to maintaining reliability of the aircraft under severe spaceenvironments. On the other hand, if a breakdown of insulation material occurs, it
may cause serious damage to an aircraft. Therefore, it is required to conduct aninsulation design with an understanding of insulation characteristics and breakdown
phenomenon of the insulation materials.
(1) A distance between conductors shall be a level that does not cause abreakdown in the ground, lift-off and orbit operating environments. In
particular, under lift-off conditions, exposure to significant change ofatmospheric pressure causes a drop of neutral gas particle density between
conductors. Insulation design requires paying attention to a breakdown
under the Paschen’s law.
(2) The distance between conductors shall be specified to be a level that does
not cause a breakdown at the maximum operating voltage that can occur
between conductors.
(3) The insulation design shall also include a specified interconductor distanceby providing clearance in processing and assembling. This shall prevent

2

JERG-2-213A

fluctuation and deformation for operating conditions on the ground, during liftoff, and in orbit, or shall account for the required distance after fluctuation ordeformation.
(4) In many cases, insulation materials are mounted by applying significant
pressure. They shall be mounted so as to avoid damage and cracks.
(5) Discharge voltages (breakdown voltages) depend on the shape of aconductor. A radius of curvature at the tip of a conductor shall be as large
as possible, and avoid making sharp edges on a conductor surface where
possible.
(6) Solid dielectrics may cause a drop of breakdown voltage under someconditions of use. Therefore, a thickness of solid dielectrics shall be
specified in full consideration of derating in accordance with material
characteristics and conditions of use.
(7) To prevent a drop in breakdown voltage caused by gas generated in thespace, the insulation design shall avoid closed spaces between conductors
where possible by taking actions such as providing a vent hole. When using
insulation material made of hardened resin, bubbles and gases in thematerial must be completely removed in order to prevent the creation of avoid.
(8) To prevent a breakdown caused by creeping discharge, barriers of pits andprojections shall be provided on solid dielectrics surface where possible.

5 Design requirements and design methods
5.1 Insulation design
This section describes the fundamental requirements of insulation design withrespect to power supply lines of a spacecraft with a bus voltage at 100 V(nominal) or lower.
The insulation design of printed circuit boards shall be compliant with JERG-0042 “Design standard of printed circuit board and assembled parts,” and shall notapplicable to this design standard.

5.1.1 Insulation design using dielectric gas
Table 5.1.1.-1 summarizes the requirements for interconductor distances ininsulation design using dielectric gas.
If values shown in Table 5.1.1-1 cannot be satisfied, a different insulation method
shall be used. The validity of each design shall be decided by analysis or3

JERG-2-213A

evaluation, for each aircraft project.

Table 5.1.1.-1 Interconductor distance with dielectric gas (Note 1)
Maximum operating
voltage (V) (Note 2)(Peak or direct current)Interconductor
distance (mm)Remarks• Devices used in anenvironment where dust
and humidity iscontrolled.
210 V
1.0 or more
• If double insulation isrequired, a different
insulation method shallbe added.
Note 1. Conditions in the air or in the vacuum are assumed.
Note 2. A maximum value of direct current voltage or pulse-like voltage
applied between electrodes when a device is operating.

4

JERG-2-213A

Table 5.1.1.-1 Explanation:
Values presented in Table 5.1.1.-1 have been specified with reference to thefollowing concepts and documents, and in consideration of the designs made in
the past and their track records in orbit.
(1) The breakdown of a gas is critical in a low pressure environment during thelaunch phase. Since a breakdown voltage of nitrogen gas, which is themain component of air, is 250 V or higher, it will not be a limiting factor in
deciding an interconductor distance at 210 V or lower.
(2) The following official standards were referenced:• IEC J60950 “Safety standard of information technology devices: Article
2.10.3, Table 2H”]
• JIS C 0704 “Insulation distance, insulation resistance and withstandvoltage of control device: Article 4.2, Table 6”(3) An interconductor distance shall be secured to reduce the risk of a criticalbreakdown caused by conductive chippings entering and remaining in
between conductors.
(The size of conductor chipping is assumed to be 1mm or less.)
5.1.2 Insulation design using solid dielectrics
In electric design today, conventional “withstanding voltage” is comprehensive andcan operate in any space environment. On one hand, this section describes theconcept of finding a final withstanding voltage on the basis of the product ofindividually set derating ratios, by breaking down the space environment intoelements such as temperature, frequency, material thickness, electron beam,
ultraviolet ray, and voltage degradation. Using this method, a breakdown of material
characteristics normally used is clearly shown, presenting indices to evaluatesensitivity for special applications of loosening or tightening under specific conditions.
In the future, excessive designing margins may be eliminated.
VEMP < VEFF: = V0  Rtemp  Rfrq  Rthick  Re  RUV  Rtime
VEMP:
VEFF:

Empirical withstand voltage
Effective withstand voltage

V0:

Mean value of nominal breakdown voltages

Rtemp: Derating ratio about temperature
Rfrq:
Derating ratio about frequency
Rthick:Derating ratio about thickness
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Re:

Derating ratio about electron beam

RUV:

Derating ratio about ultraviolet ray

Rtime:

Derating ratio about voltage degradation

Note: Note that this method of estimation does not take complex effects into
consideration, assuming that each effect contributes linearly. This method isdeveloped with an engineering approach, and has not been proved by materials
science. In materials science, some of individual effects are understood from a
unified view, which include material characteristics and theories. However, acomprehensive theory covering complex effects has not been established yet.
Note: Nominal breakdown voltage V0 refers to a breakdown voltage for a short period(20 seconds or less) on a material with no history, at a room temperature and under a
50 Hz AC current.
The factors which cause a drop in a withstanding voltage are temperature, frequency,
material thickness, electron beam, ultraviolet ray, and voltage degradation. Table
5.1.2.-1 summarizes the results on 17 materials, which are popularly used for an
artificial satellite.
The electric design compatible with the current satellite 100 V bus employs
insulation materials that withstand a peak voltage of approximately 300 V. For
almost all materials tested on this test method, effective withstand voltage VEFF was
obtained to prove the assumed fact.
Table 5.1.2.-1 Breakdown strength of solid dielectrics
Material
PTFE tape
KAPTON® tape
CHO THERM®
SOLITHANE
PARYLENE
Mica
URALANE®
LUMIRROR®
Heat-shrinkable
tube
Glass epoxy
RTV(S-691)
ETFE wire
di X CBT resin

Thickness
50 μm
25 μm
0.38 mm0.20 mm
20 μm
75 μm
1.0 mm50 μm0.18 mm

VEFF
2.5 kV
1.2 kV
7.7 kV
2.9 kV
170 V
1.4 kV
8.2 kV
2.1 kV340 V

VO
6.3 kV
5.5 kV
12.0 kV12.8 kV
3.2 kV
8.0 kV
27.0 kV6.5 kV
9.5 kV

0.2 mm
1.0 mm0.15 mm
20 μm
0.1 mm

2.7 kV
9.9 kV
2.4 kV
250 V
1.1 kV11.7 kV
29 kV
9.2 kV
2.8 kV
5.0 kV

6

Remarks

200 Hz
100 kHz
200 Hz
100 kHz

200 Hz200 HzJERG-2-213A

Arathane
RTV566
RTV1420.2 mm
0.2 mm
0.2 mm

2.1 kV
440 V
2.7 kV

10.8 kV
4.2 kV
7.7 kV

Note: Calculating conditions of VEFF:
Temperature: 100C, Frequency: 200 kHz (For a material with no dielectricstrength data obtained for 200 kHz, calculation was made on the basis of afrequency described in Remarks.)
Thickness: Nominal thickness of specimens (See above.
does not include thickness of adhesive.)

A tape thickness

Electron beam fluence: 1  1015/cm2: (Equivalent to exposure forapproximately 1 year on a stationary satellite surface. However, since a2
mechanical strength significantly drops on PTFE tape of 1  1015/cm ,calculation was made for 1  1014/cm2)
Ultraviolet irradiation: 800 ESD in wavelength band 200 to 400 nm (Applicable
only to Kapton and ETFE wire)
Application time: 100 thousand hours (Equivalent to approximately 11 years)
Concept
The conventional concept of insulation design is as follows. (See Figure 5.1-1.) It is
required to perform the withstanding voltage test, the dielectric strength test and thehigh-potential test for “maximum allowable voltages” as defined on the insulationmaterials, components, and devices in question. Every test must be passed.
“A withstanding voltage test” is specified to be sufficiently lower than a “breakdown
voltage” and sufficiently higher than a “maximum allowable voltage.” For example, on
a transformer, it is specified such as “300V for a maximum operating voltage at 100Vor lower.” This is based on a general rule of thumb, which is sufficiently effective inspite of poor quantitative evidence.
The following presents the concept described in this section. A breakdown voltageof material depends on the effects of shapes and thickness, the effects of using
conditions such as temperature and frequency, the effects of the space environment
such as ultraviolet rays or electron beams, the effects of the application of electricity
and reduction ratio due to statistical variations, which cause different effects both
independently and synergistically. Thus, an “effective withstanding voltage” is given.
This value must be higher than a “maximum allowable current.”
On the basis of this concept, quantitative grounds are given to an empirical rule. In
7

JERG-2-213A

addition, for a specific usage, an “effective withstanding voltage” can be used for adesign value by loosening a certain reduction ratio with some grounds.
However, for the time being, a satisfactory database has not been established yet
and an adequate track record is not available. It would be premature to address thisfield at this time.

Breakdown voltage
Shape factor effect (Thickness etc)

Use effect (Temperature etc)

Maximum rated voltage

Space environment effect (UV etc)

Voltage

Withstand test voltage

Voltage degradation (Time)
Effective

Withstand margin
Maximum

voltageallowable voltage

Conventional concept (example)

Figure 5.1-1

withstandConcept described in this section

Concept of insulation design

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JERG-2-213A

5.1.3 Creeping insulation design
Table 5.1.3.-1 summarizes the requirements for creeping surface distance ofsolid dielectrics.
If values shown in Table 5.1.3.-1 cannot be satisfied, a different insulation method
shall be used or validity of a design shall be decided by analysis or evaluation,for each aircraft project.
For components with values of interconductor creeping distances lower thanthose presented in Table 5.1.3.-1, their use shall be permitted by guaranteeing
the performances based on certified tests of individual components and inconsideration of the track records of their uses in orbit.

Table 5.1.3.-1 Creeping distance of solid dielectrics
Maximum operating voltage(V)(Peak or direct current)
210 V or lower

Creeping distance(mm)

Remarks

1.0 or more

Table 5.1.3.-1 Explanation:
Values presented in Table 5.1.3.-1 have been specified with reference to thefollowing concepts and documents, and in consideration of the designs made in
the past and track records in the orbits.
(1) In general, breakdown characteristics make no significant differences up to
approximately 210V, even if an interconductor distance (gap) and creeping
distance are thought to be the same.
(2) The following official standards were referenced:• IEC J60950 “Safety standard of information technology devices: Article
2.10.4, Table 2L”• JIS C 0704 “Insulation distance, insulation resistance and withstandvoltage of control device: Article 4.3, Table 8”

5.2 Measures with double insulation
5.2.1 Definition of double insulation
To prevent the critical loss of functionality caused by a failure of only one part,measures such as constructing a redundant configuration on the spacecraft must
be taken . From this a point of view, power supply lines such as the primary
power supply bus line, the battery line (including charge array line and9JERG-2-213A

pyrotechnic goods igniting power supply lines, etc.) are thought to be the parts
requiring insulation design measure. Shorting of those primary power supply
bus lines and battery lines is thought to be the most critical failure and actions
against such failures are impossible. Therefore, as a minimum requirement, ameasure to include double insulation is specified in this design standard bylimiting its application to primary power supply bus line and battery line only.
Therefore, double insulation shall be applied to the primary power supply bus line
including a shorting failure separation circuit and its upstream lines, which are, inother words, the part that may cause a short of the primary power supply line andbattery lines. However, exceptions may be made for cases where a redundant
system can be built (a redundant system is provided and a faulty system can beseparated by an overcurrent protective circuit and a current limiter). On theother hand, double insulation is not applied to semiconductor components suchas IC, transistor and diode, and internal parts of connector, relay, and capacitor(including hermetic sealing part).
Double insulation is the method of insulation that uses two different methods
chosen from 5.2.1.1 and 5.2.1.2 below. If shorting occurs on one part, themethod prevents shorting on the other part depending on a cause of the shorting.
If using the same material is inevitable due to technical limitations of electrical,
mechanical, thermal, or other conditions, the same material can be used inconstruction provided that the material has track records of flight or track recordsof evaluation and has data about insulation performances, mechanical strength,
and thermal resistance, and satisfactory safety margin is secured. In that case,also, materials shall be chosen from 5.2.1.2 (2) below.
Materials in 5.2.1.2 (3) below are not regarded as insulating materials in double
insulation.

5.2.1.1 Space insulation (interconductor insulation)
In an area to be insulated, a distance shall be included to prevent shorting
between conductors caused by foreign matters and displacement expected in thearea.
If a distance of 1mm or more is provided, it is regarded as an insulating material.
If a distance of 1mm or more is provided between two bare conductors and no
insulating material exists between them, it is regarded as single insulation. If
the gap between two conductors is filled with an insulating material, it is regarded
as double insulation.
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JERG-2-213A

5.2.1.2 Solid body insulation
Insulation is provided using the following solid body insulation material:(1) Solid body insulation material(a) Resin board(b) Resin sheet(c) Resin mold(d) Coating(e) Mica board(f) Glass fiber(g) Glass hermetic sealing(h) Ceramic board(i) Ceramic hermetic sealing(2) Items allowed for using for double insulation with the same material.
(a) Resin sheet: BT resin, Lumirror sheet(b) Resin board: GFRP, Rexolite(c) Tape: Polyester tape, Kapton ® tape(d) Wire material sheathing: Wire material shall be certified for installing on
aircraft(3) Items not regarded as insulation material(a) Solder resist on printed board(b) Surface treatment on alloy
Except for the cases where insulation is secured by types of treatment orapplication to insulation design.
(c) Ferrite

5.2.2 Parts to apply double insulation
Double insulation shall be applied to the following parts:

11

JERG-2-213A(1) Primary power supply bus line
On the panel side of the solar battery transmitting generated power to theprimary power supply bus line, double insulation shall be applied from a point
where the shorting of primary power supply bus line occurs due to a short at
only one part to the end of the input of the shorting failure separation circuitthat receives the power distribution and supply of the primary power supply
bus line. Figure 5.2.2-1shows an example of the primary power supply bus
line

Power supply
controller

Breakdown
Strength(MV/cm)
DC/DC

Load
devicesThickness of thesamples[μm]EnergyB
atteryFigure 5.2.2-1

Battery

Primary power supply bus line example(2) Battery line
The battery line shall include the charge array line (in a charge-arraysystem) , the power line inside the battery, cell case, battery cabinet, and thepyrotechnic goods igniting the power supply line, in addition to the battery
output line.
5.2.3 Implementation example of double insulation
N/A

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6. Appendix
Appendix I Mechanism of breakdown
Various types of materials are used on a spacecraft. Among them, various types of
insulation materials are used in electric and electronic devices and components of aspacecraft, which greatly contribute to maintaining reliability of a spacecraft in thesevere space environment. However, the breakdown of insulation material may
lead the spacecraft to a severe accident. It is necessary to apply materials with athorough understanding of the breakdown phenomenon and breakdown
characteristics of insulation materials. This section describes the breakdown
phenomena and breakdown mechanism, and various factors that influence a
breakdown. The insulation materials used in a spacecraft are solid body. Thissection focuses on the breakdown of the solid body itself, and creeping surface
discharge related to the surface of a solid body and vacuum (or gas).

I.1 Mechanism of breakdown [1-3]I.1.1 Conduction of solid bodyA breakdown determines the ultimate useful life of electric insulation structure.
A deep understanding of electric conduction, which is a warning sign for a breakdown,leads to providing important information about the theory of electric physicalproperties, in addition to information about its engineering applications.
A current is defined as a quantity of electricity that passes through a cross section
per unit of time. Therefore, a current density j [A/m2] is expressed as follows, withthe density of moving electric charge n [/m3]and its moving speed vd [m/s],

j  qnv d  qnE••• (I.1)

v d  E

Where, E: applying limit [V/m], q: Charge amount of electric charge [C], μ: mobility[m2/V•s]. If there are m types of moving electric charge, j is given by:mj  E  q i ni  i••• (I.2)

i 1

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JERG-2-213A

Meanwhile, conductance σ [S/m], which is one of physical values that describe
electric conduction, is defined as follows:



j
 qn
E

(

1



••• (I.3))

A reciprocal of σ is volume resistance ρ [Ωm]
The voltage-current characteristics, when increasing voltage is applied underconstant conditions such as dielectric temperature and specimen thickness, are very
important in evaluating electric insulation performance of a dielectric and in defining
electric the conduction mechanism. In general, current-voltage characteristics
changes as shown in the model of FigureI.1. The characteristics are divided into thefollowing three fields. In solid dielectrics, almost no current saturation in the area
emerges, unlike that of gases and pure liquids.
Area I is a part where electric field is relatively low. Current I increases linearly
together with voltage V, adhering to the Ohm’s law, I  V.
In area II, a current increases on a non-linear basis deviating from Ohm’s law, shifting
to a breakdown eventually. This phenomenon is known as a dielectric breakdown.
At this point, as shown in area III, a current that directly leads to a breakdown may beobserved, which is clearly distinguished in some cases. The area where Ohm’s law
is true is referred to as “low-electric field area,” and the non-linear area is referred toas “high-electric field area.”
Since a breakdown exceeds the limit of maintaining an insulation performance for adielectric, it becomes an important factor that ultimately determines the useful life ofelectric and electronic devices.

I.1.2 Breakdown mechanism of solid body
The mechanisms shown in TableI.1 are shown as breakdown mechanisms of a solidbody. They are roughly divided into short-term breakdown and long-term
breakdown. The following chart describes those breakdown mechanisms
respectively.
First, short-term breakdown is described below:

14

Current

Brakedown

JERG-2-213A

Voltage

Figure I.1

Voltage (electric field) – Current characteristics of solid body dielectric

Short timebreakdown

TableⅠ.1 Breakdown mechanism of solid body
Electronic
breakdown
process

Intrinsic breakdownElectronic avalanche
breakdown
Zener breakdown

Long timebreakdown

Thermal breakdown process
Partial discharge
Treeing
Tracking

I.1.2.1 Electronic breakdown process(a) Intrinsic breakdown
Conducting electrons exist in solid dielectrics, although there are very few. The
conducting electron obtains energy A from electric filed E and gives energy B to thegrid electron. An electric field that breaks the balance between energy A and energy
B is a breakdown of strength, or an intrinsic breakdown.
Assuming that all electrons in a solid body have identical energy W, A is found as
follows: If an electron is moving in direction z by the force of electric field E while
colliding with grid, a kinetic momentum of electron pz is:15

JERG-2-213A

(

dp z
 eE
)
dt Electric••• (I.4)

field

Where, e is charge quantity of electron (= 1.6  10-19C).
(

dp z
pz

)
 (W , T )dt grid0Collision with grid is:••• (I.5)

Where, (W,T0) is momentum relaxation time, which depends on W and gridtemperature T0. Since pz is constant under stationary state,
(

dp z
dp
( z)
0
)
Electricdt
dt grid••• (I.6)

field

Therefore, moving speed of electron vez in direction z is:

vez 

pz
e
   (W , T0 ) Emm••• (I.7)

Where, m is a mass of electron (= 9.1  10-31kg) . Setting a current density as j, A =jE and a current by one electron is expressed as, according to equation (I.1) , since j= -evez:A( E , W , T0 )  jE  evez E  (e2) (W , T0 ) E 2m••• (I.8)

Since a ratio of losing energy B by collision in a unit time depends on W and T0, it isset as B (W, T0) .
Figure 1.2 shows a conceptual chart of A and B dependency on W.

16

Value of A & BShunt

JERG-2-213A

Electron

Figure I.2

Relation between A, B and electron energy W

In Figure I.2, Von Hippel said that EH is a breakdown in strength because A > B istrue for any W when E > EH, and energy of electron increases infinitely without abalancing point. EH is referred to as Hippel’s low energy criterion.
On the other hand, if E < EH, electrons can increase under some conditions, whenelectron energy becomes to be an energy level I that is required to excite electronfrom valence band to conduction band of dielectric. Fröhlich set an electric field EF
for I as breakdown strength. This is referred to as Fröhlich’s high energy criterion.
(b) Electron “avalanche” breakdown
This is an analogy to a breakdown of a gas. Electrons accelerated by an electricfield repeats collision and ionization and the number of electrons grow to become anavalanche, which causes a breakdown. Breakdown conditions are expressed by
the following equation:

d  hc••• (I.9)

α is collision ionization factor, which is the number of collisions in a unit distance, [/m],d is the thickness of a dielectric [m], and hc is a constant, for which approximately 40
is often used by empirical means. (Seitz’s 40-generation theory) The collision
ionization factor α in a solid body is, setting the number of times an electron ionizes
in a unit time as w [/s], given as follows:



w
w

ve  e E••• (I.10)

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JERG-2-213A

Where, ve is moving speed of an electron [m/s]. Meanwhile, w is found as follows.
When an electron with average energy Wav moves in an electric filed, the time tI
required to reach ionizing energy I and distance lI of moving in the direction of theelectric filed are respectively:

2m ( I  Wav )tI ••• (I.11)

eE

and

lI 

I  WaveE••• (I.12)

Meanwhile, an electron may collide before obtaining energy I. Setting p(tI) as theprobability that an electron does not collide with an atom within time tI,, which is a
probability of obtaining energy I or higher, and setting (Wav) as collision relaxation
time (time from a collision to a next collision) , the relation is:w

p(t I )
 (Wav )••• (I.13)

If equation (I.10) and equation (I.13) are substituted into equation (I.9), and solving
the equation for E, a breakdown strength EB on the avalanche breakdown is asfollows:EB p(t I )d (Wav )  e (Wav )hc••• (I.14)

p(tI) is expressed as follows:p (t I )  exp(  

tI

0

dt
)
 (Wav )••• (I.15)

An equation of motion for an electron moving freely in an electric field is:

dv
dW
eE
 mve e  (2mW )1 / 2dt
dtm••• (I.16)

Therefore, equation (I.15) is put as follows:p(t I )  exp( E / H )••• (I.17)

Where,I
1 mdW
H  ( )1 / 2 
1/
Wav W 2 (W )e 2••• (I.18)

18

JERG-2-213A

Therefore, EB is:

EB 

H
d
ln{
}
E e  e (Wav ) (Wav )hc••• (I.19)

Breakdown Strength(MV/cm)

Breakdown Strength(MV/cm)
Breakdown Strength(MV/cm)

EB is a function of gap length d. A specimen with a low thickness is vulnerable toavalanche breakdown. Figure I.3 shows comparison between an experiment of the
breakdown strength conducted on sodium chloride (NaCl) and the electron
avalanche breakdown theory.

Thickness of the samples[μm]Thickness of the samples[μm]●100 orientation
× 111 orientation○
100 orientation(b) Calculated(20 ℃)(a)Experiment(Room Temp)

Figure I.3 Comparison between experimental values and calculated values
based on the avalanche breakdown theory pertaining to the breakdown strength of
NaCl(c) Zener breakdown
The Zener breakdown considers that electrons in a conduction band suddenly
increase due to a tunnel effect under a high electric field, and that the breakdown
occurs when the generation of heat caused by the current of electrons leads to an
exceedingly critical temperature. This model was proposed as one for a thin
specimen with a narrow band gap between a conduction band and a valence band.
For example, this is applicable to a breakdown caused by a reverse bias to a p-njunction of Ge. The breakdown of electric field strength is in the order of 10 MV/cm.

I1.2.2 Thermal breakdown process
Applying an electric field to a solid body of insulation material generates Joule heat,and the heat is lost by the rise of temperature of the solid body and diffusion to the19

JERG-2-213A

surrounding area. Figure I.4 shows the concept in relation to the heat radiated andheat generated. If an electric field E1 is applied in an ambient temperature T0, at
first the temperature rises gradually since the heat generated exceeds heat radiation,
until they balance at T1 in the figure. If an electric field becomes ETc, the balancing
point becomes T0c. With a higher electric field, the heat generated always exceeds
the heat radiated and the temperature continues rising without any balancing point.

Heat Radiation

Heat Radiation

Heat Generation

Heat Generation

In general, the temperature at which a solid body of insulation material reaches heatbreakdown T0c’ is higher than T0c in Figure I.4. However, if an electric field of ETc or
higher is applied in asteady state, the temperature of T0c’ is always maintained.
Therefore, ETc is a thermal breakdown of the strength of the solid body insulation.
However, when a pulse voltage is applied and the time required for temperature rise
to T0c’ is longer than a pulse voltage width, an electric field of ETc or higher does notcause a breakdown. Therefore, the breakdown strength is a function of the time
between applying a voltage to a breakdown.

Temperature of

Figure I.4

Insulator

Conceptual chart of heat generation and heat radiation on solid body

The fundamental equation of thermal conduction in a solid body, according to thethermal balancing conditions in unit volume of solid body,Heat value = (Heat of risingtemperature of solid body) + (Heat radiation to surroundings by thermal conduction),is, as follows:E 2  C v

T div ( KgradT )t••• (I.20)

Where, T: Temperature of solid body [K], Cv: Specific heat of solid body [J/kg･K], K:
Thermal conductivity of solid body [W/m･K], and Conductivity [S/m].
20

JERG-2-213A

In general, analytically solving equation (I.20) is difficult even under simple boundary
conditions. Here, a case where the optimum condition, which is applying a direct
current, is considered.
If a direct current is applied, the first term on the right side of equation (I.20) can beignored: div( KgradT )  E 2••• (I.21)

If it is assumed that an infinitely flat, uniform, isotropy solid body of insulation material
with a thickness d is placed in the parallel flat electrodes as shown in Figure I.5, andheat flows only in z direction, then, equation (I.21) is:

T
V
(K
)   ( )2  0
z
z
z

••• (I.22)

Where, V is a potential in the solid body [V].

And the following is true:

V
 j
z

E  

••• (I.23)

If equation (I.23) is substituted into equation (I.22) and integrated:

T
T
(K
)dz  K0 zzz

jV  z••• (I.24)

If the above is substituted into equation (.23) and integrated:
Tm

K

T



V0  2 
2••• (I.25)

dT

However, if V0: Applying voltage [V], T0: Ambient temperature [K], Tm: Temperature ofcenter of body [K], breakdown occurs when Tm = T0c. Therefore, a breakdown
voltage VB is as follows. In other words, substitute the equation above by Tm = T0c,
T = T0, VB = V0/2 (Only a half is considered since temperature distribution issymmetrical for ∵z = 0):
T 0c

K

T0



V B  8••• (I.26)

dT

If substitution can be made as(T) =exp(-φ/kT) , K(T) = K0:

2

8kT0 K 0 1 / 2
VB  () exp( / 2kT0 )••• (I.27)

 0

Where, k: Boltzmann constant (= 1.38x10-23J/K) , φ

21Work function.

JERG-2-213A

Figure I.5

Uniform and isogonic infinite flat plate specimen

I1.2.3 Electromechanical breakdown
If a Maxwell stress by applying electric filed becomes greater than the mechanicalstress of a specimen, the specimen is mechanically pressed down and electricalbreakdown then occurs. The breakdown strength in that case is found as follows:
If an electric field E is applied to a solid body insulation material, the following
Maxwell stress occurs:
Fe 



E2

2

••• (I.28)

Where, ε: dielectric constant of specimen.
On the other hand, when a thickness of specimen with Young ratio Y shrinks from d0
to d, an internal stress is, according to definition of Young ratio:d0

Fm   Ydddl Y ln 0ld••• (I.29)

If Fe and Fm balance with each other when voltage V is applied to a board specimenof thickness d, the following is obtained by substituting Fe = Fm and V = Ed:
V2 

2d 2Y



ln

d0
d

••• (I.30)

Value V becomes highest by changing d when d/d0 = exp(-1/2) 0.6, rather than dV/dd= 0. In other words, a maximum voltage VB allowed to apply without pressing downa specimen mechanically is that at d = 0.6d0, and a breakdown electric field EB is:

EB 

VB
Y
 0.6( )1 / 2d••• (I.3v)

Figure I.6 shows comparison between actual measurement values of breakdown
strength and calculated values of electrical mechanical strength for polyethylene andpolyisobutylene. It illustrates sharp drops of EB around 80C and -40C well.

22

JERG-2-213A

Breakdown Strength (MV/cm

polyethylenpolyisobutylen

Not IrradiatedPower
distributor

Temperature [℃]
Measured Value
Theoretical Value(Electro-MechanicalFigure I.6 Breakdown strength dependency of polyethylene
and Polyisobutylene on temperature

23

JERG-2-213A

I1.2.4 Partial discharge
This next section describes a breakdown over a long period of time. A solid body ofinsulation material may be destroyed by a voltage applied for a long period of time.
The three factors, partial discharge, treeing, and tracking are considered as causes.
From a practical viewpoint, it is important to understand the processes of this type of
breakdown.
First, look at a breakdown over a long period of time caused by partial discharge.
As shown in Figure I.7, if a board-like bubble (void) exists in a solid body insulationmaterial (thickness: d) with a thickness t and a cross section A, its equivalent circuit isshown as in figure (b). In the figure, assuming that t << d and a dielectric constantof the solid body is εr, an electric field in the void is Ec, and an electric field in thesolid body is Ea, the following is true:Ec   r Ea••• (I.32)

An electric filed in a void is εr time as high as that in the solid body.
following are true in the figure:Cb  0 r A
d t

Cc 

,

0 A

Meanwhile, the••• (I.33)

t

Therefore, a voltage Vc applied to the void when a potential difference Va between
electrodes is as follows:

Vc 

Cb
Va 
Cc  CbVa
1 d
1  (  1)
r t••• (I.34)

Therefore, when Breakdown strength of the gas in the void is EB, a voltage between
electrodes to cause breakdown in the void is :Vao  E B t{1 

1 d(  1)}
r t••• (I.35)

Discharge in a void is referred to as partial discharge or void discharge, and Vao isreferred to as partial discharge onset voltage.

24

JERG-2-213A

Electrode

Void

Electrode(a) Void in Sample(b) Equivalent Circuit of (a)Cc:Capacitance of void
Cb:Capacitance of Ⅰ
Ca: Capacitance of Ⅱ、Ⅲ

Figure I.7

Insulation material containing void and equivalent circuit

Figure I.8 shows voltage changes of parts when Va in Figure I.7 is a sine-wave
alternating current. If a potential difference in void Vc reaches Vc = EBt = V+ anddischarge starts once in Cc, Vc sharply drops to Vr by ⊿V. Vr is referred to as
residual voltage. This occurs because an electric charge of the opposite sign isaccumulated on the insulation material surface in the void by partial discharge, as
shown in Figure I.9(b), an electric field caused by the accumulated electric charge Es
cancels the external electric field Ea. The time required for Vc to drop by ⊿V isdecided by the continuing time of discharge. It is extremely short, such as
approximately 10-8s, no electric charge is supplied from the power supply.
Therefore, a voltage applied to Ca drops by the following value:V  VCb V
C a  Cb••• (I.36)

If Va continues to rise after discharge, Ec also rises as shown in Figure I.9(c) inproportion to the external electric field. Since the electric field caused by theaccumulated electric charge Es exists, the practical electric fields in the void (Ec+Es)
rise, and Vc rises also. If it reaches V+, partial discharge occurs again. Thissimilarly occurs when Va drops also, and changes of Vc are shown in Figure I.8. A
true electric charge Q that makes discharge in the void is

Q  V (C c 

C a Cb)  V (Cb  C c )C a  Cb••• (I.37)

25

JERG-2-213A

An apparent discharge electric charge Q’ is given by the following equation:
Q'  VC a  VC b

Figure I.8••• (I.38)

Void discharge current with alternating current voltage applied

Void Discharge
Generate

Voltage Rise

Insulator

Void

Ea:External Electric Field
Ec:Electric Field in void by External Electric Field
Es: Electric Field by accumulated electrical charge in void

Figure I.9

Change of electric field in void

Next, a partial discharge occurs when a direct current voltage is applied. As shownin Figure I.10, upon a rise of voltage, an electric field Ec rises in a void in response to
the external electric field Ea, just as the case with applying an alternating current
voltage. Moreover, a partial discharge occurs when EB is reached. If the electric26

JERG-2-213A

charge generated by the discharge adheres to the inner wall of the void, it lowers theinternal electric field. If a rise of voltage stops after that, the electric field in the void
remains low under the influence of the accumulated electric charge. However, if thiscondition continues for a long period of time, the electric charge accumulated inside
leaks toward the facing electrode because of the external electric field Ea. Therefore,
the electric charge accumulated on the inner wall decreases and the electric field Esof the accumulated charge drops, gradually increasing the electric field inside thevoid. If this rise in the electric field continues and EB is reached, a partial discharge
occurs again. As described above, a rate of partial discharge caused by a direct
current voltage is determined by the leak of electric charge. A time constant of sucha leak is decided by the product ερ of a permittivity ε and volume resistivity ρ of an
insulation material. Since the volume resistivity of an insulation material isextremely high, the time constant is high. Therefore, the frequency of discharge islower with respect to direct current, in comparison with that of alternating current.

VoltageElapsedCurrent

Long Time

ElapsedVoid Discharge
Generate

Insulator

Long Time

Void
Ea:External Electric Field
Ec:Electric Field in void by External Electric Field
Es: Electric Field by accumulated electrical charge in void

Figure I.10 Partial discharge with direct current voltage applied

As described above, if a void exists in a solid body of insulation material, an electric27

JERG-2-213A

field in the void becomes high and generates a partial discharge. Since a partial
discharge becomes the cause for energy loss in insulation material, it increases asapparent electrostatic tangent tanδ increases. If it continued for a long time,
insulation deterioration occurs as the result of the following, and a breakdown of thecomplete circuit occurs under a low voltage that is several times as low as applying a
voltage for a short time.
The following three reasons account for insulationdeterioration:
1) Cutting of molecular structure caused by collisions of charged particles to an
insulation material, and chemical reactions such as cross-linking and oxidization.
2) Thermal deterioration caused by a local temperature rise caused by collision of
charged particles.
3) Oxidization caused by products that are discharged, such as O3, NO, and NO2.
Figure I.11 shows the long-time breakdown voltage – Useful life characteristics (V-tcharacteristics) of epoxy resin, which is a representative insulation material, applying
alternating current voltage. Under such V-t characteristics, it is known that areversed n-power law is true for applying voltage (electric field) and determining a
useful life on an empirical basis:t  kV  n••• (I.39)

n=3.36
Fill the sample
with filler

Applied voltage (kV rms)

Applied voltage (kV rms)

A useful life is assessed by value of n. If a void exits, n value becomes low asshown in Figure I.12, indicating a short useful life.

n=3.62Fill the sample
without filler

Life-time (h) 60Hz
Life-time (h)Figure I.11 AC voltage V-tcharacteristics of epoxy resin

Figure I.12 AC voltage V-tcharacteristics of epoxy resin (with void)28

JERG-2-213A

I1.2.5 Treeing
If a partial high-electric field, such as a projection on an electrode, exists in the solid
body of insulation material, a discharge path (tree) made of a thin pipe is generated.
If it creates a short between electrodes, it causes a complete circuit breakdown.
Such an arborescence discharge path is referred to as treeing. It is likely to occur if
a needle-like projection, conductive fiber, or water exists on an electrode of a thickspecimen. A tree emerges at a far lower voltage than at the intrinsic breakdown
voltage of the insulation material, and leads to complete breakdown.

I1.2.6 Tracking
If salt, dust, or moisture adheres to a part of an insulation material surface where anelectric filed exists in a creeping direction, it gradually forms a conductive path (track),
which can cause a creeping flashover at a low voltage. The phenomenon of
producing a track on an insulating material surface such as this is referred to as
tracking. The phenomenon of forming a low-resistance conducting path on anorganic insulation material surface because of carbonization due to arc discharge
heat or conductive products caused by arc discharge (vapor deposition of electrodemetal) is referred to as arc tracking. Carbonization due to heat of arc discharge islikely to occur on any insulation material with a high percentage of carbon content.
On the other hand, materials with N or O are bonded in the middle of carbon
backbone, such as amino-resin and polyamide resin, which are resistant tocarbonization in general, and have good anti-arc performances. On the contrary,materials with an aromatic ring in its molecule such as phenol resin and PBT havepoor anti-arc performances. Meanwhile, with respect to materials with tracking
caused by conductive products due to an arc discharge, the time required to produce
tracking depends on the electrode materials, the discharging energy, and the numberof times of discharge.

29

JERG-2-213A

I1.3 Creeping discharge mechanism
Creeping discharge occurs along the boundary surfaces of different dielectrics.
Therefore, the phenomena are divided into discharge on a solid body surface in agas, discharge on a solid body surface in a vacuum, discharge on a solid bodysurface in a liquid, and on the boundary between a gas and a liquid. Since thissection is concerned with space devices, it describes discharge on a solid bodysurface in a gaseous phase, on a solid body in a vacuum, and between different solid
bodies.

I1.3.1 Creeping discharge in the gaseous phase
Models of creeping surface discharge, as shown in Figure I.13(a), are roughly divided
into two: An electric line of force is perpendicular to a solid dielectrics surface, and it
is parallel to the surface as Figure I.13 (b). Creeping discharge does notnecessarily start with an electrode on a high-voltage side. If an electric field near
the electrodes on the ground side is intense, the discharge starts on that side. If
creeping discharge creates a short between electrodes, it causes a creeping
flashover with a flash.
(a) Creeping discharge on perpendicular electric line of force
Because an electric field on the tip of an electrode is high, creeping discharge
occurs and develops at a low voltage and discharge expands very well.
Bushing and cable terminals have this type of structure.
(b) Creeping discharge on parallel electric line of force
Since the strength of electric fields is approximately the same at any position, adischarge that starts at a point leads to flashover instantly. A discharge starts
with a point with large electric field distortion, such as at junctions between
dielectric and electrode (triple junction) and a metal piece adhering to a dielectricsurface. This is the structure of an insulator and spacer holding a high-voltage
conductor.

30

JERG-2-213A

Electric lineof force

Electric lineof force
Solid dielectrics

Solid dielectrics(a) Perpendicular type(b) Parallel typeFigure I.13 Typical electrode structure of creeping discharge

31

JERG-2-213A

Figure I.14 shows the mechanism for a creeping discharge. The area at the tip is
ionizing intensely (electronic avalanche) while a plasma-state trunk exists at the back.
Together, they are referred to as a creeping streamer. If discharging continues for along period of time, a conductive leader is formed at the back of the streamer. The
properties of a creeping discharge include expanding with contact to a solid
dielectrics and flowing a large charging current. An electrode in which a charging
current flows is referred to as back electrode.

Gap

ElectrolengthLeaderStreamer

HV

Solid bodydielectric
HV

Figure I.14

Electronavalanche

Specific

No Back electrodei
Creeping streamerBack electrode

Chargecurrent

Mechanism of developing creeping discharge

I1.3.2 Creeping discharge in a vacuum [4,5]A solid body of insulation material is used to support a high-voltage part in a vacuum.
Performance of holding high-voltage on a solid body of insulation material in a
vacuum is lower than a vacuum gap of the same dimensions. A discharge is likely
to occur on a creeping surface of insulation material. Until now, various creeping
discharge mechanisms in a vacuum have been discussed in the following threephases:(1) Initial phase(2) Growing phase(3) Final phase(1) Creeping discharge initial phase
As shown in Figure I.15 of the conceptual diagram of creeping discharge in the initial
phase, creeping discharge on insulation material in a vacuum is triggered by thedischarging of an electron from a triple junction (junction of insulationmaterial/cathode metal/vacuum). The concentration of electric fields occurs at thejunction, and electrons are discharged to the electric field.

32

JERG-2-213A

VacuumMetal
Insulator

Metal cathode
Triple junction

Metal anode

Solid insulator

Figure I.15 Initial phase of vacuum creeping discharge(2) Creeping discharge growing phase
Various mechanisms are considered regarding how the electrons discharged fromthe triple junction begin to discharge, which include secondary electron emission
avalanche, electrons increasing in a solid body, and electron collisions. This section
describes the Secondary Electron Emission Avalanche (SEEA) which is mostaccepted.
When electrons emitted from a triple junction collide with the surface of insulationmaterial, and when the secondary electrons produced at that point collide with thesurface of the insulation material again to increase the secondary electrons, it forms
an avalanche. (Figure I.16) In general, a secondary electron emission factor on thesurface of insulation material has an energy dependency on primary electron(incoming electron) as shown in Figure I.17. If the energy of colliding electrons is in
the area of emission at a factor larger than 1 (usually 10eV to keV), it results in anemission of more electrons than electrons coming into the insulation material. Then,the part of causing secondary electron emission becomes positively charged.
Anderson’s group [6] invented a model that SEEA leads to a complete breakdown.
Insulation materials in a vacuum, particularly on a ceramic surface, are covered withmultiple layers of adsorption gas. When SEEA electrons collide with the surface of
the insulation material, the adsorption gas is emitted and it forms a partially-ionizedgas cloud from the electrons in the SEEA. Residing positive ions increases theelectric field of the triple junction. As a result, electrons emitted from the triple
junction increase and a current flowing along the surface of the insulation material
also increases. This process produces a creeping discharge on the surface of theinsulation material.

33

JERG-2-213A

VacuumInsulator

Figure I.16

Increase of electrons by SEEA electrons

Secondary electron emission factor

Primary electron energy

Figure I.17

Secondary electron emission characteristics of insulation material(3) Creeping discharge final phase
It is generally accepted that the final phase of a creeping discharge is electric
discharge in the gas emitted from the surface of the insulation material.
Figure I.18 shows the inter-electrode dependency of creeping discharge voltage on
representative materials, Teflon® and Macor. Discharge voltage increases in
response to the increase of inter-electrode distance. However, it shows a tendencyof gradual saturation. It also indicates that the discharge voltage depends on thetypes of materials.

34

JERG-2-213A

Creeping discharge voltage (kV )DC voltage●:Macor▲:Teflon

Insulation length (mm)Figure I.18 Dependency of creeping discharge voltage
on inter-electrode distance in vacuum

I1.3.3 Discharge on surface
In an insulation structure, as shown in Figure I.19, there may be a bonding surface
between different materials. On such a surface, there may be a layer of air on thebonding. Peeling is likely to occur due to thermal history after bonding. A discharge
may occur on such a surface. Figure I.20 shows the breakdown characteristics on abonding surface between Noryl resin, which is used for a small coil and silica-filled
epoxy resin. As shown, a breakdown voltage is low on the bonding surfaces
between the different materials. In addition, after a history of thermal shocks,
peeling occurs on the surface, approaching a breakdown electric field of air(3kV/mm). Moreover, if a back electrode has been formed, a breakdown voltage islikely to become even lower.
Electrode

Bonding surface
Bonding layer

Insulation material

Figure I.19

Complex insulation structure with insulation material surface

35

JERG-2-213A

Noryl/Epoxy resin adhesive sample

Epoxy resin (Silica)
Noryl resin (Pipe)
Magnet wire (Dia=0.07mm)
Beforer heat shock
After heat shock

Creeping distance (l , mm)

Figure I.20

Breakdown characteristics on bonding surface

I1.3.4 Influence of charging on breakdown
On a spacecraft, high-energy charged particles existing in space may get into a
device and may charge the solid body of insulation material in an installed device. It
may have an influence on the breakdown of a solid body of insulation material.
However, there have been few studies on insulation conducted from this point of view
because high voltages are not applied to spacecraft. There are only discussions
concerning the influences of electron beam radiation on the voltage characteristics of
creeping discharge in a vacuum. Figure I.21 shows this study. [7] This suggeststhat a glass creeping discharge voltage drops because of the radiation of the electron
beam. This is considered because the electron beam radiated on a glass creeping
surface distorts the creeping electric field, making a discharge more likely.
However, various studies have been conducted concerning the space charge effectin a solid body, as a secondary factor that influences the breakdown of a solid body.
When a high electric field is applied to an insulation material and a space charge isformed inside, the internal electric field is distorted and the breakdown aspect isgreatly changed. The formation of a space charge is based on the uneven
distribution of a positive and negative charge. A charge is supplied from electrodes(injection) and inside a solid body. In most cases, a space charge effects cause
problems with an uneven electric field. They particularly emerge in a direct current
electric field.
36

JERG-2-213A

If a formed space charge has the same sign as the electrode voltage polarity, it iscalled a homo-charge, which is primarily injected from the electrode. If it has adifferent sign, it is called hetero-charge, which is caused by the movement of acharge in a solid body. A homo-charge relaxes an electric field in front of anelectrode, and functions to prevent a breakdown. However, as it is seen in a directcurrent power transmission, reversing polarity of voltage induces a high electric field
in front, which increases the probability of a breakdown. In addition, a short circuitwith a negative electric field applied to a needle electrode with a space charge
formed may lead to local breakdown.
Without Electron Shower
With Electron Shower

Creeping discharge voltage (kV)●○Creeping length(mm)Figure I.21 Influence of electron beam radiation
on creeping discharge voltage characteristics of glass in vacuum
References[1] The Institute of Electrical Engineers of Japan, Discharge Handbook Publishing
Committee, “Discharge Handbook” (Ohmsha) (2003)[2] Inuishi, Nakajima, Kawabe, Ieda “Dielectric Phenomena Study”,(The Institute of Electrical Engineers of Japan) (1973)[3] Hara and Akiyama, “High-voltage Pulse Power Engineering”,(Morikita Publishing) (1991)[4] R.V.Latham, “High Voltage Vacuum Insulation: The physical Basis”,(Academic Press, Inc.) (1981)[5] H.C.Miller, IEEE Trans. Electrical Insulation, Vol.24 (1989) p.765[6] R.A.Anderson and J.P.Brainard, J.

Appllied Physics, Vol.51 (1980)

p.1414[7] Fujii, and Koh The Institute of Electrical Engineers of Japan Magazine,
Vol.119-A (1999) p.177

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Appendix II Influence of concentrating an electric field and an electron
emission from electrode material

II1 Relation between electrode shape and electric field [1]Take a look at a system in Figure II.1, which is composed of electrodes and a space
between them. Setting φ = φ (x, y, z) as optional at an arbitrary point p (x, y, z) inthis space, φ satisfies the following Poisson equation if a space charge density is ρ:div(grad )  ••• (II.1)

Where, ε is a dielectric constant of the space. If there is not a space charge, setting
ρ = 0 makes the following Laplace equation:div (grad )  0••• (II.1)

Therefore, solving the equation (II.1) or (II.2) gives an electric field distribution in
space under the following boundary conditions on electrode surface:  Vi(i: Electrode number)••• (II.1)

Table II.1 provides solutions of analysis on electric fields in an electrode alignmentthat is frequently used in handling high-voltage phenomena.
Electric fields with complex electrode shapes and complex boundary shapes ofspace media are solely found using numerical calculations. The numericalcalculations include the calculus of finite differences, the finite element method, thecharge simulation technique, and the surface charge method.
Electrode2

Electrode1

Space
Electrode3

Figure II.1 Potential in space between electrodes

39

JERG-2-213A

TableII.1 Electric field with fundamental electrode alignment(The field amplification factor ƒ = Emax/Eav, where
Emax: Maximum electric field, Eav: Average electric field)
Electrode alignment
Parallel flat platesElectric field

E

V
d

f 1
Coaxial cylinders

Concentric sphere

Cylinder and flat plateEV
r ln( R2 / R1 )f 

R2 / R1  1
ln( R2 / R1 )

E

R1 R2
V
r 2 ( R2  R1 )

f 

R2
R1

E

2 h2  R2h  h2  R2
(h  R  x ) ln(
R22

h2  R2f 
R ln

Cylinder and half-cylinder projection in
uniform electric field2h  h2  R2
RR2
E  {1 }E 0( R  x) 2

40

VJERG-2-213A

Oval and oval projection in uniform
electric field

II2 Electrode material and electron emission
If a voltage is applied between electrodes placed in a vacuum, the current- voltage
characteristics shown in Figure II.2 are obtained. If a voltage of high electric fieldthat can cause breakdown is applied, a field emission of electron from the cathodeoccurs due to the tunnel effect. The electric field emission current Ie is a stablecurrent decided by a work function φ of an electrode material and an electric field E,
which is expressed by the equation of Fowler-Nordheim (F-N). However, if Ie ismeasured between electrodes with an area larger than a certain size, the value issignificantly larger than the theoretical value in general. An Ie value of the actual
measurement is explained by the fact that the electric field on a local cathode surface
is thought to be 100 times as high as the macro electric field E. Here, the conceptof field enhancement factor β is introduced, expressing Ie by the following equation:

Ie 

1.54  10 6 ( E ) 2 Ae

 6.83  10 7  3 / 2
 exp
E

［A］ ••• (II.4)

Where, Ae is an electric field emission area.
The cause why β occurs is, in general, thought to be a micro projection (protrusion,
whisker, projection, irregularity) of approximately several μm. The existence of suchprojections is observed under various conditions. Those projections are thought tohave formed from the melting of electrodes due to an arc discharge, melting due tothe collision of micro particles, ion etching, the growth caused by electrostatic field oran electric field emission, or the elimination of impure particles.
An actually-measured leader current before a breakdown is verified to be an electricfield emission current by the fact that the F-N plot becomes linear. (Figure II.3
shows an example of F-N plot.) According to the gradient of this line, a value for β isdetermined.

41

JERG-2-213A

In general, a cathode having an area of a certain size has two or more sources of
emitting an electric field, which are projections. In this case, also, it is verified that Ie
plot is roughly linear based on experiment and theory. On the basis of the F-N plot,
the effective field enhancement factor and effective electric field emission area are
determined as effective and average factors. Even if a shape of projection isconstant, β fluctuates depending on inter-electrode distance d. If d becomes
smaller, it approaches the uniform electric field, and β decreases. Ultimately, whend = 0, the value becomes β = 1 regardless of the shapes of the projections. Figure
II.4 shows an example of d-β characteristics.
It is reported that if the micro electric field βE at the tip of projection reaches
approximately 5  107 V/cm, the electric field emission is restricted due to the spaceelectric charge effect of electric field emission electrons. In this case, the F-N plot
deviates from a line at high-electric field area as shown in Figure II.3.
Even under an extremely high vacuum on the order of 10-9 Torr, gas adsorption
molecules cannot be removed completely while the electric field emission is
influenced by gas adsorption of the emission source. The following effects of gas
adsorption are considered:
1) Fluctuation of Ie due to changes of work function φ and effective electric fieldemission areas.
2) Occurrence of superimposed noise on Ie

Breakdown

Micro discharged-Decrease
Dielectric breakdown

Figure II.2 Voltage-current
characteristics between electrodes invacuumFigure II.3 F-N plot in vacuum gap

42

Electric field multiply coefficientβ

JERG-2-213A

Gap length (cm)Figure II.4 Gap length in vacuum and electric field multiplication factor

II3 Influence of work function in metals
The electric field emission current Ie, is influenced by work function φ as it is clearlyfound by (II.4). If work function φ becomes greater, Ie becomes smaller. Table II.2
shows work functions of metals in general.
Table II.2 Work functions of metals
Metal

Work function φ(eV)Melting point
(K)

C

4.34

3800

Co

4.41

1490

Cs

1.38

302

Fe

4.21

1540

Mo

4.20

2630

Ni

4.01

1725

Pt

5.32

2047

Ta

4.10

3123

W

4.52

3655

II4 Surface treatment of electrode
Since discharge voltages (breakdown voltages) change depending on conditions of
the electrode surface, the following treatments are effective on electrodes,

43

JERG-2-213A

particularly for those used under high electric fields and under vacuum conditions:(1) Providing surface treatments:• Cleaning with solvent• Chemical etching• Cleaning by discharge• Inorganic coating(2) Performing conditioning• Current conditioning• Glow discharge conditioning• Spark conditioning

Voltage

Figure II.5 shows an example of current conditioning. Between electrodes placed ina vacuum, a voltage that is lower than an expected breakdown voltage is applied.
Then, after the current flow between the electrodes becomes stable, the voltage isincreased on a step-by-step basis. A leading current gradually drops while a voltage
is applied, and the frequency of current spikes and current pulses caused by microdischarge gradually decreases. According to this method, projections on electrodes,
adhesives on electrodes, and micro particles are removed.

Current

Time

Time

Figure II.5 Current conditioning
References[1] Hara and Akiyama, “High-voltage Pulse Power Engineering”, (Morikita
Publishing) (1991)

44

JERG-2-213A

Appendix III Influence of neutral molecular on discharging
III1 Paschen’s law in gas [1,3]A voltage that produces a spark discharge in a uniform electric field is referred to as aspark voltage. A spark voltage Vs is the function of product pd, of gas pressure p,
and gap length d, under a constant temperature of gas. Since Paschen discovered
this through experimentation, this is referred to as Paschen’s law.

Vs  B

pd
pd
B
Apd
const.  ln( pd )ln{}1ln(1  )••• (III.1)



Where, A and B are constants.
Figure III.1 shows the spark voltages of representative gasses as functions of pd.
With a constant gap length, lowering the pressure results in gradual drop of spark
voltages. This is because average free paths (the average value of the distance inwhich electrons in gas flies freely between successive two collisions) are prolonged,
and then acceleration by an electric field increases, encouraging collision andionization. The spark voltages show a minimum value at a certain pd value, andbecome sharply high at pressures lower than at that point. This is because average
free paths become too long, decreasing the number of collisions of electrons
between electrodes, making collisions and ionization inactive again.
By differentiation of equation (III.1) with pd, a minimum spark voltage (Vs)min(Paschen minimum) is found to be:(Vs ) min  2.718B
1ln(1  )
A••• (III.2)

Setting (pd)c as a pd that gives a minimum spark voltage, ((Vs)min):1
2.718 ln(1  )( pd ) c ••• (III.3)



A

Table III.1 shows an example of (Vs)min and (pd)c of an actual measurement.

45

JERG-2-213A

If the pressure drops below the Paschen minimum value, the discharge voltage
suddenly rises. Figure III.2 shows the dependency of discharge voltages onpressure in a parallel flat-plate electrode system under a high vacuum environment.

Spark Voltage(Max) [V]

Air

Air

Gap length  Atmospheric pressure (pd) [mm･Torr]
Figure III.1 Example of Paschen curve

Table III.1 Minimum spark voltages ((Vs)min) andpd that gives the value ((pd)c)
Gas(Vs)min (V)(pd)c (mm･Torr)

Air

330

5.67

H2

270

11.5

O2

450

7.0

N2

250

6.7

He

Approximately
156

Approximately
40

Ar

233

7.6

Ne

186

3.0

Na vapor

335

0.4

CO2

420

5.4

46

JERG-2-213A

Breakdown voltage(kV)

Plane-Plane Cupper
Electrode

Pressure (Torr)

Figure III.2 Dependency of inter-electrode discharge voltage
on pressure in vacuum [2]III2 Issues and measures on hot launch
During the lift-off of a spacecraft, pressure in the unit sharply drops from theatmospheric pressure (approximately 760 Torr). Turning on the power supply
mounted in the unit under those conditions, may cause a discharge according toPaschen’s law, depending on a level of voltage. Usually, the gas in the unit isthought to be air. According to Table III.1, conditions to produce a voltage at 330 Vor higher should be avoided.

References[1] The Institute of Electrical Engineers of Japan, Discharge Handbook Publishing
Committee, “Discharge Handbook” (Ohmsha) (2003)[2] R.V.Latham, “High Voltage Vacuum Insulation: The physical Basis”,(Academic Press, Inc.) (1981)[3] Otori, Sekiguchi, Kono, “Ionization Gas Study”, (The Institute of ElectricalEngineers of Japan) (1969)47

JERG-2-213A(blank)48

JERG-2-213A

Appendix IV Creeping distance and discharge voltage
IV1 Creeping distance and discharge voltage [1]Creeping discharge is significantly influenced by the alignment of electrodes. The
cases are roughly divided into those on a dielectric with back electrode and those
without a back electrode.
(1) Cases without a back electrodeIn the atmosphere, generally, flashover characteristics are similar in most cases tothose of metal electrodes in the air. Therefore, a creeping flashover voltage can be
expected on the basis of the data, or creeping distance can be found in the reverse.
However, it may be more complex if the distortion of electric field is significant due tothe existence of a dielectric, or if surface conditions are difficult. In general, in manycases, a creeping flashover voltage drops sharply due to dirt, damage or moisture on
the dielectric surface, under conditions of utility frequency voltage and direct current.
Sufficient margins must be given.
(2) Cases with a back electrode
Since these conditions are extremely complex, a flashover may occur at an
unexpected distance. Extra attention must be paid in the design phase. Flashovervoltages when applying a sharp rising impulse voltage span a significant range, andare well approximated by either of the following two equations. In other words,
setting a creeping flashover voltage as V [V], a specific capacitance as C0 [F/m] (C0 =ε/d, ε: Dielectric constant of dielectric, d: Thickness of dielectric), a creeping distanceas l [m], and a wave front steepness of a voltage as dV/dt [V/s]:

Kb

V 
8

C0

3

5 l

••• (IV.1)

Where, Kb is 73.6 against a positive impulse voltage and is 74.25 against a negative
impulse voltage.

Kd

V 
5

C 0 20

dV
dt

5 l

••• (IV.2)

Where, Kd is 135.5 against a positive impulse voltage and is 139.5 against a negative
impulse voltage.
49

JERG-2-213A(a) Influence of creeping distance
As shown in Figure IV.1, and Figure IV.2, a longer creeping distance does notnecessarily lead to a rise of creeping flashover voltage. In the figure, the solidline indicates the calculation by equation (IV.1), and the dotted line indicates thecalculation by an equation (IV.1). The circles indicate experimental values.
(b) Influence of dielectric constant
As shown in Figure IV.1, a greater dielectric constant leads to a drop of thecreeping flashover voltage.
(c) Influence of dielectric thickness
As shown in Figure IV.2, increasing thickness is effective in increasing a creeping
flashover voltage.
(d) Influence of specific capacitance
Figure IV.3 shows the relation between impulse creeping flashover voltages andspecific capacitances. On the basis of the gradient of the line, an index of
specific capacitance C0 in the equation above is found to be -0.4. The glass inFigure IV.3 is fiberglass.
(e) Comparison of creeping discharge characteristics between air and a vacuum
environment

Inpulse Creeping
Flash over voltage(kV)

Inpulse Creeping
Flash over voltage(kV)

Table IV.1 indicates creeping flashover voltages using various spaces. Figure
IV.4 shows the dependency of streamer length on pressure, found on the basis ofa creeping discharge figure. It indicates that a streamer length changes inproportion to the reciprocal of atmospheric pressure. In addition, at the same
voltage, a drop of atmospheric pressure leads to a higher rate of developing acreeping streamer.

Creeping Distance (m)Creeping Distance (m)Figure IV.1 Influence of dielectric
constant of dielectric (r1: Inner diameter,r2: Outer diameter)

Figure IV.2 Influence of thickness ofdielectric (r1: Inner diameter, r2: Outer
diameter)

50

Figure Diameter(mm)JERG-2-213A

Flash over voltage(kV)

Positive Polarity
Negative Polarity

Glass-Air
Smooth Ceramic Cylinder

Glass+ distilled water
Regular Ceramic
CylinderGlass+Ethyl alcohol

Positive Polarity

Negative Polarity

Fibre Cylinder
Titanium oxide Cylinder⊥/Pressure(1/Torr)
Capacitance(x10-10F/m2 ) (In case of l=0.1m)

Figure IV.3 Relation between creeping
flashover voltage and specificcapacitance

Figure IV.4 Relation between pressure
and streamer length (diameter/2)

51

JERG-2-213A

Table IV.1

Creeping discharge characteristics in vacuum andin air (Electrode: Stainless-steel SUS)

Lengthof
Spacer(mm)

Spacer
Diameter(mm)

Vacuum（10-6Pa）BreakdownElectricField（kV/mm）
AC
InpulseDC
1.2/50μs（peak）

Air（10-5Pa）BreakdownElectricField（kV/mm）
AC
InpulseDC
1.2/50μs（peak）

Material

Relative
permittivity

Teflon

2.1

2.0
4.0

7.0
7.0

16.0
10.9

15.1
10.3

12.8
8.5

2.75
2..45

2.80
2.50

2.80
2.45

Plastic
Glass

3.2

4.0

7.0

12.0

12.3

9.9

2.25

2.37

2.55

Quartz

3.8

5.0
6.5

9.5
9.5

8.6
8.2

8.6
8.3

7.8
7.1

2.9
2.7

2.9
2.7

2.7
2.7

Pyrex
Glass

4.6

2.0
4.0

6.35
6.35

16.5
11.5

15.5
11.1

14.0
9.9

3.75
2.36

3.75
2.45

3.75
2.63

Ceramic

5.8

2.0
4.0

7.0
7.0

16.6
12.6

16.2
12.5

14.25
10.25

2.50
2.50

2.50
2.45

2.75
2.63

Sapphire

12.0

3.1

12.55

9.0

8.4

7.3

1.81

1.87

1.94

εr

IV2 Concept of preventing creeping surface discharge
In general, a creeping barrier is used to prevent a creeping discharge. A creeping
barrier, as shown in Figure IV.5, is composed of the insulation base with a surface on
which the originally creeping discharge develops, and a creeping barrier installed
approximately perpendicular to the base, which is called a “shade” or a “pleat.” Theinsulation base, in practical application, corresponds to the “core” of a support
insulator or the “shell” of bushing. An electrode covering the insulation basemounted on one side is called a back electrode. A support insulator has no backelectrode, although it corresponds to the core electrode of a bushing. In general, acreeping discharge is likely to be longer in a structure with a back electrode. From
viewpoint of practical insulation, increasing the distance in creeping direction stillleads to a drop in the discharge voltage.
Also, it is effective to cover the whole creeping electrodes with coating. However,enough consideration must be taken to prevent the enclosure of a layer of air or theoccurrence of cracks due to its thermal history.

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JERG-2-213A

HV

electrode

HV

Creeping barrier
HV

Creeping streamer

Insulation base

Creeping streamer
Grounding electrode(Creeping streamer is reverse polarity to applied voltage)(a)

No Back electrode(c)

Back electrode grounded

Gap length
HV

HV

Back electrode(b)

Back electrode is connected to HV

Figure IV.5(d)Back electrode is insulated

Example of barrier insulation structure53

JERG-2-213A

References[1] The Institute of Electrical Engineers of Japan, Discharge Handbook Publishing
Committee, “Discharge Handbook” (Ohmsha) (2003)

54