MO SURGE ARRESTERS - METAL OXIDE RESISTORS AND SURGE ARRESTERS FOR EMERGING SYSTEM CONDITIONS

696
MO SURGE ARRESTERS -
METAL OXIDE RESISTORS
AND SURGE ARRESTERS
FOR EMERGING SYSTEM CONDITIONS
WORKING GROUP
A3.25
AUGUST 2017

Members
B. RICHTER, Convenor CH M. COMBER US
A. DELLALLIBERA BR R. GOHLER DE
F. GREUTER CH V. HINRICHSEN DE
M. HOLZER AT S. ISHIBE JP
Y. ISHIZAKI JP B. JOHNNERFELT SE
M. KOBAYASHI JP L. FAN CN
I.M. RAWI MY Y. SPACK-LEIGSNERING DE
M. TUCZEK DE M. NAKAJIMA JP
J. WOOWORTH US R. OSTERLUND SE
Corresponding Members
T.M. OHNSTAD NO
A.M. HADDAD UK
WG A3.25
Copyright © 2017
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“CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the
MO SURGE ARRESTERS – METAL
OXIDE RESISTORS AND SURGE
ARRESTERS FOR EMERGING SYSTEM
CONDITIONS
ISBN : 978-2-85873-399-6

MO SURGE ARRESTERS - METAL OXIDE RESISTORS AND SURGE ARRESTERS FOR EMERGING SYSTEM CONDITIONS
3
EXECUTIVE SUMMARY
In 1991 the first Technical Brochure about MO surge arresters was published (TB 60), describing
effects on gapless metal oxide surge arresters (MO arresters) from various electrical stresses
encountered in 3-phase a.c. systems.
The Working Group A3.17 of SC A3 took the task to evaluate the stresses on MO arresters and to
review the existing international standards about MO surge arresters. An experimental research
project was initiated to evaluate the energy handling capability of MO resistors under current impulse
stresses. The results were published in 2013 in TB 544.
The present Technical Brochure summarises in 9 chapters the work done by the Working Group A3.25
of SC A3.
Chapter 1, History of MO Surge Arresters, gives a very brief overview about the development of
gapless MO surge arresters. The first MO arresters, based on Matsushita’s patent on MO varistors,
were developed and installed in Japan in 1975 (Meidensha, 66 kV system). In the 1980s, the
performance of MO resistors was greatly improved with regard to protection performance, life
performance and energy handling capability. Leading manufacturers in Europe and USA started their
own development and production. In USA, GE started on the LV level and announced first LV MOV
elements based on the Matsushita licence in 1972 from where they moved on towards MO elements
for HV applications . In Europe, ASEA offered the first full HV arrester portfolio based on MO
technology in 1979/1980, including a first commercial HVDC arrester followed by the first UHV
arrester in 1981. In 1982 Siemens supplied their first MO surge arresters, a SF6 gas insulated (GIS)
arrester. There is no other arrester type where the introduction of MO resistors lead to such a
simplification of the design as for the metal-clad, SF6 gas insulated arresters. In 1994 the first 1050 kV
GIS arrester worldwide was delivered by ABB for the 1050 kV GIS pilot plant from ENEL in Italy.
Nowadays gapless MO surge arresters with porcelain, polymeric and metallic housings in various
designs are used in all system voltages as well as in special applications such as FACTS or HVDC
converters, in traction systems, as line arresters and many more.
Chapter 2, Energy Handling Capability of MO Resistors, continued with the experimental
research work initiated by WG A3.17, concentrating on the energy handling capability of MO resistors
when stressed with repeated and multiple current impulses of different wave shapes and changing
polarities. Main results of this multiple stress study are:
The mean sum failure energy, up to mechanical failure or according to the introduced “complex failure
criterion” of the tested MO resistors was equal for single long duration current energy injections and
for double long duration current energy injections with up to 3 s time interval between the two
impulses.
Energy injections with high current impulses of the wave shape 4/10 µs and high current densities can
lead to a dramatic change of the “characteristic voltage”, which is similar to the reference voltage and
reflects the U-I-characteristic in the leakage current range. These changes however do not seem to
affect the energy handling capability of the MO resistors measurably. They only have to be taken into
consideration regarding the determination of the thermal stability of surge arresters.
None of the analysed repeated energy injections (with a.c., long duration current impulses or 90/200
µs impulses) at energy levels, which are close to the rated energies of standard surge arresters,lead
to mechanical failures or relevant changes of the residual voltage or “characteristic voltage”. Thus the
thermal stability and the protective level of standard surge arresters should not be effected by the
above mentioned kind of repetitive energy impacts.
Chapter 3, Long Term Ageing of Metal Oxide Resistors. The present report is one of the first to
address the long-term stability of current MO-resistors in a broader context, including a comparison of
the different international standards, unique ageing results extending over more than 10 years of
continuous accelerated testing, an experimental survey on the stability of commercial MO resistors
available today on the market as well as a discussion of recent new test requirements under
demanding applications like AC-stressing at or above the reference voltage and in d.c. systems.
Chapter 4, High Field MO Resistor Development in Japan, summarises the development and
application of “high field” (HF) MO resistors. As the field strength is increased, energy handling

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capability, thermal stability and voltage withstand capability of MO resistors are generally decreased.
But these disadvantages can be technically covered, for example, by increasing the resistor diameter,
by using heat sinks, or by making use of of the enhanced cooling of the resistors in insulating gases
like SF6 in GIS arresters. Therefore the advantages and disadvantages of increasing field strength are
dependent on the type of surge arrester and manufacturer’s design philosophy. Examples of designs
with HF resistors are given.
Chapter 5, Simulation Approaches for Resistors and Arresters. In this chapter modeling and
simulation of MO resistors and arresters are addressed. Simulation of MO resistor and arrester
behavior is especially challenging for different reasons. The geometric scales vary over many orders of
magnitude - from the internal microstructural dimensions of the MO material up to that of complete
UHV arresters of more than ten meters height. The MO material itself has distinct non-linear
behaviour, all electric and dielectric properties are dependent at least on the applied electric field and
on temperature. In general, electrical, thermal and mechanical coupled problems have thus to be
solved. It further makes a difference if the MO resistor/arrester shall be investigated for its behavior in
the transient voltage limiting or in the continuous operation mode. First simulation results for
complete arresters are shown as examples. The chapter highlights the current state of simulation
approaches, which are quickly developing.
Chapter 6, Insulation Withstand - Tests and Calculations. The intention of this chapter is to
discuss the possibility of calculating the external withstand voltage of an MO arrester housing as an
alternative to testing, especially in the case of EHV and UHV MO arresters.
As for any other piece of electrical equipment in transmission and distribution systems a surge
arrester must function properly without any breakdown of external insulation when exposed to
lightning current impulses, switching current impulses or power-frequency overvoltages. To verify the
adequacy of the insulation, tests must be performed, providing that the tests represent as closely as
possible the voltage distributions that would be the case in a field service situation when the arrester
is exposed to these types of stresses. Especially for high voltage multi-unit arresters, a very extensive
series of tests may be necessary to verify the adequacy of the insulation for all possible design
configurations.
In certain cases, where dry arcing distances (air gaps) across arrester units or from line end of the
arrester to ground are very large, it may be possible to assert that the arrester has inherently
sufficient insulation withstand strength without the need for testing. This would be possible if, for a
given arrester design, the rules proposed are adhered to. This approach is considered to be an
indirect means for verifying insulation withstand requirements and has been adopted in the current
version of the standard IEC 60099-4, Ed. 3.
Chapter 7, MO Surge Arresters for UHV Systems. Though not being a standardized term, “UHV”
stands for all system voltages of Us > 800 kV or highest voltage for equipment of Um > 800 kV.
Standardized rated UHV system voltages are 1100 kV, implemented e.g. in China, and 1200 kV,
intended for the future Indian UHV grid. Basically the surge arresters in these systems are “standard”
arresters. However, some requirements need special considerations. Main concerns are the excessive
height of more than ten meters, resulting in axial voltage, power and temperature unbalance effects
(apart from mechanical problems), the necessary low switching impulse protection level, resulting in
very high energy handling requirements, the huge number of MO resistors in the arresters, requiring
very low failure probabilites of the individual MO resistors during energy injection, and the limited
protection distance, because part of the typically provided protection distance is used by the arrester
and its installation itself. Most of these problems have been covered by the latest standard IEC 60099-
4, Ed. 3. It should, therefore, not be an issue any more to qualifiy “UHV” arresters by standardized
type tests. There are still some issues, though, which do not require standardization but nevertheless
ask for careful consideration and even some further research work.
Chapter 8, Line Surge Arresters. Line surge arresters (LSA) are commonly used to address
lightning-related phenomena with the intent to improve the overall reliability of transmission lines.
LSAs have to avoid uncontrolled flashovers of line insulators in order to prevent earth faults and short
circuits within the system component “transmission line”. Two types of LSAs are used: non gapped
line arresters, NGLAs, and externally gapped line arresters, EGLAs. Each approach has its merits.
Benefits and disadvantages of both types are explained, and appropriate applications are shown, as
well as test procedures for EGLAs. Realized applications and their effect of increasing the power
system reliability are given.

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Chapter 9, Consequences of the Investigations Performed by WG A3.17 and A3.25. This
chapter points out how the work of the CIGRE Working Groups A3.17 and A3.25 influenced the latest
revision of the international standard IEC 60099-4. A completely new classification system for MO
surge arresters has been introduced, replacing the former Line Discharge Classes (LDC) by a new
concept of charge transfer and energy handling schemes. Consequently, new definitions were
introduced and test procedures had to be developed. Some aspects, as for instance the realistic and
quantitative simulation of the time-, voltage- and temperature-behavior of MO resistors and MO
arresters and the long term performance of MO resistors, are still under discussion and need further
investigations and research.

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CONTENT
EXECUTIVE SUMMARY ............................................................................................................................... 3
FOREWORD...............................................................................................................................................11
1. HISTORY OF MO SURGE ARRESTERS........................................................................................13
1.1 INTRODUCTION................................................................................................................................................................13
1.2 FIRST MO SURGE ARRESTERS DEVELOPED IN JAPAN.............................................................................................13
1.3 MO SURGE ARRESTERS IN USA AND EUROPE, SOME MILE STONES..................................................................14
2. ENERGY HANDLING CAPABILITY OF MO RESISTORS...........................................................15
2.1 SUMMARY..........................................................................................................................................................................15
2.2 INTRODUCTION................................................................................................................................................................15
2.3 RESULTS, WHICH WERE INITIATED BY A3.25............................................................................................................16
2.3.1 Test Setup..................................................................................................................................................................16
2.3.2 Single Impulse Energy Handling Capability......................................................................................................16
2.3.3 Double Impulse Stress.............................................................................................................................................23
2.3.4 Energy handling capability of repetitive stresses............................................................................................26
2.4 CONCLUSION AND OUTLOOK....................................................................................................................................43
3. LONG TERM AGEING OF METAL OXIDE RESISTORS ............................................................45
3.1 BACKGROUND OF THE AGEING TEST IN THE IEC STANDARD ............................................................................45
3.2 COMPARATIVE OVERVIEW ON EXISTING STANDARDS........................................................................................46
3.3 OVERVIEW ON THE AGEING CHARACTERISTICS OF TODAY`S COMMERCIAL MO RESISTORS: AN
EXPERIMENTAL STUDY.....................................................................................................................................................51
3.4 VERY LONG-TERM ACCELERATED AGEING TESTS OF MO RESISTORS: UNIQUE EXAMPLES........................56
3.5 ACCELERATED AGEING TESTS (AC) AT OR ABOVE REFERENCE VOLTAGE.......................................................58
3.6 ACCELERATED AGEING TEST UNDER D.C.-OPERATION .........................................................................................62
3.7 PRESENT KNOWLEDGE OF THE LONG-TERM AGEING PHENOMENA OF MO RESISTORS..........................67
3.8 CONCLUSIONS, OPEN ISSUES AND SUGGESTIONS FOR FUTURE WORK.......................................................74
4. HIGH FIELD MO RESISTOR DEVELOPMENT IN JAPAN...........................................................77
4.1 CONCEPT OF HF RESISTORS.........................................................................................................................................77
4.2 GENERAL EFFECTS OF INCREASING FIELD STRENGTH OF MO-RESISTORS......................................................78
4.2.1 Protection performance..........................................................................................................................................78
4.2.2 Ageing performance ..............................................................................................................................................79
4.2.3 Energy absorption capability...............................................................................................................................80
4.2.4 Thermal stability ......................................................................................................................................................81
4.2.5 Voltage withstand capability along resistor .....................................................................................................81
4.3 ADVANTAGES AND DISADVANTAGES OF INCREASING FIELD STRENGTH......................................................82
4.4 EXAMPLE OF SPECIAL CONSIDERATION TO OPTIMIZE PERFORMANCE OF HF RESISTORS .........................83
4.4.1 Optimization of composition of materials..........................................................................................................83
4.4.2 Improvement on energy absorption capability ................................................................................................85
4.5 APPLICATION EXPERIENCE OF HF RESISTORS TO SURGE ARRESTERS ...............................................................87
4.6 CONCLUSION..................................................................................................................................................................89

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5. SIMULATION APPROACHES FOR MO-RESISTORS AND ARRESTERS ..................................91
5.1 INTRODUCTION AND OVERVIEW ...............................................................................................................................91
5.2 MODELING AND SIMULATION OF INDIVIDUAL MO RESISTOR ELEMENTS .......................................................94
5.2.1 Equivalent circuit models........................................................................................................................................94
5.2.2 FEM Models and Simulation Examples ...............................................................................................................95
5.3 MODELING AND SIMULATION OF COMPLETE MO SURGE ARRESTERS.............................................................99
5.3.1 MO Arresters in the Transient Voltage Limiting Mode....................................................................................99
5.3.2 MO Arresters in the Continuous Operating Mode ...........................................................................................99
5.3.3 Modeling and Simulation Approaches for MO Arresters in the Continuous Operating Mode ........... 102
5.3.4 Simulation of a Substation Class Arrester in Continuous Operation.......................................................... 111
5.4 CONCLUSION AND OUTLOOK................................................................................................................................. 113
6. INSULATION WITHSTAND – TESTS AND CALCULATIONS................................................. 115
6.1 INTRODUCTION............................................................................................................................................................. 115
6.2 ASSESSMENT OF SURGE ARRESTER INSULATION WITHSTAND........................................................................ 116
6.2.1 Lightning impulse withstand................................................................................................................................ 118
6.2.2 Switching impulse withstand............................................................................................................................... 120
6.2.3 Power frequency withstand................................................................................................................................ 122
6.3 SUMMARY....................................................................................................................................................................... 123
7. MO SURGE ARRESTERS FOR UHV SYSTEMS ........................................................................ 125
7.1 CONSEQUENCES OF EXCESSIVE HEIGHT............................................................................................................... 127
7.1.1 Non-uniform axial potential, power and temperature distribution........................................................... 127
7.1.2 Dielectric withstand of the housing ................................................................................................................... 129
7.2 CONSEQUENCES OF LOW SWITCHING IMPULSE PROTECTION LEVEL......................................................... 129
7.3 CONSEQUENCES OF LIMITED PROTECTION DISTANCE...................................................................................... 131
7.4 CONCLUSIONS AND OUTLOOK............................................................................................................................... 132
8. LINE SURGE ARRESTERS............................................................................................................ 135
8.1 INTRODUCTION............................................................................................................................................................. 135
8.1.1 Non-gapped Line Arrester (NGLA).................................................................................................................. 136
8.1.2 Externally Gapped Lines Arresters (EGLA) .................................................................................................... 136
8.1.3 IEC Standard 60099-8 for EGLA..................................................................................................................... 138
8.2 APPLICATION OF LINE SURGE ARRESTERS ............................................................................................................. 144
8.2.1 General considerations and literature............................................................................................................. 144
8.2.2 Application of line arresters in the Tenaga Nasional Berhad (TNB) transmission network .................. 145
9. CONSEQUENCES OF THE INVESTIGATIONS DONE BY WG A3.17 AND A3.25......... 147
9.1 INFLUENCE ON STANDARDIZATION ........................................................................................................................ 147
9.2 FURTHER WORK NEEDED ............................................................................................................................................ 150
CONCLUSION ........................................................................................................................................ 151
REFERENCES............................................................................................................................................ 153
APPENDIX A............................................................................................................................................ 165

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Abbreviation/Acronyms Definition
AC Alternating current
AIS Air insulated switchgear
CCOV Crest value of the continuous operating voltage
CFD Computational Fluid Dynamics
CIGRE International Council on Large Electric Systems
(in French: Conseil International des Grands Réseaux Électriques
DC Direct current
DCOV D.C. component of continuous operating voltage
ECOV Equivalent continuous operating voltage
EGLA Externally gapped line arrester
EHV Extra high voltage
EM Electromagnetic
EMTP Electro Magnetic Transients Program
EQS Electro-quasistatic
EQST Electro-quasistatic-thermal
FACTS Flexible alternating current transmission system
FEM Finite element method
FFO Fast-front overvoltages
GB Guobiao standards, traditional Chinese
GFD Ground flash density
GIS Gas insulated switchgear
HF High field
HV High voltage
HVDC High voltage direct current
IEC International Electrotechnical Commission
IEEE Institute of Electrical and Electronics Engineers
JEC Japanese standard
LDC Line Discharge Classes
LIPL Lightning impulse protective level
LIWV Lightning impulse withstand voltage
LSA Line surge arrester
LV Low voltage
MO Metal oxide
MOSA Metal oxide surge arrester
MT Maintenance team
MOV Metal oxide varistor
NGLA Non gapped line arresters
PCOV Peak value of the continuous operating voltage

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PECOV Peak value of equivalent continuous operating voltage
SC Study Committee
SF6 Sulphurhexafluoride
SFO Slow-front overvoltages
SiC Silicon carbide
SIPL Switching impulse protective level
SPICE Simulation Program with Integrated Circuit Emphasis
SVU Series varistor unit
TC Technical committee
TEM Transverse-electric-magnetic
TFR Tower footing resistance
TLA Transmission line arrester
TNA Transient network analysis
TOV Temporary overvoltages
TSC Thermally stimulated discharge current
UHV Ultra high voltage
VFFO Very-fast-front overvoltages
WG Working Group
ZnO Zinc oxide

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FOREWORD
The first CIGRE Technical Brochure on surge arresters was published in 1991 (TB 60) describing
effects on gapless metal oxide surge arresters (MO arresters) from various electrical stresses
encountered in 3-phase a.c. systems. Since then, continued improvements in material and equipment
technologies coupled with the interest of the deregulated power industry in maximizing utilization of
existing infrastructure has revolutionized MO surge arrester applications and their expected
performance in a higher stressed system environment.
The Working Group A3.17 of SC A3 took the task to evaluate the stresses on MO surge arresters and
to review the existing tests and procedures in the international IEC standards and several national
standards. Further on, the current state of MO surge arrester design was investigated as well as
various applications in different types of electrical networks. Emphasis was given to the MO resistors
as the active part of the MO surge arresters. An experimental research project was started to
investigate the energy handling capability of the MO resistors under different impulse current stresses.
The results of the WG A3.17 were published in August 2013 in the TB 544 “MO Surge Arresters,
Stresses and Test Procedures.”
The Working Group A3.25 of SC A3 continued with the research project on energy handling capability
of MO resistors and addressed further aspects of MO resistors and surge arresters under emerging
system conditions. In particular, the following items were investigated and reported in the present
Technical Brochure:
- Energy handling capability of MO resistors under multiple and repeated impulse stresses
- Long-term performance of MO resistors
- Development and application of “high field” MO resistors
- Current state of simulation approaches for MO resistors and surge arresters
- Possibility of calculation of insulation withstand as an alternative to testing
- MO surge surge arresters for UHV systems
- Application and testing of line arresters
In addition, a brief history of MO surge arresters is given in the first chapter and in the last chapter it
is shown how the results of the research projects and experimental investigations of the two Working
Groups influenced the international standardization of MO surge arresters within IEC. Necessary future
work is addressed as well.
The appendix gives an overview about CIGRE Technical Brochures related to MO surge arresters and
their application.
Considering the current development and discussions in the field of MO surge arresters, specific
subjects have been addressed in more detail than others. The results of the research project on
energy handling capability of MO resistors and surge arresters lead to a completely new classification
system for MO resistors and arresters with new definitions and test procedures. The long-term
performance of MO resistors was for the first time addressed in a broader way, which also resulted in
new test requirements and procedures in the IEC standard. Simulation approaches for MO resistors
and complete surge arresters are described in detail, considering combined electrical and thermal
problems and giving examples for simulations of a complete MO surge arrester for the 500 kV system.
The content of this Technical Brochure was discussed and agreed by the members of the Working
Group. The individual sections were written by one or more authors in charge. Each section starts
with a short introduction to the specific subject and ends with a short conclusion. Therefore, each
chapter can be read by itself without necessarily reading the complete Technical Brochure.
In the large number of publications on MO resistors and MO surge arresters different wording is used
for basically the same object (ZnO varistor, ZnO resistor, MO varistor, MO resistor, varistor, ZnO or
MO arrester, etc.). This has historical reasons and also depends on the technical community or the
kind of research and development performed. Just as an example, the term “varistor” stands for an
individual non-linear component of an arrester in one technical community and for a complete arrester
bank made up of tens of arresters connected in parallel in another one. In this Technical Brochure,
the technical terms MO resistor and MO surge arrester are used, following the wording in the
international standards of IEC TC 37 that are directly related to the subjects of this Technical

12
Brochure. Few exceptions are made only if necessary, e.g. in citations and the reference list, where
the original titles are given.

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1. HISTORY OF MO SURGE ARRESTERS
Author in charge: Bernhard Richter
1.1 INTRODUCTION
This chapter contains a brief, and for sure incomplete, overview about the history of surge arresters,
focusing on gapless MO surge arresters for a.c. systems.
Lightning protection of structures began with the invention of the lightning rod in the 1750s. The first
telegraph systems arrived about 1837, and in the following years the first devices for lightning
protection were invented and installed on telegraph lines. These gapped devices were then for the
first time called “arresters”. Surge arresters for the protection of electrical power systems were first
developed and installed in the 1880s. Since then were several steps in design and technology until in
the 1970s the first gapless metal oxide surge arresters (MO arresters) appeared in the market, and in
the 1980s the first completely molded polymer housed arrester for application in medium voltage
systems. For a brief summary of the history of overvoltage protection and arrester design in the USA
see [Woo 2011].
1.2 FIRST MO SURGE ARRESTERS DEVELOPED IN JAPAN
The importance of surge arresters in Japan had been increased for the purpose of improving system
reliability against the background of the rapid growth of power networks in the 1950s and later. Valve
type arresters with silicon carbide (SiC) resistors and series gaps had been mainly used instead of
Aluminum-cell surge arresters and Oxide-film surge arresters used before. The great efforts had been
made to improve the performance of gaps for a lower protection level and superior cut-off of the
follow current. Current-limiting gaps by extending the arc with the flux of its current or moving the arc
with the flux of magnets had been developed and used in the 1970s. These arresters seemed to be
close to the ultimate surge arrester under the technology at that time.
In 1968, Matsushita Electric Industrial Co., Ltd. developed ZnO varistors for application in television.
The study on the applicability of the varistor for electronic applications to high voltage surge arresters
was started in 1970, and then the world's first Metal-oxide surge arrester (MOSA), developed and
produced by Meidensha, was installed in the 66 kV system of the Kyushu Electric Power Company in
1975, see Figure 1.1, [Hay 2008] and [Kob 2016]. Due to the various advantages in protection level,
durability, anti-pollution performance, simple construction and compactness, Japanese manufacturers
started to develop MOSAs and the higher rated MOSAs were developed in a short period in Japan. The
first 500 kV GIS-MOSA was installed in 1978 in the system of the Kyushu Electric Power Company,
and the first porcelain type 500 kV MOSA was installed in 1979 in the system of the Kansai Electric
Power Company.
In the 1980s, the performance of MO resistors was greatly improved on protection performance, life
performance and energy absorption capability, which made it possible to decrease the residual voltage
of arresters for 66–500 kV systems in Japan by 15–30%. As a result, the lightning impulse withstand
voltages (LIWV) of switchgears and transformers were reduced and their compact and economical
designs were realized in Japan. Moreover, their applications had been diversified: MOSA in 6.6 kV pole
mounted transformers since 1985 and externally gapped transmission line arresters (EGLA) since
1987.
Meanwhile, the concept of increasing field strength, reference voltage per unit thickness, of MO
resistors was developed and presented in 1984. In the early 1980s, the field strength of normal MO
resistors was generally in the level of 2 kV/cm at a given reference current. Since then, the high field
resistors (HF resistors) of approximately 2.7-6 kV/cm have been used in various types of surge
arresters in Japan, GIS arresters, transmission line arresters, and liquid-immersed arresters in 6.6 kV
distribution apparatus.

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Figure 1.1 The first MO surge arrester in the world: MOSAs at Hayato SS of Kyushu Power Co: 66 kV
heavy pollution type, Japan 1975.
1.3 MO SURGE ARRESTERS IN USA AND EUROPE, SOME MILE STONES
Following the invention of the ZnO based MO resistors and the first gapless MO surge arresters put in
service in Japan, development, based on the Matsushita patent, started also in Europe.
In 1972 BBC (now ABB) started the development of MO resistors based on their own recipe. In 1976
the first HV MO resistors were available, and 1980 the first gapless MO surge arresters. In the same
year ASEA (now ABB) entered the market with a complete HV surge arrester portfolio.
In USA, GE started on the LV level and announced first LV MO resistor elements based on the
Matsushita licence in 1972 from where they moved on towards MO elements for HV applications.
In 1982 Siemens supplied their first MO surge arrester, a SF6 gas insulated (GIS) arrester. There is no
other arrester type where the introduction of MO resistors lead to such a simplification of the design
as for the metal-clad, SF6 gas insulated arresters: omitting of N2 space, sealed tube and additional
capacitive grading resulting in a reduction of both the arrester diameter and length and finally the
arrester costs. In the same year ASEA came up with a gapless GIS MO surge arrester for a 400 kV
system. In 1994 the first 1050 kV GIS arrester worldwide was delivered by ABB for the 1050 kV GIS
pilot plant from ENEL in Italy.
Nowadays gapless MO surge arresters with porcelain housings, polymer housings and metallic
housings in various designs are used in all system voltages, in traction systems, in wind power parks
and a lot of other specific applications.
In 1979 the technical committee 37 of IEC decided that the test standard for gapless MO surge
arresters will not be published as an additional chapter within the existing standard for gapped
arresters, but that there is a need for a separate standard. For preparation of this standard the new
Working Group WG4 (MT4 since 2002) was founded. The first edition of IEC 60099-4 was published in
1991.
Influenced by the research work on MO resistors and arresters, initiated by WG A3.17 and WG A3.25
of CIGRE SC A3, a completely revised edition 3.0 of IEC 60099-4 was published in 2014.

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2. ENERGY HANDLING CAPABILITY OF MO
RESISTORS
Authors in charge: Max Tuczek and Volker Hinrichsen
2.1 SUMMARY
This part of the Brochure covers the energy handling capability of MO resistors. In [Cig 2013a], the
single impulse energy handling capability of MO resistors has been discussed; within this section, the
results of [Cig 2013a] are summarized and subsequently expanded by new investigations on the
multiple energy handling capability. The CIGRE WG A3.25 supported and supervised a research
program at Technische Universität Darmstadt where recent MO resistors from seven different well
established manufacturers were stressed with single and multiple energy injections to analyze the
limits of energy handling capability.
The investigations were carried out on MO resistors of approximately 60 mm in diameter and 45 mm
height, as typically applied in HV surge arresters and of approximately 40 mm in diameter and 45 mm
height, as used in high duty distribution surge arresters. The energy for the measurement of the
single impulse energy handling capability and the multiple stresses were injected using a.c. (50 Hz),
long duration current and double exponential energy injections (mainly with standard impulses).
In the first section, the single impulse energy handling capability of different MO resistor sizes at
different current density with different failure criterions are analyzed and determined. After that, the
differences between single and double impulse energy handling capability (the second impulse being
delayed up to 3 s) are depicted. For the long duration energy injection at the used current density, no
difference between the energy handling capability of the single and the double energy injections
(given that the energy impact of the two impulses is totaled up) could be observed. In the following,
repeated energy stresses, between which the MO resistors cooled down to ambient temperature, were
examined. Measuring the single energy handling capability, it was observed that double exponential
energy injections with very high current densities lead to serious changes in the pre-breakdown region
of the U-I-characteristic. However, at three makes, it could be shown that this change does not seem
to affect the energy handling capability. To get further information on the durability/fatigue of MO
resistors caused by energy injections, energy injections up to 100 pre-stresses per MO resistor were
repeatedly carried out by using a.c. and long duration energy injections. These investigations show
that the amount of repeatedly injectable energy is close to the single impulse energy handling
capability for a.c. energy injections (with small current density) where it differs from impulse energy
injections (with higher current density). These results were further examined by observing the effect
of different types of energy injections (with different current densities, impulse length and polarity
sequences) on the same energy level regarding the change of the U-I-characteristic and the energy
handling capability.
2.2 INTRODUCTION
The energy handling capability is an important ratio of an MO arrester, but the value is different for
different kinds of stresses. Due to the reason that the voltage in modern MO surge arresters stays at a
certain level in the event of an overvoltage, an MO resistor had to withstand the current flow of the
surge (or the transferred charge) and also the absorbed energy. The absorbed energy is converted
into heat which can damage the MO resistor directly (if an inhomogeneous temperature distribution
causes partially melted MO resistor bulk material) or indirectly if the temperature of the MO resistor in
combination with the applied system voltage leads to a thermal runaway of the MO surge arrester.
These different aspects of energy handling capability are commonly divided into:
 "impulse" energy handling capability (the impulse damages the MO resistor directly),
o "single" impulse stress,
 withstand values (deterministic approach),
 values related to a certain failure probability (statistical approach),
o "multiple" impulse stress, i.e. impulses in time intervals too short to obtain an
approximately uniform temperature distribution in the MO resistors,

16
o "repeated" impulse stress, where the time interval between impulses is long enough
to obtain a cooling of the MO resistors close to their initial temperature (this includes
durability and fatigability aspects),
 "thermal" energy handling capability (the MO resistor is not damaged directly by the energy
injection of a surge but by additional stress of the applied system voltage).
The different aspects of the energy handling capability as well as the determination of the single
energy handling capability are described in detail in the Technical Brochure of A3.17 [Cig 2013a]
(section 3). In the following chapter, a short overview of these previously published results of the
single energy handling capability will be given and extended by results regarding multiple energy
injections is presented in 2.3.3 to 2.3.4.
For these studies (including the measurements of the single energy handling capability), more than
4000 MO resistors were tested at Technische Universität Darmstadt. This could only be achieved with
the help of the members of A3.25.
2.3 RESULTS, WHICH WERE INITIATED BY A3.25
2.3.1 Test Setup
All tests which are presented in the following chapter were performed with commercially available MO
resistors of normal field strength (no “high field MO resistors”) from seven well-established American,
European and Japanese manufacturers. Two different sizes of MO resisters were used:
Size 1: The height is 40 mm to 45 mm (except for one make of only 26 mm) and their diameter
around 60 mm (which are typically applied in 10 kA station class arresters of line discharge class 3,
acc. IEC 60099-4, Ed. 2.2)
Size 2: The height is from around 30 mm to roughly 40 mm and their diameter around 40 mm
(typically applied in 10 kA distribution class arresters)
To anonymize the results in each of the following sections special characters (e.g. T-Z, α-γ, I-III, 1-2
etc.) are used/introduced to indicate the different makes of different manufacturers. There is no
relation between these characters and those of other sections.
The MO resistors were tested in a pneumatic test fixture with a contact force of 3,0 N/mm² which
prevents it from bouncing off the electrodes during the energy impact. To ensure comparable contact
conditions and avoid flashovers starting at the contact electrode, new aluminum discs of 5 mm
thickness, 1-2 mm smaller in diameter than the tested MO resistor and with rounded edges were used
for each test. To reduce the heat flow from/through the MO resistor, a thermal insulation (fiber
silicate) was installed between the test fixture and the contact electrode.
2.3.2 Single Impulse Energy Handling Capability
The Single impulse energy handling capability as a function of current density was first published by
[Rin 1997]. He found out that the single impulse energy handling capability increases with increasing
current density of the energy impact. This dependence could be confirmed in a study which was
conducted by the CIGRE WG A3.17 ten years later. The study was published in [Cig 2013a], [Rei
2008a], [Rei 2008], [Tuc 2009] and [Hin 2009] and will be summarized within this section. For this
study, more than 3000 pieces of commercially available MO resistors from seven well established
American, European and Japanese manufacturers were tested.
In contrast to earlier studies, the single impulse energy handling capability was measured not only in
terms of mechanical failure of the MO resistor, but a “complex failure criterion” was introduced. This
“complex failure criterion” takes a change of the U-I-characteristic into account as failure mechanism.
A change in the U-I-characteristic can occur because of non-visible pre-damages and/or because of
degradation of the U-I-characteristic in consequence of the energy impact. Such kind of failure could
result in a failure during the next energy impact, or, in the case of degradation of the U-I-
characteristic it could jeopardize the thermal stability of an entire arrester. The “complexe failure
criterion” was introduced and described in detail in CIGRE TB 544 MO Surge Arresters – Stresses and
Test Procedures [Cig 2013a].

17
The flowchart of the test and evaluation procedure is shown in Figure 2.1. At the beginning of each
test sequence, all test samples (for each current level of each manufacturer at least 40…50; for some
sequences, up to 80 MO resistor samples were tested) were marked unambiguously and afterwards
an initial measurement was performed to gather information on the characteristics of the MO resistor
at two current levels. In the low current region (leakage current range), the so called “characteristic
voltage” was measured at a peak current density of 0,12 mA/cm² (50 Hz), five seconds after the
application of voltage and in the high current region, the residual voltage was measured at a nominal
discharge current of 10 kA, 8/20 µs. The “characteristic voltage” is, for comparison reasons, a
normalized voltage independent of the size of the tested MO resistors [Cig 2013a]. It is not to be
mixed up with the reference voltage Uref defined by manufacturers for their specific MO resistors.
Afterwards, the energy impact was performed. An overview of the different energy impacts which were
performed with the two MO resistor sizes is given in Table 2.1.
Table 2.1 Energy impacts performed with the two different MO resistor sizes to determine
the single impulse energy handling capability
Energy impact Size 1 Size 2
Alternating
current
(50 Hz)
≈10 A performed
≈100 A performed
≈300 A performed
Long -
duration
current
4 ms performed performed
Double
exponential
current
90/200 µs performed performed
4/10 µs performed
After the energy impact, the MO resistors were cooled down to ambient temperature. If they showed
no visual damage, an exit measurement was performed. Within these exit measurements, the two
initial measurements and an additional 8/20 µs current impulse with a current density of 1.5 kA/cm²
were performed. By this last high current impulse, MO resistors which were pre-damaged but did not
fail during the previous energy impact could be identified. A MO resistor does not pass an energy
injection if it failed mechanically during the energy impact, changed its residual voltage or its
“characteristic voltage” more than 5 % or failed mechanically during the exit measurement. The
mechanical failures were distinguished in cracking, flashover or puncture. This procedure was time
consuming, but in comparison to earlier studies (which were often performed up to mechanical failure
of the MO resistor), different failure modes of the different energy impacts could be observed.
Due to the restrictions of the test setup, the energy impacts with alternating current could only be
performed up to the mechanical failure of the tested MO resistor. Based on the two different failure
criteria for different energy impacts, two different statistical evaluation methods were used to
calculate the mean failure energy at a defined current level (these statistical evaluations are described
in [Rei 2008]).

18
The results of the single impulse energy handling capability are shown in Figure 2.2, where the mean
failure energy is plotted over the amplitude of the current density (on average, the coefficient of
variation is smaller than 10 %). In the current range up to 10 A/cm², the mean failure energies were
determined with alternating current energy injections (up to mechanical failure of the MO resistor). In
the range of 100 A/cm², long duration current energy injections were used and in the current range
of 1000 A/cm², the currents were injected using a 90/200 µs double exponential current impulse.
Each dot in this diagram represents a calculated mean failure energy which was determined by
impressing a specified current level to at least 40 new MO resistors. The differently colored lines
Impulse test
(energy injection)
Visual inspection:
mechanically failed?
OK defect
Initial measurements
Uch,1 at Jch = 0,12 mA/cm² (after 5 s)
Ures,1 at I = In
ch,1 ch,2 ch,195% 105% ?U U U   
Measurement of characteristic voltage
Measurements at lightning current impulse
Ures,2 at I = In
Imd at J = 1,5 kA/cm²
Exitmeasurements
yes
no
no
yes
yes
res,1 res,2 res,195% 105% ?U U U   
no
yes
no
Visual inspection:
Impulse test
(energy injection)
Visual inspection:
OK defect
Initial measurements
Ures,1 at I = In
ch,1 ch,2 ch,195% 105% ?U U U   
Measurement of characteristic voltage
Measurements at lightning current impulse
Ures,2 at I = In
Imd at J = 1,5 kA/cm²
Exitmeasurements
yes
no
no
yes
yes
res,1 res,2 res,195% 105% ?U U U   
no
yes
no
Visual inspection:
Figure 2.1 Flowchart of the test and evaluation procedure, [Cig 2013a]

19
represent different resistor makes of several manufacturers which were tested. The orange line shows
the results of [Rin 1997] of a similar study in which one make was taken into consideration ([Rin
1997] and tested always up to mechanical failure. This is the reason why the results in the current
density range up to 10 A/cm² can be directly compared.
The following conclusions can be drawn from Figure 2.2:
 The mean failure energies vary approximately from 400 J/cm³ up to 1200 J/cm³, for very fast
impulses even up to 1700 J/cm³. Additional evaluation shows that the transferred charge is
also not constant but increases with increasing current density of the energy impact. It should
be noted that these energy values must not be confused with the rated (design) energy of
standard surge arresters, which is in the range of 200 J/cm³.
 The tendency of an increasing energy handling capability with increasing current density of
the energy impact (which was for the first time reported in [Rin 1997]) can be confirmed. It
should be considered that, for long duration and double exponential energy impacts, the
“complex failure criterion” was applied, which leads to lower energy values compared to
testing up to mechanical failure (which will be shown later in this section).
 At very high current densities, two MO resistors show a lower energy handling capability of
approximately 500 J/cm³ (which is, however, far above the rated (design) energy of standard
surge arrester, as already mentioned). The reason for this behavior is the coating of these MO
resistors which was obviously not designed to handle the high voltages which occur at the
applied current densities. In this case, the performance of the ZnO bulk material does not
limit the energy handling, but the coating does. The 90/200 µs impulse is a rather new
current stress in [IEC 2009] and therefore the MO resistors “U” and “S” might not be
optimized for this kind of stress (this stress is primarily relevant for transmission line arrester).
 Furthermore, an interesting outcome of the evaluation of the failure modes of the different
energy stresses was that the Ures at î = 10 kA was nearly never affected by any kind of energy
stress. The residual voltage thus seems to be a very weak indicator concerning the aging by
Figure 2.2 Mean failure energy vs. amplitude of current density for “Size 1” MO resistors, [Cig 2013a]

20
energy impact and this is why it was not taken into consideration for the later described multi-
stress studies. However, the “characteristic voltage” seems to be a sensitive indicator and
occurs often at very fast energy impacts with high current densities as it is described in [Cig
2013a].
To determine the impact of the “complex failure criterion”, a study (on new blocks) was started in
which the energy handling capability (long duration and 90/200 µs impulses) of the make “T” were
tested up to mechanical failure. The research was implemented in the same way the study of [Rin
1997] and the studies with alternating current energy impacts had been before. The comparison of
the energy handling capability of make “T” with and without “complex failure criterion” and the results
of [Rin 1997] are shown in Figure 2.3. It can be seen that the difference between with and without
“complex failure criterion” is negligible for lower current densities (long duration current energy
impacts of 4 ms length) but that, on the other hand, the difference gets larger for higher current
densities. The reason for this is that higher current densities lead to more electrical aging of the MO
resistors and therefore the proportion of “failure” due to change of the “characteristic voltage”
increases (if the blocks do not fail because of flashover). If the blocks were tested up to mechanical
failure, this failure mechanism is not considered and therefore the energy handling capability is higher
by testing up to mechanical failure. Furthermore, it has to be mentioned that the difference between
the energy handling capability by testing with alternating current and long duration current in
Figure 2.2 is not related to the difference in current shape, but can be explained by the different
applied failure criteria.
0
200
400
600
800
1000
1200
1400
1600
1800
0.1 1 10 100 1000 10000
meanfailureenergyinJ/cm³
peak current density in A/cm²
T, complex failure criterion
[Rin 1997], until mechanical failure
T, until mechanical failure
AC
≈ 8 s
≈ 100 ms 4 ms
90/200 µs
Diameter ≈ 60 mm
Heigth ≈ 40..45 mm
1 ms
2 ms
Figure 2.3 Mean failure energy vs. amplitude of current density for “Size 1” MO resistor of make “T”;
comparison of failure criteria “until mechanical failure” and “complex” for the long-duration current
impulse stress, [Cig 2013a]

21
As initially explained, two sizes of MO resistors were tested. The MO resistors of “size 2” are resistors
which are typically used in distribution class surge arresters. The MO resistors were tested with long
duration and double exponential impulses as shown in Table 2.1. The results of the tested single
impulse energy handling capability (all research on size 2 MO resistors was done with “complex failure
criterion”) are shown in Figure 2.4. The following can be concluded:
 The energy handling capability for this MO resistors (except for make “X”) in the current
range of long duration current energy injections is approximately 15 % to 25 % lower than
the energy handling capability of size1 MO resistors in the same current range.
 For the faster 90/200 µs impulse stress, the energy handling capability increases, but only
very rarely.
For the high current impulse stress with 4/10 µs (which is used in [IEC 2009] to test the thermal
stability of the distribution class surge arrester), the energy handling capability in some cases sinks to
very low values of only 200 J/cm³ to 600 J/cm³. The dominating failure mechanism for this impulse
was the change of the “characteristic voltage”. It is acknowledged that the manufacturers do know
this behavior and take it into account when Ur is specified for the operating duty test. If this is done,
the operating duty test can be passed even though electrical aging of the MO resistor would take
place in the preceding energy injection. The change of the “characteristic voltage” depending on the
energy impact is shown in Figure 2.5. Each dot in the diagram was found by stressing one MO resistor
with a defined energy impact which causes a change in the “characteristic voltage”. It is remarkable
that the trend line is linear for all makes but shows different slopes. Some makes seem to be
optimized to have a small change of “characteristic voltage” for high energy impacts whereas other
manufacturers seem to have found a way to live with the change of “characteristic voltage”. The
highest measured energy levels of each make are not in all cases equal to the energy at which the MO
resistor mechanically failed, because the performance of the used test generators does not allow for a
testing of all MO resistors up to mechanical failure with a high current impulse of 4/10 µs.
Figure 2.4 Mean failure energy vs. amplitude of current density for “Size 2“ MO resistors, [Cig 2013a]

22
It could be shown that the energy handling capability of “size 1” and “size 2” resistors differs, but it
has to be mentioned that the manufacturing process may vary for different MO resistor sizes (even for
those made by the same manufacturer). The single impulse (alternating current) energy handling
capability of the three MO resistor sizes which were manufactured equally are shown in Figure 2.6.
Each point was found by stressing 50 MO resistors with alternating current up to mechanical failure
and calculating the mean failure energy (as described above and in [Cig 2013a]). The result of this
study was that MO resistors of bigger diameter have a lower specific energy handling capability at the
same current density (of course the large standard deviation has to be taken into account). [Cig
2013a] shows that this is not only a question of failure probability, but, because of the difficult
manufacturing process, MO resistors of higher diameter seem to be more inhomogeneous.
Another important outcome of this study was that there is a certain risk of outliers with a lower
energy handling capability in nearly every delivered batch of MO resistors. Figure 2.7 shows an
-40
-35
-30
-25
-20
-15
-10
-5
0
5
0 100 200 300 400 500 600 700 800 900 1,000
ChangeofUchin%
Energy in J/cm³
S
U
V
W
X
Y
"Size 2"
Diameter ≈ 40 mm
Height ≈ (30..40) mm
Figure 2.5 Change of “characteristic voltage” vs. energy injection by 4/10 µs impulse current,
[Cig 2013a]
Figure 2.6 Mean failure energy (incl. standard deviation) vs. current density amplitude for MO resistors of
same make and same height but different diameters, [Cig 2013a]

23
example of a test series of 50 MO resistors of the same batch in which all but one MO resistors failed
at approximately 1000 J/cm³. The resistor marked red, number “32”, failed at a very low energy level
(below the design energy). Because these outliers are rare and standard arresters are not often
stressed to their limits, this outcome is not really an issue for standard applications but more critical
for applications such as UHV arresters or large arrester banks for overvoltage protection of series
capacitor banks. In these applications, thousands of MO resistors are stressed up to their design
energy at the same time with low current densities (which is unfortunately the most critical stress as
initially explained) and therefore the risk of an arrester failure is higher for these applications.
As already mentioned, the outlier phenomenon can be observed for nearly all in tested ceramics
shown in Figure 2.2 and it will be discussed in more detail in section 2.3.4.4.
2.3.3 Double Impulse Stress
A standard surge arrester has to withstand not only single but also multiple energy injections. Most of
the results presented in this section are also covered in [Tuc 2013] and [Tuc 2014].
It is widely known that lightning flashes are very often multiple events (with time intervals of several
milliseconds between the individual strikes), restriking circuit breakers and also multiple earth faults in
open or compensated neutral systems may impose multiple stresses on surge arresters. Also, for
reason of standardization, the knowledge of how long the time intervals between two stresses have to
be to consider them independent of each other in an operating duty test (acc. to [IEC 2009]) is
important. In the past various studies on multiple double exponential current stresses of MO resistors
were carried out (e.g. [Dar 1997], [Dar 1998] and [Kle 2004]) but only rarely on multiple long
duration current stresses and multiple alternating current stresses. The outcome might be different,
because we know from the single impulse stresses that the failure mechanism differs for different
current densities and impulse types. For longer energy impacts with lower current density (e.g. long
duration current impacts and alternating current impacts) the risk of flashovers is lower than for
double exponential stresses. The flashovers observed by [Dar 1998] during alternating current energy
injections could not be observed during the single impulse energy injections of section 2.3.2. The
reason for this could be the pneumatic test fixture which had been improved for this study by applying
a contact force of 3,0 N/mm² which prevents bouncing off the electrodes. As already discussed and
simulated for example in [He 2007] the current in the MO resistor bulk material is not homogenously
Figure 2.7 Example of failure energies at alternating current tests up to mechanical failure, [Cig 2013a]

24
distributed for lower current densities but it is concentrated in current paths which leads to hot spots
during an energy impact. For multiple energy injections with long time intervals between the energy
impacts, these hot spots may dissipate heat to the cooler enclosed MO resistor material. In this case,
the sum failure energy of two energy impacts where the second follows the first after a long delay
may be larger than the single impulse energy handling capability. During very long delays between
the energy impacts, the MO resistor will dissipate a significant amount of heat to its surroundings and
this is why the design of a surge arrester has an important impact on the cooling behavior.
In order to get a basic understanding of the potential difference between multiple and single energy
injections, a study was carried out in which the time delay between two long duration current
impulses was varied. The longest chosen delay was 3 s (as explained above, the surroundings of the
MO resistor are only negligible if the longest delay is short enough to prevent the MO resistor from
dissipating a significant amount of heat) and if no delay was applied, a long duration current impulse
of double wavelength was chosen. To prevent bouncing off the electrode and to avoid axial heat flow,
the same test fixture as in the single impulse energy handling capability measurements was used.
To simulate double long duration stresses, two long duration current generators were used in parallel.
The first long duration current generator was connected to the specimen via a spark gap. To prevent
a pre-triggering of the second generator due to voltage oscillations of high steepnesses at the end of
the first energy injection, the second current generator was connected to the specimen via a
semiconductor switch.
The research was carried out with regards to the “complex failure criterion” and in another test series
with energy injections up to mechanical failure of the MO resistors. Both studies will be described in
the following subsection.
2.3.3.1 Double long duration current impulse stresses, with “complex failure criterion”
The difference between a double and a single long duration current impulse stress is the delay
between the first and the second impulse during which the temperature in the MO bulk material can
be equalized (depending on the length of the delay) and also that for a double impulse stress two
current rises occur (the current rises have a low di/dt in comparison with standard double exponential
current stresses). It was discussed whether this secondary current rise (at a high temperature level of
the MO resistor) has any effect to the failure mode or whether it can change the U-I-characteristic of
the MO resistor. To check possible effects, this investigation was performed with “complex failure
criterion” (as described in section 2.3.2) at MO resistors of “size 2” (typically applied in 10 kA
distribution class arresters) and afterwards with another set of MO resistors with energy injections up
to mechanical failure (this is described in the subsection 2.3.3.2).
An example of two long duration current stresses with a delay of approximately 80 ms is given in
Figure 2.8
Figure 2.8 Example of a double long duration current stress with a delay of approximately 80 ms (y-
axis is not scaled for the purpose of neutralization)

25
The results of this investigation are shown in Table 2.2. Two series of double impulses with different
delay lengths between the two impulses were performed and the results were compared with two
series of single impulses; one with the same wavelength (and the doubled current density) and one
with the double wavelength (but same current density) as one of the double impulse stresses. In the
second row of the table, the duration of one long duration current impulse and the delay between the
two impulses is given for the double impulse series. Concerning the single impulse test series, only the
duration of the long duration current impulse is given. In the third row, the approximate current
amplitude in p.u. is given. For the single 1.9 ms long duration current impulse, the current amplitude
has to be twice as high as the 4 ms long duration current impulse to inject approximately the same
amount of energy. Regarding the double impulses of 1.9 ms impulse duration each, approximately the
same current value as the single 4 ms long duration current impulse is needed. The mean failure
energy is given in the fourth row and it is compared to the mean failure energy of the 4 ms single
long duration current impulse. For the double impulse stresses, the sum of the energy which is
injected in the two separated long duration current impulses is used to calculate the mean failure
energy for this type of stress. In the last row of the table, the coefficient of variation of the failure
energy is given.
Table 2.2 Failure energies for single and double impulse stresses (with “complex failure criterion”)
on “size 2” MO resistors of one manufacturer.
Double impulse Single impulse
Impulse length/
time duration
1.9 ms/
3 s
1.9 ms/
80 ms
4 ms 1.9 ms
Current amplitude
in p.u.
1.0 1.0 1.0 2.0
Mean failure energy
(sum) in p.u.
0.95 0.95 1.0 0.94
Coefficient of variation
0.21 0.15 0.17 0.11
The value of the coefficient of variation is very high, which is indeed a disadvantage of the complex
failure criterion as explained before. Taking this into account, the differences between the mean
failure energies of the different test series are very small and no tendency for the different lengths of
delay between the double impulses is observable. The rather long delay of 3 s (test series of the
second column in Table 2.2) does not seem to improve the temperature distribution in the bulk
material of the tested MO resistors because it does not increase the energy handling capability
significantly (measurably).
The failure modes of the double impulse stresses were the same as those of the single impulse
stresses for this make (an overview of the failure modes of single impulse stresses for different makes
is given in [Cig 2013a]). This also indicates that a double impulse stress with long duration current
stresses of the used current density with the used length of delay between the impulses has the same
impact on MO resistors as one long duration current stress with the same current density and the
same energy value.
2.3.3.2 Double long duration current impulse stresses up to mechanical failure
Due to the mentioned high standard deviation of the measured failure energy in double long duration
current impulse stresses with “complex failure criterion”, the test was repeated with a different make
of the same size but without “complex failure criterion”. The MO resistors were stressed up to
mechanical failure and only the handled energy until the MO resistor failure could be observed. For
this test, a current value was chosen which leads to mechanical failure in all tested resistors within the
last quarter (of the impulse length) of the second long duration current impulse (considering a double
impulse stress). Again, double long duration current stresses with an impulse length of 1.9 ms and a
delay between the two impulses of 80 ms and 3 s were compared with single long duration impulses
(also tested up to mechanical failure) of 1.9 ms and 4 ms impulse length. The results are presented in
Table 2.3 (which is structured like Table 2.2).

26
The result of this test is that the coefficient of variation is reduced in comparison to the results of
section 2.3.3.1, but the differences between the mean failure energies (which are shown in the fourth
row) are still less than the coefficient of variation. This is why the results of this approach should be
handled with care, too and the small increase in the mean failure energy for longer delays between
the two impulses should not be overestimated. The failure modes of the different energy stresses
differ only negligibly.
Taking the results of both double long duration impulse tests, which were compared with single
impulse stresses of the same failure criterion, into consideration, a MO resistor seems to be able to
handle the same energy stress within an interval of 3 s regardless of how many parts the energy
injection is split into. Only the sum of the injected energy is relevant for a mechanical failure (or, in
case of the investigated “complex failure criterion”, the amount of electrical and/or mechanical pre-
damage) of the MO resistor. Regarding thermal stability issues, only the sum of the injected energy
(the heat of the MO resistor) is relevant if the impulses do not change the U-I-characteristic. This has
to be considered when an operating arrester has to handle multiple energy injections.
Double impulse Single impulse
Impulse length/
time duration
1.9 ms/
3 s
1.9 ms/
80 ms
4 ms 1.9 ms
Current amplitude
in p.u.
1.0 1.0 1.0 2.0
(sum) Mean failure
energy in p.u.
1.04 1.02 1.0 1.02
Coefficient of
variation
0.09 0.12 0.07 0.10
2.3.4 Energy handling capability of repetitive stresses
As described in the section above, a single energy stress affects MO resistors just as much as the
overall energy impact of impulses with short delays between the individual impulses. The next step is
to understand the aging/fatigue effect of energy impacts regarding different stresses. In doing so,
users can be enabled to decide whether an arrester should be replaced after certain energy injections
or not.
In the following tests, the fatigue of MO resistors was identified by analyzing the change of
“characteristic voltage” (see chapter 2.3.2) and the change of the energy handling capability (in
comparison to non-pre-stressed MO resistors of the same make). Between the energy impacts (after a
pre-stress), the MO resistors were always cooled down to ambient temperature and, after a defined
number of impulses, the “characteristic voltage” was measured. After a defined number of energy
injections, the energy handling capability was also measured by a destructive energy impact. The test
procedure of every investigation is explained in each subsection. The tests which are presented in this
section are also published in [Tuc 2014].
2.3.4.1 Energy handling capability after 4/10 µs pre-stress
As depicted in Figure 2.5 (and described in section 2.3.2), some makes of “size 2” (which are typically
applied in 10 kA distribution class arresters) showed major changes in the “characteristic voltage”
after a 4/10 µs energy injection. Whether a change of the “characteristic voltage” by one 4/10 µs pre-
Table 2.3 Failure energies for single and double impulse stresses (until mechanical failure of the MO
resistors) on “Size 2” MO resistors of another manufacturer (different to Table 2.2)

27
stress has any effect to the energy handling capability of the MO resistor (in this case a change of the
“characteristic voltage” would indicate a pre-damage) will be discussed in this subsection. As
described in [Cig 2013], lightning strikes with a current amplitude in the range of 100 kA and above
(which is necessary to change the “characteristic voltage” by 20 % and more) occur only very rarely.
For this reason, only one 4/10 µs pre-stress with high current amplitude was applied and no multiple
pre-stresses of this kind were considered.
The structure of the test is shown in the flowchart of Figure 2.9. At the beginning of each test series
of each make, an initial measurement of the “characteristic voltage” at a peak current density of
0.12 mA/cm² was conducted (5 s after voltage application). After that, a 4/10 µs double exponential
pre-stress of a current value was performed, which did not lead to a failure of any MO resistor of this
make in any of the tests, but it changed the “characteristic voltage”. Subsequently, the MO resistors
were cooled down to ambient temperature and at the exit measurement the “characteristic voltage”
was measured again. At the end of the test series, the energy handling capability was measured by
applying a 4 ms long duration energy injection up to mechanical failure and compared with the energy
handling capability of new MO resistors (also measured up to mechanical failure). To identify whether
the polarity of the current flow of 4/10 µs pre-stress in relation to the destructive 4 ms long duration
current has any impact on the energy handling capability, both polarities of the 4 ms impulse were
tested.
Initial measurement
Uch.1 at Jch = 0.12 mA/cm² (after 5 s)
Cool down to
ambient temperature
Determine
4 ms long duration
energy handling capability
Exit measurement
Uch.x at Jch = 0.12 mA/cm² (after 5 s)
4/10 µs pre-stress
Figure 2.9 Flowchart of the investigation to determine the energy handling capability after
4/10 µs pre-stresses

28
The energy handling capability of MO resistors of three different makes at a 4 ms long duration
current stress (up to mechanical failure) after a 4/10 µs pre-stress (which leads to a certain change of
the “characteristic voltage”) is shown in Figure 2.10. Each mark in the diagrams was identified by
averaging the measurement results of ten MO resistors. Due to the small number of MO resistors per
mark, the variation of the energy handling capability (which is also shown in Figure 2.10) is high.
Regarding the blue marks labeled “+ pre-stress”, the current flow of the destructive 4 ms long
duration energy impact goes into the same direction as the current flow of the 4/10 µs double
exponential pre-stress; the opposite holds true for the green marks labeled “- pre-stress”. The value
of the mean failure energy is compared to the energy handling capability of new (unstressed) MO
resistors whose variance is depicted in black and labeled “0 % change of Uch”. The changes of the
“characteristic voltage” are on different levels for all makes, because the levels were chosen to assure
that no MO resistor of this make failed at the pre-stress at the highest value of change of the
“characteristic voltage”.
It can be concluded that even major changes of the “characteristic voltage” have no measurable
impact to the mean failure energy regarding long duration current stress and therefore a change of
the “characteristic voltage” in these kind of impulses is no indicator for a change of the single impulse
energy handling capability. Furthermore, the current flow polarity of the pre-stress in relation to the
current flow of the destructive long duration impulse neither has any measurable impact on the
energy handling capability. Only the make which is shown in Figure 2.10 (b) shows a small tendency
of reduction of the energy handling capability for major changes of the “characteristic voltage” which
should not be overestimated due to the high variance of the mean failure energy. The failure modes
of the destroyed MO resistors do not differ when different pre-stress levels are being applied.
(c)
Figure 2.10 Result of the test in order to identify the energy handling capability after 4/10 µs
pre-stresses. (a)-(c) are different makes, “+ energy pre-stress” indicates measurements in
which the current flow of the pre-stress has the same direction like the current flow of the
destructive 4 ms long duration energy injection (the opposite is true for “- energy pre-stresses”
current flow)
(a) (b)

29
2.3.4.2 Repeated a.c. energy injection
In series compensation applications, surge arresters are exposed to repeated a.c. energy injections up
to their design energy. But in standard applications and more often in UHV systems (see also [Hin
2012]), surge arresters are stressed with temporary overvoltages which results in a.c. energy impacts.
To determine the impact of a.c. energy impacts to aging/fatigue of MO resistors, ten MO resistors of
three makes (named I, II and III) of “size 1” (which were typically applied in 10 kA station class
arresters) were exposed to 100 a.c. energy pre-stresses per MO resistor with an energy value of 80 %
of their average failure energy (of the same current density) in these test series.
The Flowchart of this test series is shown in Figure 2.11. The first step was to measure the
“characteristic voltage” at a peak current density of Jch = 0.12 mA/cm³ (5 s after voltage application)
in the initial measurement. After that, the first a.c. pre-stress with 50 Hz at a peak current density of
3.5 A/cm² was performed. An adequate length of the impulse was chosen by assuring that the energy
impact of each pre-stress amounted to no less than 80 % of the single impulse mean failure energy of
the stressed ceramic (see section 2.3.2). This led to energy impacts of approximately 600 J/cm³ to
700 J/cm³. After the pre-stress, the MO resistors were cooled down to ambient temperature within
15 min (by forced cooling). An exit measurement was performed every fifth pre-stress in which an
initial measurement was repeated, to determine the change of the “characteristic voltage”. After this,
the next a.c. energy pre-stress was performed.
After 100 pre-stresses, the a.c. energy handling capability was determined by applying a destructive
a.c. energy injection (of the same current density as the pre-stresses).
The results of the change of the “characteristic voltage” by a.c. pre-stresses of the three measured
ceramics (I, II and III) compared to the “characteristic voltage” of the non-stressed MO resistors are
given in Figure 2.12. It becomes clear that the “characteristic voltage” increases at the first impulses
and seems to rise until a certain limit (1 % to 4 % higher than the starting value) for all makes. It is
important to notice that apparently no deterioration but, on the contrary, a formation (improvement)
of the U-I-characteristic in the leakage current region takes place during this kind of pre-stress.

30
The a.c. energy handling capability of all MO resistors which withstood 100 a.c. energy pre-stresses
were identified and the results were compared with the energy handling capability of new MO
resistors. The results are shown in Table 2.4. In the second row of this table, the quantity of MO
resistors which withstood 100 pre-stresses is depicted. The values are: ten for “make I”, six for “make
Initial measurement
Uch.1
at Jch
= 0.12 mA/cm² (after 5 s)
a.c. energy pre-stress
Cool down to
ambient temperature
Perform exit measurement?
no
Further pre-stress?
yes
Determine
a.c. energy handling capability
Exit measurement
Uch.x
at Jch
no
yes
Figure 2.11 Flowchart of the repeated a.c. pre-stress procedure [Tuc 2013]
0
1
2
3
4
5
0 20 40 60 80 100
MeanchangeofUchin%
Number of pre-stresses
I
II
III
Figure 2.12 Mean change of “characteristic voltage” Uch by a.c. energy pre-stresses
[Tuc 2013]

31
II” (three failed at pre-stress number 4, 45 and 50 and one had to be removed due to improper
handling) and nine of ten for “make III” (one failed at pre-stress 40). The “characteristic voltage”,
which was measured after every fifth pre-stress, never showed an indication for a failure of an MO
resistor such as a decrease in the “characteristic voltage”, but it has to be mentioned that there never
was a failure of an MO resistor at a pre-stress directly after a measurement of the “characteristic
voltage”. The energy handling capability of these MO resistors was determined by an a.c. energy
impact of 3.5 A/cm² up to mechanical failure. The failure modes of the destroyed MO resistors (which
were pre-stressed before using 100 a.c. energy injections) show no significant difference compared to
failure modes of new MO resistors destroyed by an equivalent energy injection. The mean failure
energy and the coefficient of variation are given in the last two rows of Table 2.4. It can be seen that
the energy handling capability of the MO resistors which withstand 100 a.c. pre-stresses is not
affected by the pre-stresses. The small increase of the mean failure energy should not be
overestimated and could be explained like this: the results of all tested MO resistors were used to
calculate the single impulse energy handling capability, even of those of the outliers mentioned in
section 2.3.2 which have a lower energy handling capability. These outliers might have failed during
the 100 a.c. pre-stresses and therefore do not decrease the value of the mean failure energy of the
pre-stressed MO resistors. This is why the mean failure referred to in Table 2.4. shows an increase of
the mean failure energy of pre-stressed MO resistors in relation to the mean failure energy of non-
pre-stressed ones.
Table 2.4 a.c. energy handling capability of MO resistors after 100/one hundred a.c. pre-stresses
compared to the a.c. energy handling capability of non-pre-stressed MO resistors at the same
current density.
Make I Make II Make III
Number of tested
MO resistors
10 6 9
Mean failure
energy in p.u.
1.1 1.1 1.2
Coefficient of
variation
0.038 0.014 0.029
2.3.4.3 Repeated long duration energy injection
Long duration current stresses are very important regarding arresters because this kind of stress is
closer to the “normal” stress of standard surge arrester applications than the previous mentioned a.c.
stresses. Furthermore, many surge arrester standards use long duration current impulses for energy
injections for example to test the thermal stability of surge arresters or impulse durability of MO
resistors and many manufacturers use this impulse for routine test of MO resistors. In this section, the
durability/fatigue of MO resistors of “size 1” (typically applied in 10 kA station class arresters) will be
identified concerning repeated long duration energy injections.
The main differences between an a.c. and a long duration energy injection in the tests mentioned
here are that a.c. energy injections have a bipolar current flow, a lower current rise time, lower
current density and a longer interval of energy injection in comparison to long duration current
impulses. As mentioned in section 2.3.2, the energy handling capability of a 2 ms long duration
current impulse with a current density of approximately 200 A/cm² is more than 50 % higher than the
a.c. energy handling capability at a current density of 3.5 A/cm² if the same failure criterion is applied
(see Figure 2.3). This higher energy handling capability can be explained with a more homogeneous
current flow (more current paths) through the MO resistor bulk material.
In pre-measurements with five makes (of only one MO resistor per make) it could be shown that the
multiple energy injections for long duration current impulses (with higher current densities) is lower
than for a.c. energy injections. In a pre-test with a repeated injected energy of 50 % of the mean
single impulse failure energy (leads to 450 J/cm³ to 550 J/cm³ pre-stress energy), four of five makes
could not handle more than 20 long duration pre-stresses, whereas in section 2.3.4.2 it is shown that
two of three makes could handle 100 a.c. repeated energy injections of 80 % of the mean single

32
impulse failure energy (leads to 600 J/cm³ to 700 J/cm³ pre-stress energy) without pre-damaging the
MO resistor bulk material.
At a different pre-test it could be shown that it is possible to find in nearly every make at least one
MO resistor which could handle 100 pre-stresses with 300 J/cm³ long duration energy injections each.
The aim of the following test was to identify the energy level for three makes at which a group of 20
MO resistors had not failed during 20 repeated pre-stresses. If one or more failures had occurred, the
test was repeated with new 20 MO resistors at an energy level which was 50 J/cm³ lower that the old
one and if no failure had occurred, the test was repeated with 20 new MO resistors at a 50 J/cm³
higher energy level. The flowchart of a test series at one energy level is shown in Figure 2.13. At the
beginning, an a.c. pre-stress at 3.5 A/cm² with an energy value of 80 % of the mean failure energy
(at 3.5 A/cm²) was performed to exclude the previously mentioned outliers of this measurement (as
shown in section 2.2.4.2, this measurement does not decrease the energy handling capability).
Afterwards, the initial measurement was performed and the “characteristic voltage” at a peak current
density of Jch = 0.12 mA/cm³ was measured (5 s after voltage application). Regarding the pre-
stresses, a long duration current stress with a virtual duration (as defined in [IEC 2009]) of 1.9 ms
Initial measurement
Uch.1
at Jch
Cool down to
ambient temperature
no
Further pre-stress?
yes
Determine
Exit measurement
Uch.x
at Jch
a.c. energy pre-stress
20 times
Long duration current pre-stress
Figure 2.13 Flowchart of the repeated long duration current impulse test procedure at one
energy level [Tuc 2013]

33
was used. Between each pre-stress, the MO resistors were cooled down to ambient temperature
within 15 min (by forced cooling) but no intermediate measurement of the electrical characteristics
(such as measurement of the “characteristic voltage”) was performed. After 20 pre-stresses, the
“characteristic voltage” was measured again and, after this, the energy handling capability of each
pre-stressed MO resistor was measured by an a.c. energy injection (unit mechanical failure of the MO
resistor).
Table 2.5 shows the energy levels of the three different makes (α, β and γ) at which no MO resistors
failed and, additionally, the tested higher energy levels at which failure occurred. In row 3 and 4, the
quantity and the impulse number of the MO resistor failure are depicted. It is worth mentioning that
the energy level which could be handled repeatedly by the three makes is quite close to (or partly
even below) the typical rated energies of standard surge arresters and far from the mean single
impulse energy handling capability at the same current density of 800 J/cm³ to 1000 J/cm³ in “size 1”
MO resistors (see also chapter 2.3.2).
Table 2.5 Energy level and quantity of MO resistor failures (20 samples per make) for repeated 2 ms
long duration current impulse [Tuc 2013]
Make α Make β Make γ
Pre-stress level
in J/cm³
150 200 250 300 300 350
Quantity of failures (of
20 MO resistors)
0 1 0 2 0 8
Impulse at which the
MO resistors failure
occurred
- 5 - 3, 6 -
1, 1, 3,
4, 4, 4,
19, 20
In Table 2.6, the results of the exit measurement of the MO resistors at the energy level at which no
failure occurred are given. In the third row, the mean increase of the “characteristic voltage” after the
last pre-stress in comparison to the initial measurement is depicted. This value shows a nearly
negligible change which is positive for all makes; this indicates that the “characteristic voltage” after
the pre-stresses is higher than before and therefore it can be said that an improvement of the
electrical characteristic took place. Thus, a thermal stability test could be handled easier after pre-
stresses of the tested kind than without pre-stresses.
The mean a.c. failure energy of the MO resistors which were pre-stressed by applying 20 long
duration energy injections (of the value shown in the second row) is depicted in the fourth row and it
is nearly not affected by the pre-stresses. The small increase should not be overestimated and could
be explained by statistical uncertainties (as explained before), it is only certain that the mean failure
energy is not decreased, but it should not be concluded that it will even be increased by pre-stresses.
The failure modes of pre-stressed and new MO resistors show no significant differences when being
destroyed by an equivalent energy injection.

34
Table 2.6 a.c. energy handling capability of MO resistors (20 samples per make) after 2 ms long
duration current impulse pre-stresses (at a different energy level) compared to the a.c. energy
handling capability of not pre-stressed MO resistors [Tuc 2013]
Make α Make β Make γ
Pre-stress level at which
no failure occurs
150
J/cm³
250
J/cm³
300
J/cm³
Increase of Uch during
20 LD pre-stresses
0.5 % 1.3 % 2.2 %
mean a.c. failure energy
in p.u.
1.09 1.07 1.05
Coefficient of variation 0.078 0.032 0.044
2.3.4.4 Comparison of Different Pre-Stresses on the Same Energy Level
The results of the tests which were presented in the last two sections seems to show that the energy
handling capability concerning repeated injectable energy is not related to the single impulse energy
handling capability (at the same current density) but is effected by the current density (or the wave
shape) of the energy injection. In order to verify these results, to improve the statistical base, to
homogenize the test results for different types of energy injections and to get a better understanding
of these phenomena, another test was started: the same specific energy was injected using three
different kinds (wave shapes) of energy impacts and the quantity of failures during 20 pre-stresses as
well as the energy handling capability of the MO resistors which survived 20 pre-stresses was
observed. To understand whether the electrical characteristic of the MO resistors is affected by the
different pre-stresses, the “characteristic voltage” was measured after every pre-stress.
In this test, the two makes ”1” and “2” (of “size 1” MO resistors which are typically applied in 10 kA
station class arresters) were stressed with repeated energy injections. Only two makes were stressed
because the results of the last sections show that there are no fundamental differences in the failure
modes of different makes which were stressed with different kinds of repeated stresses. The flowchart
of this new test series is shown in Figure 2.14. At the beginning, the residual voltage at the nominal
discharge current of 10 kA, 8/20 µs and the “characteristic voltage” at a peak current density of
Jch = 0.07 mA/cm³ (5 s after voltage application) was measured, a lower current density was applied
than in the previous measurements to reduce possible recovery effects during/for the “characteristic
voltage” measurement. Afterwards the MO resistors of the different makes were repeatedly pre-
stressed with 20 energy injections of one impulse type (current density) and one energy level.
Between each pre-stress, the MO resistors were cooled down to ambient temperature (in
approximately 15 min by forced cooling) and the “characteristic voltage” was measured. After 20 pre-
stresses, the residual voltage at the nominal discharge current of 10 kA, 8/20 µs for the MO resistors
which survived the pre-stresses were measured again and the energy handling capability was
determined by applying an a.c. energy injection with a peak current density of 3.5 A/cm² up to
mechanical failure of the MO resistors.
For the pre-stresses, a.c. energy injections (four cycles), long duration energy injections with a virtual
duration of the peak of 2 ms and double exponential current discharge with a wave shape of
90/200 µs were used. In order to get different energy levels for the pre-stresses, the current density
of the different energy injections was varied. To inject 300 J/cm³ with a 90/200 µs impulse
approximately 550 A/cm² (the injected charge was in the range of 2.8 C) was needed, for 2 ms long
duration current about 50 A/cm² (the injected charge was in the range of 3.5 C) and for a.c. (4
cycles) approximately 5 A/cm² (the injected charge was in the range of 3.8 C). 200 J/cm³, 300 J/cm³
and 400 J/cm³ were used as energy levels because the pre-measurements (with long duration current
energy injections) had shown that the first MO resistor failures would be occurring for 20 pre-stresses
in the above mentioned range of energy level and this range is also important for the design of surge
arrester.

35
In Table 2.7, the quantities and impulse numbers of the MO resistor failures during the pre-stresses
for different types of energy impacts and energy levels are given for both of the tested makes. It can
be observed that the quantity of failures increases with increasing energy level of the pre-stresses
(the missing MO resistor failure for make 1 at 300 J/cm³ at the 90/200 µs impulse should not be
overestimated) but with a different gradient for the different types of energy impact. At the faster
(higher current density) 90/200 µs energy impact, a rapid increase of the quantity of failures between
two energy levels could be observed for both makes, whereas the failure quantity is smaller for the
longer 2 ms long duration and a.c. energy injections. Furthermore, with some exceptions (for example
for make 1), faster energy impacts cause higher failure values at the same energy level than longer
energy impacts with lower current density.
Figure 2.14 Flowchart of the repeated energy injection test with three different kinds of energy
injections
Pre-stresses with a.c.
long duration or 90/200 µs impulse
Initial measurement
Ures,1 at I = 10 kA
Uch.1 at Jch = 0.07 mA/cm² (after 5 s)
Cool down to
ambient temperature
no
yes
Determine
Intermediate measurement
Uch.x at Jch = 0.07 mA/cm² (after 5 s)
20 x
Exit measurement
Ures,1 at I = In
Further pre-stress?

MO SURGE ARRESTERS - METAL OXIDE RESISTORS AND SURGE ARRESTERS FOR EMERGING SYSTEM CONDITIONS

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