Critical Analysis of IS 1255-1983, reaffirmed 1996
for “Code of Practice for Installation and
Maintenance of Power Cables up to and including 33
kV rating”
“Stop hurting your extruded power cables! Do not perform DC high-potential test”
Ravi Kumar
Premjeet Nair
Senior Application Engineer- Cable Infrastructure
Megger India Pvt Ltd
Kolkata, India
Ravi.kumar@megger.com
Product Manager- Cable Infrastructure
Megger India Pvt Ltd
Kochi, India
Premjeet.nair@megger.com
Abstract— Traditionally power cables, during installation
or maintenance, are tested with a DC high voltage for its
electrical strength as prescribed in IS 1255-1983,
reaffirmed 1996. Modern extruded dielectric power cables
such as XLPE have inherent space charge accumulation
tendencies when subjected to a high voltage DC. The
accumulated space charge leads to overstressing of
insulation resulting in cable breakdown with high
downtime and repair costs. As IS 1255-1983 is silent on the
harmful effect of DC on XLPE cables, power companies in
India are facing revenue losses as well as the quality of
service to the consumer is affected. This paper analyses the
demerits of IS 1255 and compares it with the other
available test methods such as Very low frequency and
Damped Alternating Current test, as well as condition
assessment methods, such as Dissipation factor and Partial
Discharge. A case study highlighting the advantages of
newer test methods has been presented and a cost to
benefit analysis is undertaken.
Keywords—Power cables, XLPE, DC test, VLF test, DAC test,
Partial Discharge test, Dissipation factor test, condition monitoring
I. INTRODUCTION
Power cables are arteries and veins of a power distribution
network, it transports power from a transmission or
distribution substation to another distribution substation or to a
residential distribution transformer at a consumer premises. In
a highly populated areas, underground power distribution
network is the only option to provide electricity, it is also
costlier than the overhead power distribution network.
Underground cables constitute a major investment in electric
power network. These cable networks are required to have a
good life expectancy and high reliability.
This article investigates into techniques for commissioning (or
installation/acceptance) and maintenance tests on up to 33 kV
rated power cables as described in IS 1255-1983 and, various
other international practices based on IEEE field guides and
IEC standard documents.
II. HISTORY & EVOLUTION OF EXTRUDED POWER CABLES.
First power cables were laid more than 90 years ago, since
then the dielectric material used in the power cables to provide
the desired voltage insulation has changed considerably.
Cables of the older days were mainly oil filled laminated
dielectrics, like Paper Insulated Lead Covered (PILC).
Though, these cables were robust and immune to minor
defects with high tolerance to partial discharge (PD), their
performance was affected by defects such as leakage of fluid
at gaskets or due to hole in outer sheath, ingress of moisture,
waxing of fluid, dry or brittle paper, etc. One or more of these
factors led to frequent breakdown of PILC cables requiring
more maintenance and higher losses.
In order to overcome the limitations of laminated cables,
extruded polymeric dielectric cables such as Cross Linked
Poly Ethelene(XLPE) were introduced in 1980s. Initial XLPE
cables did overcame majority of the issues present in PILC
cable, however it was found that XLPE cables had more
susceptibility towards moisture ingress resulting into water
trees, hence degradation of cable insulation was faster and
service life expectancy was low. Modern XLPE cable are Tree
retardant or TR-XLPE which have significantly improved the
problem of water trees in it.
Majority of the power cables used in India are of low and
medium voltage category networks i.e. rated up to 33 kV.
Recently, few power cables for transmission network up to
400kV are also being installed. India has a significant size of
PILC cable installation majorly in metro cities, some of which
are more than 40 years old, but majority of the cable
installations are XLPE, as the number of XLPE installation in
last twenty years has increased considerably.
III. WHAT IS A HIGH VOLTAGE WITHSTAND TEST/ DIAGNOSTIC
TEST?
In a high voltage withstand test, a cable is subjected to a
pre-determined voltage, for a pre-determined time. If cable
survives the test voltage and duration it is taken into service, if
it fails, it is then repaired and retested.
A diagnostic test such as partial discharge or dissipation
factor (Tan Delta), carried along with voltage with stand test or
separately, provides additional useful information about the
condition of insulation of cable. A partial discharge test would
reveal the localised defects on the cable and its accessories
such as joints and termination, so that corrective action can be
taken. A Tan Delta test (on a service aged cable) would provide
overall insulation quality so that cable or cable section
replacement decision can be planned.
IV. WHY TO PERFORM A HIGH VOLTAGE
WITHSTAND/DIAGNOSTICS TEST?
A. Manufacturing defects
The newly manufactured cable may have certain defects like
micro voids, protrusion, impurities or moisture in the
insulation. Thus, it is important to carry out a high voltage
withstand test of cable at site to verify the integrity of the cable
insulation.
B. Installation defects
Improper handling of cable during installation will weaken the
insulation and may create additional defects. Power cables
comes in a standard length of 250m or 500m drum, so two or
more power cables are joined to make a longer network. In
order to connect the power cable within power network,
terminations are installed at both end of the network. Creating
joints and terminations is a manual process, subjected to human
error during installation (Fig. 1 & 2). So, the quality of
insulation of joint and termination need to be verified. Thus,
before energising the power cable into service, it is very
important to perform a high voltage withstand test.
defects in the cable, rendering it to be unreliable. Thus, a
periodic predictive maintenance test is also required to be
carried out on power cables to assess the condition of
insulation for carrying out corrective actions.
D. Breakdown
Lastly, if a cable insulation breakdown failure occurs in
service, it is required to perform a voltage withstand test on the
cable after repair work is done and before it is taken back into
service again.
In addition to the high voltage withstand test, if a diagnostics
tests such as partial discharge measurement or Tan Delta
measurement (on aged cables) is performed, it would reveal
more valuable insights into quality of insulation of cable and
its accessories (joints and terminations).
V. HIGH VOLTAGE WITHSTAND TEST AS PER IS 1255: 1983
“IS 1255 recommends DC test on power cables after jointing
and terminations are completed. The recommended values of
test voltages are closer to 3 UO i.e. three times rated phase to
earth voltage. It specifies a test duration of 5 minutes. DC test
voltage for service aged cable is 1·5 times rated voltage or less
depending on the age of cable, repair work or nature of jointing
work carried out, etc.” [2].
VI. WHY DC IS DANGEROUS ON XLPE CABLE.
In an extruded polymeric insulation, electrical charges such
as electrons, holes or ions have a tendency to get
electrostatically attracted to specific location within the
insulation to form chemical bonds. These are known as space
charges. It accumulates due to difference in the rate at which
charge enters and leaves a region [3].
Applying a high DC voltage to a polymeric dielectric
insulation for a longer duration, for e.g. 3 UO or 1.5 times rated
voltage, for a duration of 5 minutes during a commissioning or
maintenance test will create space charges.
A cable may fail during reenergization if it is not
completely discharged after a DC high voltage test, due to
presence of trapped space charges, which have a longer
discharge time constant [4]. It has been observed that after the
DC test is conducted, these trapped space charge do not
discharge even after 24 hours.
IEC 60502-2: 2014 categorically warns that “a DC test may
endanger the insulation system under test and recommends that
where ever possible an AC test should be used”
While IEEE 400.2: 2013 state that if an aged cross-linked
polyethylene (XLPE) cable is exposed to high-voltage DC
tests, then there are major negative issues affecting its integrity,
after it is taken back into service.
C. Service ageing
Cable manufacturers as well as the field experiences
suggests normal life expectancy of a cable to be around 40 to
50 years [1]. But, power cables in service withstands many
stresses during its operation such as thermal stress, electrical
over voltage and over current, moisture, etc. Combination of
these factors reduces the insulation quality and creates many
It is further observed that IEEE 1234-2019 unequivocally
cautions that “some utility or industrial standards do not yet
consider the potentially damaging effects of DC testing. It is
not recommended to DC test on an extruded cable”
In addition to space charge phenomena, it is also known
that on extruded cables used in ac systems, DC test is not
effective in detecting many forms of gross defects, e.g. voids,
sharp cuts etc that may be present in a cable system that will
otherwise be detected by other test voltage such as very low
frequency (VLF) or at operating frequency [5]
Acknowledging the above, it is worldwide well established
that high voltage DC test is harmful as well as ineffective in
some cases for extruded dielectric cables such as XLPE and it
should not be carried out.
VII. INTERNATIONAL STANDARD FOR HIGH VOLTAGE TEST ON
EXTRUDED CABLES.
Ideally, power cables should be tested with same power
frequency ac at higher voltages as used in service for e.g.
50Hz ac. However, the charging current required for such test
because of the inherent capacitance of longer length of
concentric shielded cable, is very high and it requires a very
large and bulky system to generate required charging current
and power. So, the field testing is extremely challenging,
inconvenient or not possible with 50Hz ac.
Another option is to use AC resonant test systems which
operates in frequency range from 20Hz to 300Hz but they are
also extremely bulky and costly, so transportation is a
challenge and it also requires a larger space and setup time at
test site which is not feasible or desirable for a medium
voltage cable.
during commissioning test. It also recommends around 1.7 UO
rms voltage for a duration of 30 minutes on a service aged
cable. IEC differs in its recommendation as it suggest a 0.1Hz
VLF test voltage of around 3 UO rms for a duration of 15
minutes. It also says that a lower voltage and/or shorter
duration should be applied on a service aged cable depending
on its age, history of breakdown and purpose of test.
0.1 Hz VLF test is the most popular choice with utilities
across the worldwide.
It must be noted that IEC 60502-2 recommends test frequency
of 0.1Hz only. IEEE 400.2 also recommends test frequency
0.1 Hz but it also mentions that for longer circuits if the test
kit doesn’t meet the required capacity then frequency may be
reduced till 0.01Hz and test time need to increase suitably.
However, it doesn’t suggest how much time to increase.
Experience reveals that when a test is performed at a 0.01Hz
frequency, the stress on cable is much lower than 0.1Hz test
and it is not very effective, as it can miss out many significant
defects present in the cable.
0.1 Hz VLF CR power sources due to their design has higher
limit test capacitances than that of 0.1Hz VLF sine power
sources. Hence it is possible to test considerably longer cable
networks as compared to 0.1 Hz VLF sinusoidal without the
need of reducing frequency.
If no other test is possible then standard also mentions about
possibility of testing with power frequency 50 Hz at nominal
voltage UO for 24 hours. This is known as soak testing where a
cable is charged under no load condition at rated nominal
voltage for 24 hours, if cable survives then it is taken into
service. However, this test method doesn’t take into account of
the overvoltage stresses that a cable may encounter while in
service. So cable behavior above rated operating voltage is
unknown.
Very Low Frequency (VLF) 0.1Hz ac voltage testing is an
alternative method of test on a power cable. Moreover, it is
also a portable and economical solution and onsite testing is
possible with convenience.
Unlike DC voltage, VLF test voltage continuously changes its
polarity so it does not produce harmful space charges [6].
Also, at 0.1 Hz, the charging current required is only 1/500 of
that at 50 Hz so it allows significantly smaller and more
portable VLF power sources to test even longer lengths cable
network.
Two kind of VLF waveforms are described in both IEEE
400.2 as well as in IEC 60552 namely VLF cosine-rectangular
waveform (VLF CR) and VLF sinusoidal waveform (VLF
sine). In cosine rectangular electrical stress distribution is
closer to the power frequency as the polarity reversal takes
place around 8 msec (Fig 3.). While sinusoidal may have a
very different stress distribution than that of power frequency.
IEEE 400.2 recommends a 0.1Hz VLF test voltage of around
2 UO rms for minimum of 1 hour to be applied on a new cable
VIII. DAMPED ALTERNATING CURRENT (DAC) TEST- A NONDESTRUCTIVE METHOD FOR CABLE TESTING AND DIAGNOSTICS.
DAC voltage testing is one of the popular alternative methods
of ac voltage testing and is applicable for a broad range of
medium-voltage (MV), high-voltage (HV), and extra-highvoltage (EHV) cable types [7]. It is also a very convenient,
portable and economical onsite test method.
DAC voltages are generated by charging the cable to a
predetermined voltage level and then discharging the
capacitance of cable through a suitable inductance which in
turns causes the damping of applied voltage.
The frequency of DAC oscillation depends upon the value of
HV inductor in the test equipment and the capacitance of the
cable. Normally the frequency of a DAC test lies in between
20Hz to 500Hz [7].
Newly installed cable systems can be tested with 50 DAC
excitations in monitored and/or non-monitored ways.
Acceptable voltage level for a commissioning test is 2 UO.
Cable systems in service or after repair or refurbishment, can
be tested with 50 DAC excitations in a monitored and/or nonmonitored way but at a reduced voltage level, typically 1.7 UO
[7].
rectangular voltage and, Dissipation Factor with the help of
VLF sinusoidal waveforms.
The protocol followed by company to carry out the test is
described in Table 1.
Table 1. Test protocol
Since the frequency of a DAC high voltage test lies in between
20Hz to 500Hz, the test result is closer to that of a power
frequency (50/60Hz) test, DAC test results are also relatable to
that of a power frequency test (Fig.4)
Cable
Type
New
Old
Tests
performed
Voltage
withstand and
Partial
Discharge
Voltage
withstand,
Partial
Discharge and
Tan Delta
Voltage
withstand
test
parameters
Up to 2 UO
rms voltage
for 60 minutes
Partial
Discharge
test
parameters.
From 0.5 UO
to 2 UO in step
of 0.2Uo
Tan Delta
test
parameters
Optional (At
0.5 UO UO,
and 1.5 UO)
Up to 1.6 Uo
rms voltage
for 30 minutes
From 0.5 UO
to 1.6 UO in
step of 0.2 UO
At 0.5 UO
UO, and 1.5
UO
Decision on the test results is taken as per guidelines described
in Table 2.
Table 2. Test result analysis guideline
IX. WHY A DIAGNOSTIC TEST IS VERY IMPORTANT?
A high voltage withstand test is a Go No-go test in which if
the cable survives the duration of the test then it is taken into
service, should the cable fail it is repaired and retested. In
order to determine more information on condition of cable,
diagnostic tests such as Dissipation Factor (Tan Delta) and
partial discharge is performed which provide more insights
into the nature of defects in cable, if any.
Both IEEE and IEC standards suggests partial discharge and
Tan Delta test to be carried out on a cable, during a high
voltage withstand test or separately, at the time of
commissioning, or during periodic/breakdown maintenance.
While IS 1255 is silent about any field diagnostic test.
Tan Delta is mostly carried out on a service aged cable, it
provides insights into overall insulation condition of cable and
can reveal information such as moisture, water trees or
presence of wet joints so that cable or section replacement
may be planned, while partial discharge is frequently carried
out during commissioning test as well as maintenance test as it
provides localised information about weak points such as
voids, contamination or bad joints/ terminations etc so that
corrective action can be taken.
X. CASE STUDY
A major petrochemical company in India has hundreds of
power cable installed within its premises, majority of which
are below 33kV. Most of the cables were installed in 1998 and
are in their mid-service life. During installation the company
had followed IS 1255 and only DC tests were conducted. As
the cable started ageing, they felt the need of test and
diagnostics methods as per international practices, in order to
reduce their cable failure and to increase remaining life. Few
years back they adopted testing methodology according to
IEEE 400 series under which they now carry out VLF
withstand test as well as partial discharge with cosine
Test Result
Cable occur insulation breakdown
Cable survives test voltage and duration
as well as partial discharge activities
within limit
Cable survives test voltage and duration
but high partial discharge activities.
Cable survives test voltage and duration
with acceptable PD level but high Tan
Delta values
Action
Cable is repaired and retested.
Cable taken into service
Partial Discharge spots are
repaired, and PD test is
reconducted.
If possible, Tan Delta is
performed on cable sections and
cable replacement decision is
taken accordingly.
A 2.4 km, 33 kV rated XLPE cable (Fig. 5) feeding to a
critical unit tripped, subsequently a joint failure was located at
240m in phase L3 while other two phases were healthy. The
joint was repaired, and cable was tested with DAC voltage up
to 1.6 UO rms (Phase to earth) for 30 minutes (Fig. 6). Cable
successfully passed the withstand test but high-level of partial
discharge was observed at the same joint of 240m.
Tan Delta was also performed on the cable and the result were
found to be within acceptable limit as per IEEE 400.2,
indicating no presence of moisture in the cable. Results are
summarized in table 3.
Table 3. Tan Delta test result.
Tan δ Values x 10-3
Phase
0.5
UO
Stability
UO
L1
L2
L3
1.0
1.0
1.0
0
0
0
1.0
1.0
1.0
Stability
1.5
UO
Stability
0
0
0
2.3
2.3
2.9
0
0
0
∆
Tan
δ
1.3
1.3
1.9
From the PD test result in Fig. 7. It was evident that the
jointing work at L3 was not properly conducted, so decision
was taken to open and rework on the joint with more
supervision. Hence a possible in-service failure was averted
with the help of a PD test.
partial discharge or Dissipation factor diagnostics are added
into VLF test sets, the costs will increase by another 50% [8].
Though, VLF & DAC test sets are relatively costlier than DC
test sets but increase in reliability and life expectancy of
cables, by use of proven VLF & DAC methods results into
much larger savings in breakdown maintenance and
replacement costs as well as downtime revenue losses.
XII. CONCLUSION
Recommendations of IS 1255 were valid and applicable on
older PILC cable where DC test was acceptable. Technology
progress has led to change in insulation materials of power
cables, modern power cables are mostly XLPE upon which
DC test is proven to be dangerous as well as ineffective and it
is not recommended by international standards such as IEC &
IEEE.
Very Low Frequency (VLF) test and Damped Alternating
Current (DAC) tests are two well proven, convenient and
recommended technologies for high voltage test on XLPE
power cables.
It is high time Indian utilities migrates from DC test to
VLF/DAC test as well as adopt diagnostic techniques such as
partial discharge and Tan Delta measurements, which will
reduce the failure of their vital cable assets as well as would
enhance the quality of power supply to consumers.
REFERENCES
[1]
[2]
[3]
XI. COST ANALYSIS
DC test sets up to 60 kV which is required for a test on a 33kV
power cable as per IS 1255 are a combination of a transformer
and a rectifier, so they are very cheaply available.
A sinusoidal VLF test set with 60 kV peak output would be in
general two to three times costlier than a DC test set. A cosine
rectangular VLF test set and Damped AC test sets with similar
output would be 25% higher than a sinusoidal VLF test set. If
[4]
[5]
[6]
[7]
[8]
Central Electricity Authority of India (CEA): “Guidelines for use of
Under Ground Cable System and overhead conductor system- 2018”
IS 1255: 1983 reaffirmed 1996 “Code of Practice for Installation and
Maintenance of Power Cables up to and including 33 kV rating”
John C Fothergill “The Coming of Age of HVDC Extruded Power
Cables”
IEC 60502-2: 2014 “Standard for Power Cables with Extruded
Insulation and their accessories.”
IEEE 400.2- 2013: “Guide for Field Testing of Shielded Power Cable
Systems using Very Low Frequency (VLF)”
IEEE 400.3- 2006: “Guide for Partial Discharge Testing of Shielded
Power Cable Systems in a Field Environment”
IEEE 400.4- 2015 “Guide for Field Testing of Shielded Power Cable
Systems rated 5kV and Above with Damped Alternating Current (DAC)
Voltage”
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