High voltage dc field testing
In 1996, the insulated conductor industry determined that
dc withstand testing of the plastic (XLPE) insulation systems either in the cable factory
as a routine production test or after installation as the higher voltage proof test was
detrimental to the life of the insulation and therefore discontinued recommending dc
testing. Medium voltage EPR insulating systems are not subject to the same aging
characteristics and, therefore, can be dc tested as required in accordance with the tables
When an insulated cable arrives on the job site, the
recipient should be able to confidently assume it will attain the designed service life.
This means it must arrive free of internal discontinuities in the dielectric such as voids
or inclusions, as well as freedom from air pockets at the interfaces between the shielding
systems and the dielectrics surfaces. It is, however, the specter of mechanical
damage, or substandard splicing and terminating that could cause the engineers responsible
for continuity of service to desire a field applied proof test to establish the
cables serviceability. The time-honored methods of proof testing in the field
involve high potential direct current (dc). The advantage of the dc test is obvious. Since
the dc potential does not produce harmful discharge as readily as the ac, it can be
applied at higher levels without risk or injuring good insulation. This higher potential
can literally sweep-out far more local defects. The simple series circuit path
of a local defect is more easily carbonized or reduced in resistance by the dc leakage
current than by ac, and the lower the fault path resistance becomes, the more the leakage
current increased, thus producing a snow balling effect which leads to the
small visible dielectric puncture usually obvserved. Since the dc is free of capacitive
division, it is more effective in picking out mechanical damage as well as inclusions or
areas in the dielectric which have lower resistance.
Field tests should be utilized to assure freedom of
electrical weakness in the circuit caused by such things as mechanical damage, unexpected
environmental factors, etc. Field tests should not be used to seek out minute internal
discontinuities in the dielectric or faulty shielding systems, all of which should have
been eliminated at the factory, nor should the dc potential be excessive such that it
would initiate punctures in otherwise good insulation.
For low voltage power and control cables it is general
practice to use a megger for checking the reliability of the circuit. This consists
essentially of measuring the insulation resistance of the circuit to determine whether or
not it is high enough for satisfactory operation. For higher voltage
cables, the megger is not usually satisfactory and the use
of high voltage testing equipment is more common. Even at the lower voltages, high voltage
dc tests are finding increasing favor. The use of high voltage dc has many advantages over
other types of testing procedure.
dc field acceptance testing
It is general practice, and obviously empirical, to relate
the field test voltage upon installation by
using a percentage of the factory applied dc voltage. This means that prior to being connected to other
equipment, solid extruded dielectric insulated shielded cables rated 5kV and up may be
given a field acceptance test of about 300 volts per mil. The actual test values
recommended for the field acceptance test are presented in the Table below. If other
equipment is connected it may limit the test voltage, and considerably lower levels more
compatible with the complete system would be in order.
High voltage field
prior to being placed in service
dc Hi-Pot Test
dc Hi-Pot Test
Wall - mils
Wall - mils
Note: If the leakage current quickly
the duration may be reduced to 10 minutes.
The dc leakage can be affected by external factors such as
heat, humidity, windage, and water level if unshielded and in ducts or conduits, and by
internal heating if the cable under test had recently been heavily loaded. These factors
make comparisons of periodic data obtained under different test conditions very difficult.
If other equipment is connected into the cable circuit this makes it even more difficult.
In the event hot poured compound filled splices and terminations are involved, testing
should not be performed until they have cooled to room temperature.
The relays in high voltage dc test equipment are usually
set to operate between 5 and 25 milliamperes leakage. In practice, the shape of the
leakage curve, assuming constant voltage, is more important than either the absolute
leakage current of a go or no go withstand test result.
From the standpoint of safety as well as data
interpretation, only qualified personnel should run these high voltage tests
After the voltage has been applied and the test level
reached, the leakage current may be recorded at one minute intervals. As long as the
leakage current decreases or stays steady after it has leveled off, the cable is
considered satisfactory. If the leakage current starts to increase, excluding momentary
spurts due to supply-circuit disturbances, trouble may be developing and the test may be
extended to see if the rising trend continues. The end point is, of course, the ultimate
breakdown. This is manifested by an abrupt increase in the magnitude of the leakage
current and a decrease in the test voltage. It should result in relay action to
trip the set off the line, but this assumes the equipment has enough power to
maintain the test voltage and supply the normal test current. Since the total current
required is a function of cable capacity, condition of dielectric, temperature, end
leakage and length, the test engineer must be sure that relay action actually
signifies a local fault, rather than being merely an indication that the voltage had been
applied too quickly or one of the other factors contributing to the total current had been
At the conclusion of each test, the discharge and grounding
of the circuit likewise requires the attention of a qualified test engineer to prevent
damage to the insulation and injury to personnel.
Maintenance proof testing
It may be justifiable in the case of important circuits to
make periodic tests during the life of the installation to determine whether or not there
had been significant deterioration due to severe and perhaps unforeseen operational or
environmental conditions. The advantage of a scheduled proof test is, of course, that it
can frequently anticipate a future service failure, and the necessary repair
or renewal can be made without a service interruption, usually during a major shutdown.
Furthermore, a dc test failure is seldom burned-out, and
visual analysis may disclose the cause and permit corrective action.
As a note of caution, once a complete circuit has been
connected and all exposed ends sealed, it is not desirable when maintenance proof-testing
to remove these seals, disconnect the conductors, and it is sometimes impossible to
provide ends with adequate clearance and length of insulation surface to
permit high voltage testing even at the levels specified in the following table. Further,
there is the danger of mechanically injuring the dielectric during the seal removal and
end preparation. This is a major reason why a megger test is often used in
maintenance checking of the numerous circuits in a power plant.
High voltage maintenance
cables in service less than five years*
Phase to Phase
dc Proof Test
(5 Minutes) kV
*Consult manufacturer when cables
are in service over five years.
Frequency of tests
In the case of power plants, it is customary to schedule
desired maintenance proof tests to coincide with planned major shutdowns. It is not
necessary or justifiable to check every circuit each year. The following schedule is
suggested as a guide.
Frequency of proof
Installation Acceptance Test
12 - 18 months
4 - 5 years
Other Test Methods
Test methods such as VLF (Very Low
Frequency) and Field Partial Discharge Testing are acceptable
alternatives to the DC Hipot test. Refer to IEEE Guides 400.2 and 400.3
for additional information.