Overview of Electrical Testing and Test Methods

1. Testing of electrical equipment is usually performed in the field on new
equipment after installation and on existing equipment to assess its
condition. The manufacturer conducts electrical tests on equipment before
it leaves the factory; these tests, known as factory tests, are outside the
scope of this text and therefore will not be discussed. Field tests are
conducted to see whether newly installed equipment has been damaged,
to indicate whether any corrective maintenance or replacement is
necessary on existing equipment, to indicate if the equipment can
continue to perform its design functions safely and adequately, to chart the
gradual deterioration of the equipment over its service life, and to check
new equipment before energization. In view of these objectives, the
electrical testing of equipment can be divided into the following:
• Types of tests
• Types of testing methods
Overview of Electrical
Testing and Test Methods

2. Types of Tests
The types of fi eld tests are acceptance tests, routine maintenance
tests, and
special maintenance tests that are conducted for specific purposes.
Acceptance Tests
These tests are known as start-up or commissioning tests and are
performed on
new equipment, usually after installation and prior to energization.
When
these tests are repeated within a year, that is before the warranty
period expires,
then these tests are referred to as proof tests. Tests of this type are
made at 80%
of the final factory test voltage value. They are run to determine the
following:
Whether the equipment is in compliance with the specification
To establish a benchmark for future tests
To determine that the equipment has been installed without damage
To verify whether the equipment meets its design intent and limit

3. Routine Maintenance Tests
These tests are performed at regular intervals over the service life of the
equipment. They are made concurrently with PM and at 60% of the final
factory test voltage value. In the course of routine maintenance tests, it is
very helpful to record the information as it is found on the equipment and to
also record the condition in which the equipment is left. Therefore, these
tests can be further subdivided into the following:
As-found tests: These tests are performed on equipment on receipt or
after it has been taken out of service for maintenance, but before any
maintenance work is done.
As-left tests: These tests are performed after maintenance has been
performed and just before re energization. They can indicate the
degree of improvement in the equipment and service as a benchmark
for comparison for future tests.

4. Special Maintenance Tests
These tests are performed on equipment that is known to
be defective or has been subjected to adverse conditions
that may affect its operating characteristics.
An example might be the fault interruption by a circuit
breaker, which requires inspection, maintenance, and
tests before it can be put back into service.

5. Types of Testing Methods
The testing of electrical power system equipment involves checking
the insulation
system, electrical properties, and other factors as they relate to the
overall operation of the power system. Therefore, testing of electrical
equipment
can be divided into the following types:
• Solid insulation testing
• Insulating liquid testing
• Relay and protective device testing
• Circuit breaker time–travel analysis
• Grounding electrode resistance testing
• Fault gas analysis testing
• Infrared inspection testing

6. Solid Insulation Testing
Insulation can be either solid, liquid, or gaseous dielectric
materials that prevent the flow of electricity between points of
different potential. Insulation testing is done to determine the
integrity of the insulating medium.
This usually consists of applying a high potential (hi-pot)
voltage to the sample under test and determining the leakage
current that may flow under test conditions. Excessive
leakage current flows may indicate a deteriorated condition or
impending failure of the insulation. Insulation testing can be
performed by applying either direct current (DC) voltage or
alternating

7. AC hi-pot test and DC hi-pot test
The AC hi-pot test
The AC hi-pot test is primarily a “go” or “no-go” test. The voltage is raised
to a specified level. If the equipment fails or shows excessive leakage current,
the equipment under test is unusable. If the equipment does not fail, it has
passed the test. This test can only indicate whether the equipment is good or
bad. It cannot indicate with what safety margin the test was passed. However,
there are nondestructive tests that can be performed with AC voltage, such as
power factor (PF), dissipation factor (DF), capacitance, etc.,
The DC hi-pot test
The DC hi-pot test can indicate more than a “go” or “no-go” condition.
It can indicate that equipment is all right at the present time but may fail in
the future. DC testing is done to obtain information for comparative analysis
on a periodic basis. With dc testing, the leakage current is measured during
the progress of the test and compared to leakage current values of previous
tests. However, the DC hi-pot test is considered to be a destructive test if the
test voltage is not applied in a predetermined control-voltage steps. The DC
voltage tests can be performed at lower voltages, which are nondestructive
tests, such as insulation resistance, dielectric absorption ratio, and
polarization index.

8. Insulating Liquid Testing
Insulating liquids used in transformers or other electrical apparatus are
subject
to deterioration and contamination over a period of time. These contaminants
have a detrimental effect on the insulating properties of the fluid,
as well as on the solid insulation system of the transformer winding. Basically,
the elements that cause the deterioration of the insulating fluids are moisture,
heat, oxygen, and other catalysts that result in a chemical reaction that
produces acid and sludge, which in turn attack the insulating fluids. The
main insulating fluids that are in use today for transformers are oil, silicone,
and RTemp and Wecosol. Askarel was used in the past, but its use was
banned by federal regulations owing to its high toxicity; however, there may
be installations that still may have this fluid at their plant sites. Regular tests
are recommended to monitor the condition of the insulating liquid. Samples
should be taken from the transformers on periodic basis to perform various
tests in accordance with American Society of Testing Materials (ASTM)
Methods.

9. Protective Device Testing
Protective device testing involves the testing and
maintenance of protective relays, low-voltage draw out power
circuit breakers, low-voltage molded-case breakers, and
associated equipment such as instrument transformers and
wiring.
The function of protective relays and devices maintenance
and testing is to assure that a particular breaker or protective
relay is able to perform its basic protective function under
actual operating conditions. The tests on
relays, protective trip devices, and circuit breakers can be
classified as commissioning tests, routine maintenance
testing, and verification testing.

10. Circuit Breaker Time–Travel Analysis
The circuit breaker time–travel analysis test is performed to
determine if the operating mechanism of the circuit breaker
is operating properly.
This test is usually performed on medium- and high-voltage
circuit breakers and depicts the position of breaker contacts
with relation to time. This relationship can then be used to
determine the operating speed of the circuit breaker for
opening and closing and contact bounce, and the interval
time for closing and tripping.
The breaker operating time data can be used to evaluate
the condition of mechanical parts of breakers, such as
closing mechanism, springs, and shock absorbers.

11. Grounding Electrode Resistance
Testing
The integrity of the grounding system is very important in
an electrical power system for the following reasons:
To maintain a reference point of potential (ground) for
equipment and personnel safety
To provide a discharge point for traveling waves due to
lightning
To prevent excessive high voltage due to induced voltages
on the power system Therefore, to maintain ground
potential effectiveness, periodic testing of grounding
electrodes and the grounding system is required.

12. Fault Gas Analysis Testing
Fault gas analysis testing comprises of dissolved gas analysis
and total combustible gas tests. The dissolved-gas analysis
provides information on the individual combustible gases
dissolved in the insulating oil. The total combustible
fault gas analysis test provides information on incipient faults in
oil filled transformers by measuring the total combustible gases
present in the nitrogen cap of the transformer. Because of
excessive heat due to loading of the transformer, or arcing and
sparking inside the transformer insulating oil, some of the oil in
the transformer decomposes and generates combustible gases,
which then are dissolved in the oil, and eventually become
liberated where they mix with the nitrogen above the top oil.

13. Infrared Inspection Testing
There are many different devices available using infrared
technology to check hot spots in switchgear and other
energized parts of the power system.
They are very useful in routine maintenance and
inspection for finding bad connections and joints and
overloaded terminals or lines.
ELECTRICAL POWER
EQUIPMENT
MAINTENANCE
AND TESTING
SECOND EDITION
Paul Gill