Test Equipment needed in measuring Electrical/Electronic Devices

1. The Basics of Electrical Testing
(Electrical Test Measurements)
Mr. Christian Ace M. Solidum, LPT

2. Introduction
• Electrical testing in its most basic form is the act of applying a
voltage or current to a circuit and comparing the measured
value to an expected result. Electrical test equipment verifies
the math behind a circuit and each piece of test equipment is
designed for a specific application.
• It is the job of a test technician to know which piece of test
equipment to use for the task at hand and also understand
the limitations of the test equipment they are using. In this
article, we take a look at the most common pieces of test
equipment used in the field.

3. Multimeter
• Also known as a VOM (Volt-Ohm meter), a multi-meter is a
handheld device that combines several measurement
functions (such as voltage, current, resistance and
frequency) into a single unit.
• Multi-meters are mainly used to troubleshoot electrical
problems in a wide array of industrial and household
devices such as electronic equipment, motor controls,
domestic appliances, power supplies, and wiring systems.
• Digital multi-meters are the most common form of meter
used today; however analog multi-meters are still
preferable in some cases, like when monitoring a rapidly
changing value or sensitive measurements, like testing for
CT polarity.

4. Megohmmeter
• Most commonly referred to as simply a “megger”, a
megohmmeter is a special type of ohmmeter used to
measure the electrical resistance of insulators.
• Resistances values by megohmmeters may range from
several megohms to several million megohms (teraohms).
Megohmmeters produce high voltages via battery powered
internal circuitry or a manually operated generator with
outputs ranging from 250 to 15,000 volts.
• Megohmmeters are one of the most frequently used pieces
of test equipment and can be used to measure the
insulation of various types of apparatus such as circuit
breakers, transformers, switchgear and cables.

5. Low-Resistance Ohmmeter
• Often called a DLRO in the field, the low-resistance ohmmeter is
used for making high-precision resistance measurements below 1
ohm. Low-Resistance ohmmeters produce low voltage DC currents
via battery power with outputs of up to 100A.
• Resistance measurements are achieved with four terminals, called
Kelvin contacts. Two terminals carry the current from the meter
(C1, C2), while the other two allow the meter to measure the
voltage across the resistor (P1, P2). With this type of meter, any
voltage drop due to the resistance of the first pair of leads and
their contact resistances is ignored by the meter.
• Low-Resistance ohmmeters are one of the most frequently used
pieces of test equipment and can be used to measure the
resistance of various types of apparatus such as circuit breaker
and switch contacts, cable and busway, transformers and
generators, motor windings, and fuses.

6. Hipotential Test Set (AC/DC/VLF)
• Dielectric withstand (or hipot) testing checks for good insulation in medium and high-
voltage apparatus, the opposite of a continuity test. Insulation is stressed above
nominal values to ensure minimal current leaks from the insulation to ground.
• Hipot test sets consist of a high voltage lead, a return lead, and a ground lead. The
high voltage lead is connected to the device under test with all other components
grounded and the resulting current is measured through the return.
• If too much return current flows, the test set internal protection will trip. The hipot
test is a “go, no go” test, meaning leakage current must not trip the test set but there
is no minimum acceptable value.
• Output voltage can range anywhere from 1kV-100kV+ ac at line frequency or dc
depending on the device under test. Very low frequency (VLF) withstand testing is
the application of an AC sinusoidal waveform, generally at 0.01 – 0.1 Hz, to assess
the quality of electrical insulation in high capacitive loads, such as cables.

7. High Current Test Set (500A to
15000A+)
• A high current test set may consist of two pieces known as a “control unit” and an
“output unit”, or these functions may be combined in a single package. Low voltage,
high current outputs are used for primary-injection testing of low voltage circuit
breakers.
• The high current or “primary-injection” test set consists of large transformers that
step down line voltage (ex. 480V) to a very low level, such as 2-15V. The large
reduction in voltage allows for a large increase in available current output (15kA+),
especially for a short duration.
• Current output is controlled by a tap changer and variable resistor. Integrated timers
display the period between current on and current off to indicate how long it takes
for a circuit breaker to trip.
• Circuit breakers may be connected directly to the high current test set via bus or
cable. Depending on the size, this type of test equipment can also used to test
ground fault and other current relays by connecting directly to switchgear bus.

8. Secondary Test Set
• Circuit breakers with solid state and microprocessor trip units can be
tested by injecting secondary current into the trip unit directly rather than
passing primary current through the CT’s using a high current test set. The
main shortcoming of the secondary current injection test method is that
only the solid-state trip unit logic and components are tested.
• Secondary test sets are designed by trip unit manufacturers to be used
with a single style or family of trip unit using a proprietary connection. Test
kits can range from simple hand held, push button style in design to more
sophisticated “suitcase” units that operate similar to a primary injection
test set.
• Hand held units are often used to defeat trip unit protective functions,
such as ground fault, when testing circuit breakers via primary-injection

9. Relay Test Set
• These are power system simulators used for testing protection
devices used in industrial and power systems. Relay test sets are
fitted with multiple sources to test solid-state and multi-function
numerical protection, each voltage and current channel is
operated independently to create different power system
conditions.
• High end relay test equipment can test not only simple voltage,
current, and frequency relays but also complex protection
schemes, such as communication-assisted line protection, and
protection schemes that use IEC61850-compliant IEDs (intelligent
electronic devices).

10. Power Factor Test Set
• Power Factor Test Sets provide a comprehensive AC insulation diagnostic test for high
voltage apparatus, such as transformers, bushings, circuit breakers, cables, lightning
arrestors, and rotating machinery.
• Test voltages are generally 12kV and below, the power factor test set measures
voltage and current of the device under test using a reference impedance. All
reported results – including power loss, power factor, and capacitance – are derived
from the vector voltage and current.
• Tests are made by measuring the capacitance and dissipation factor (power factor) of
a specimen. The values measured will change when undesirable conditions exist,
such as moisture on or in the insulation; presence of conductive contaminants in
insulating oil, gas or solids; presence of internal partial discharges etc.
• Test connections include a single high voltage lead, (2) low voltage leads and a
ground. Safety switches and a strobe light are included for operator protection and a
temperature sensor is used to correct test values. Power factor test sets are usually
operated with a laptop computer connected via USB or Ethernet.

11. Winding Resistance Test Set
• Winding resistance measurements are an important diagnostic tool for assessing
possible damage to transformer and motor windings. Winding resistance in
transformers will change due to shorted turns, loose connections, or deteriorating
contacts in tap changers.
• Measurements are obtained by passing a known DC current through the winding
under test and measuring the voltage drop across each terminal (Ohm's Law).
Modern test equipment for this purposes utilizes a Kelvin bridge to achieve results;
you might think of a winding resistance test set as a very large low-resistance
ohmmeter (DLRO).
• Winding resistance test sets have (2) current leads, (2) voltage leads and (1)
ground lead. Typical current range of a winding resistance test set is 1A-50A.
Higher currents have been found to reduce test times on high current secondary
windings.

12. Current Transformer Test Set
• CT test sets are small, multi-function units designed to perform demagnetization,
ratio, saturation, winding resistance, polarity, phase deviation, and insulation tests
on current transformers. High-end CT test equipment can directly connect to multi
ratio CT’s and perform all tests on all taps with the push of a button and without
changing leads.
• Current transformers can be tested in their equipment configuration, such as being
mounted in transformers, oil circuit breakers or switchgear. Modern CT with
multiple voltage and current outputs can double as a relay test set when operated
with a laptop computer.

13. Ground Resistance Test Set
• The ground resistance test set works by injecting a current into the earth between
a test electrode and a remote probe, measures the voltage drop caused by the soil
to a designated point, and then use Ohm's Law to calculate the resistance.
• Ground resistance test sets come in a variety of styles with the most common
being the 4-terminal unit for soil resistivity testing and the 3-terminal unit for fall-
of-potential testing. Copper rods or similar stakes are used to make contact with
the earth along with spools of small stranded wire to cover long distance
measurements.
• Clamp-On Ground Resistance Testers measure ground rod and grid resistance
without the use of auxiliary ground rods. They offer accurate readings without
disconnecting the ground system under test but come with limitations.

14. Power Recorder
• Power recorders are devices used to collect voltage and current data which can be
downloaded into software in order to analyze electrical system conditions. These
are troubleshooting tools used to pinpoint electrical problems such as voltage
swells, sags, flicker and poor power factor.
• Power recorders may also be used to measure power consumption over a period
of time, which is useful for engineers planning to expand a system or customers
who wish to audit their energy bills. There are many different types of power
recorder which range in size, accuracy and storage capability.
• Installation of a 3-phase power recorder involves wrapping conductors with split-
core CT’s and clipping a set of leads to system voltage and ground. The recorder is
set up to measure according to the system configuration for a specified time
period and can also be viewed in real time using a PC or integrated screen.

15. Infrared Camera
• Thermal imagers are camera that detect invisible infrared radiation and convert
that data into a colored image on a screen. Infrared cameras are most commonly
used for inspecting the integrity of electrical systems because test procedures are
non-contact and can be performed quickly with equipment in service.
• Comparing the thermal signature of a normally operating piece of equipment to
the one being evaluated for abnormal conditions offers an excellent means of
troubleshooting. Even if an abnormal thermal image is not fully understood, it can
be used to determine if further testing may be required.
• Thermal imagers are classified based on their accuracy and detector resolution.
High end infrared cameras feature high resolution image capture and temperature
accuracy down to a tenth of a degree or less.

16. Vibration Tester
• Vibration analyzers are used to identify and locate the most common mechanical
faults (bearings, misalignment, unbalance, looseness) in rotating machinery. As
mechanical or electrical faults develop in motors, vibration levels increase. These
increases in vibration and noise levels occur at different severity of a developing
fault.
• Accelerometers are used to take vibration measurements with the equipment in
service and data is loaded into software for analysis. As the machine under test
operates, the accelerometer detects its vibration along three planes of movement
(vertical, horizontal and axial).