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.
Electrical test equipment should be considered a source of lethal electrical energy. Technicians must observe all safety warnings and follow all practical safety precautions to prevent contact with energized parts of the equipment and related circuits, including the use of appropriate Personal Protective Equipment.
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.
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.
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.
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.
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.
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.
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).
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.
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.
Transformer Turns Ratio (TTR) Test Set
The TTR test set applies voltage to the high-voltage winding of a transformer and measures the resulting voltage from the low voltage winding, this measurement is known as the turns ratio. In addition to turns ratio, the units measure excitation current, phase angle deviation between the high- and low-voltage windings and percent ratio error.
Transformer turns ratio test sets come in a variety of styles and test connections, however all turns ratio testers have at least two high leads and two low leads. The excitation voltage of a TTR test set is generally less than 100V.
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.
Magnetron Atmospheric Condition (MAC) Test Set
Traditional field testing of vacuum interrupters utilizes the hi-potential test in order to evaluate the dielectric strength of the bottle, this test produces a go/no-go result that does not determine when, or if, the gas pressure inside the bottle has dropped to a critical level. Unlike the hipot test, testing vacuum interrupters utilizing magnetron atmospheric condition (MAC) principles can provide a viable means for determining the condition of vacuum interrupters prior to failure.
The magnetic field test is set up by simply placing the vacuum interrupter into a field coil, which will produce a DC current that remains constant during the test. A constant DC voltage, usually 10 kV, is applied to the open contacts, and the current flow through the VI is measured.
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.
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.
Infrared cameras are available in a variety of styles and resolutions. Which camera is best for an inspection depends on the type of equipment to be inspected and the environmental conditions. Photo: TestGuy
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.
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).
Arcing, tracking and corona all produce ionization which disturbs the surrounding air molecules. An ultrasonic tester detects high frequency sounds produced by these emissions and translates them down into human audible ranges.
The sound of each emission is heard using headphones and the intensity of the signal is observed on a display panel. These sounds may be recorded and analyzed through ultrasound spectral analysis software for a more accurate diagnosis.
Normally, electrical equipment should be silent, although some equipment such as transformers may produce a constant hum, or some steady mechanical noises. These should not be confused with the erratic, sizzling frying, uneven and popping sound of an electrical discharge.
Ultrasonic detectors are also useful in the detection of air leaks in transformer tanks and gas insulated circuit breakers.
Load banks are used to commission, maintain and verify electrical power sources such as diesel generators and uninterruptible power supplies (UPS). The load bank applies an electrical load to the device under test and dissipates the resulting electrical energy through resistive elements as heat. The resistive elements are cooled with motorized fans within the load bank construction.
Multiple load banks can be connected together if required. Some load banks are purely resistive while others may be purely inductive, purely capacitive, or any combination of the three. Load banks are the best way to replicate, prove and verify the real-life demands on critical power systems.
Battery Impedance Tester
Battery Impedance Test Equipment is mainly used in substation and UPS applications to determine the health of lead-acid cells by taking measurements of important battery parameters such as cell impedance, cell voltage, inter-cell connection resistance and ripple current. All three tests can be performed with a single unit.
The battery impedance tester works by applying an ac current signal across an individual cell and measuring the ac voltage drop caused by that ac current as well as the current in the individual cell. It will then calculate the impedance. The standard lead set used is dual-point, Kelvin-style. One point is for applying the current and the other for measuring the potential.
The time-domain reflectometer (TDR) is used to determine the characteristics of electrical paths by transmitting a signal and observing reflected waveforms along a conductor. If the conductor is of a uniform impedance and is properly terminated, then there will be no reflections and the remaining incident signal will be absorbed at the far-end by the termination. If there are impedance variations, due to a fault or poor terminations, then some of the incident signal will be reflected back to the source. The operating principle of a TDR is similar to that of radar.
An optical time-domain reflectometer (OTDR) is the optical equivalent of an electronic time domain reflectometer (TDR). The OTDR injects a series of optical pulses into the optical fiber under test and extracts light that is scattered or reflected back from points along the fiber. The strength of the return pulses is measured and integrated as a function of time, and plotted as a function of length of the fiber. This is equivalent to the way that an electronic time-domain meter measures reflections caused by changes in the impedance of the cable under test.