Inside the electrical tester's toolkit

Figure 1. The hand-held digital multimeter is the workhorse of the circuit testing tools available.

Digital multimeter

Digital multimeters (Figure 1) are the most common type of electrical test equipment and are the workhorses for electrical testing. Applications for multimeters are numerous and include testing voltages at both power and control levels, measuring the current to various loads, checking the resistance of motor and transformer windings and other components, and testing continuity of relay or switch contacts and wiring.

Almost every multimeter measures AC and DC voltage and current, and resistance. Voltage measurements generally range from millivolts to 1,000 volts. Current ranges generally run from milliamps to as much as 10 amps. Resistance measurements typically are from less than one ohm to tens of megohms. Many models have additional functions such as continuity buzzers, temperature, capacitance, frequency and diode testing. More advanced models include peak hold, inrush current, max/min recording, data logging and PC communication for setup and data storage.

Meters that measure true RMS (TRMS) values are more accurate than average-responding meters when waveforms contain harmonics, such as the voltages and currents in motor drives and power supplies.

Non-contact voltage detector

Figure 2. The non-contact tester indicates whether a circuit or component is deenergized.

These instruments are simple and effective for quickly determining whether voltage is present on a circuit or in a panel (Figure 2). However, you must verify that they’re working properly if you want to rely on them for confirming voltage absence (see sidebar, “Safety takes three steps”). A follow-up test with a multimeter is recommended to confirm that conductors are deenergized before performing work on the circuit. Various models use lights, buzzers or both as the indicating signal.

Solenoid voltage detector (Wiggy)

These instruments have been around for many years and are still popular with many electricians. The original design uses a solenoid to move a pointer, which indicates the nominal system voltage level. They don’t give precise readings of actual voltage. In addition to the pointer, solenoid vibration provides an audible indication of voltage presence. They require no batteries and are rugged enough to bounce around in a tool box without damage.

Now for the bad news. While many of these units are still in service, they don’t comply with contemporary test equipment safety standards. Also, the solenoids consume significant power from the circuit under test and produce a voltage transient when making and breaking the connection. As a result, they can damage any sensitive solid-state devices on the circuit. They also can overheat if energized for longer than the recommended duty cycle marked on the instrument, and they are subject to mechanical wear that might render them inaccurate. I don’t recommend using them.

Newer designs mimic the original style instrument, but use indicator lights rather than the solenoid. Some of them actually include a vibrator to provide the feedback of the solenoid. These instruments have acceptable safety ratings and do not cause damage to the circuit under test.

Current clamp

Figure 3. The measuring loop on a flexible current probe attachment for a multimeter is capable of being opened up to encircle large conductors or bus bars.

While almost every multimeter has a current measuring function, it’s only usable for relatively low currents. In addition, the need to break into the circuit to connect the meter in series with the load makes many measurements inconvenient or impossible. Most industrial current testing is done with a spring-loaded current clamp adapter that is clamped around a conductor temporarily, extending the amp range of the meter and allowing convenient current measurements on almost any circuit.

Historically, current clamps operated on the transformer principle and were limited to AC measurements. Today, however, many use electronic devices, such as Hall-effect sensors, and can measure both AC and DC current.

Current clamps are rated from several amps to thousands of amps, and there are flexible probes available that install around large conductors or multiple bus bars (Figure 3). These allow current measurements at any location in a power distribution system from the main service to the smallest branch circuit.

Clamp meter

 Figure 4. The clamp meter measures the current flowing through smaller conductors.

These are becoming popular, with a wide selection available from a number of manufacturers (Figure 4). They combine the functions of a multimeter with a built-in current clamp, eliminating the need for a separate adapter and the interconnecting wires. The only function missing from many of them is DC current, although there are some AC/DC models. Current measuring ratings can be as high as 1,000 amps. Consider one of these if you make many current measurements routinely and don’t need the flexibility of the larger current probes.

Insulation tester/megohmmeter

Although multimeters measure resistance, they’re not capable of making the very high resistance measurements needed for testing insulation on components such as bus bars, panels, motors and transformers. For this task you need an instrument that measures tens or hundreds of gigohms (1 gigohm = 1,000 megohms) at test voltages from 500 to several thousand volts (Figure 5). Insulation testers apply the test voltage between the conductors or windings and the metal frame of the equipment under test, measure the very small current that flows through the insulation and display the result as resistance.

Figure 5. The insulation tester monitors the current flowing through the insulation between a conductor and its environment.

Modern digital testers automatically run insulation tests — dielectric absorption ratio, polarization index, step voltage and dielectric discharge. These non-destructive tests involve calculations based on changes in the insulation resistance during a measurement period of several minutes. When performed regularly, megohm tests can indicate insulation deterioration long before it becomes a problem.

Hi-pot tester

Like megohmmeters, hi-pots measure insulation integrity by applying a high voltage across the insulation. Unlike megohmmeters, however, hi-pots don’t measure insulation resistance. They measure the resulting leakage current and sense the presence of breakdown (arcing) either across a surface or through air. As a result, hi-pots not only determine insulation effectiveness, but they also can detect reduced spacing or crimped wires. Manufacturers often used them for production line testing or in the field for component-acceptance testing.

Hi-pots are available for both AC and DC testing, with voltage ranges from 2.5 kV to more than 100 kV. Because the hi-pot can deliver enough current to produce arcing, improper testing can be destructive to the device under test. Therefore, it’s important to follow the recommendations of the OEM for the device being tested or use standard industry practice. While AC tests are used for many applications, DC tests are necessary if high capacitance exists to ground, such as filters or long cables that would cause excessive leakage current with an AC tester.

While some testers are controlled manually with analog meters, available microprocessor-controlled digital testers automatically control the voltage ramp-up, test voltage and test duration. They also log the results in memory for later uploading to a PC.

Low-resistance ohmmeter/milliohmmeter

Testing the resistance of switch or relay contacts, large motor or transformer windings or bonding jumpers and connections requires an instrument that can measure accurately down to micro-ohms (1/1,000,000 of an ohm). Multimeters simply can’t measure such small resistances, and the test leads and clips typically have a resistance of several tenths of an ohm, which would make any such low-resistance readings meaningless.

Figure 6. A low resistance ohmmeter can accurately measure winding resistance in transformers and motors.

A low-resistance ohmmeter uses a four-wire Kelvin connection to the device under test. Two wires inject a test current, while the other two measure the resulting voltage across the device, which is then converted to ohms and displayed (Figure 6). Because the test leads aren’t included in the voltage reading, their resistance has no effect. Test currents can vary from 1 A to 100 A or more, depending on the instrument design and measurement range.

Loop impedance tester

Loop impedance testers are useful for verifying the integrity of branch circuits and equipment grounds. They draw a current through the circuit (in short pulses to minimize the power dissipation) and measure the resulting voltage drop to calculate the total circuit impedance. This impedance reading can be used to determine what the short-circuit current is at that location or confirm that protective devices will trip properly. Bad connections in a series of receptacles can be located by identifying the point where the loop impedance changes significantly.

Power quality analyzer

Figure 7. The power quality monitor is valuable when troubleshooting equipment that is sensitive to harmonic distortion of the electrical waveform.

For those who need to take their power system measurements to the next level, a power quality analyzer fits the bill (Figure 7). Measurement capabilities include most multimeter functions such as voltage, current and resistance, plus power, power factor energy. They also display voltage or current waveforms on an oscilloscope screen, capture and display transients, and calculate and display harmonic levels and total harmonic distortion. This can provide information that is simply not available on a multimeter, and makes the devices indispensable for troubleshooting motor drives, transformer overheating problems or just general power quality analysis. Harmonic measurements, for example, can indicate transformer saturation or loads that need filtering to reduce harmonic heating of upstream equipment.

Safety

Any test equipment, test leads or accessories that connect to energized conductors must meet the IEC category rating appropriate for the test location, and must carry an independent third-party agency approval, such as UL, ETL or CSA. Industrial electrical testing requires a minimum 600 V CAT III rating, although some measurements will require a CAT IV rating (main service bus bars or outdoor circuits, for example). Many new instruments are 600 V CAT IV/1000 V CAT III, making them suitable for virtually any measurement within their voltage rating. To ensure safety while making measurements, replace older test equipment that doesn’t carry these ratings.

Anyone who works on or near energized systems must be qualified on those systems by virtue of training and knowledge, in accordance with the OSHA definition, must wear appropriate personal protective equipment and must follow appropriate work practices for the tasks performed as defined in NFPA 70E.

This discussion has covered the more common pieces of test equipment for industrial power system testing. There are other more specialized test instruments as well. Having the appropriate equipment for the type of testing you do will make your tasks safer, more effective and more efficient.

Ben Miller is an electrical consultant and safety trainer, and president of B. Miller Engineering. E-mail him at [email protected] or call (847) 948-7746.