If your work includes installing, maintaining, or repairing electrical systems, you need the right test eqipment.
Whether you're trying to determine if a circuit is safe to work on or what kind of corrective action to take, your analysis is only as good as your test equipment. Over time, maintenance departments, contracting firms, and the individuals they employ acquire a wide range of test instruments. Some are specialized, while some are just plain essential for jobs that require adequate test equipment, like installation and startup testing, predictive maintenance, preventive maintenance, troubleshooting, corrective maintenance, and extended monitoring.
But what if you have to replace all of your test equipment on a tight budget? Or suppose you need to outfit a new crew or update an existing crew's equipment? What test equipment would you — or they — be unable to do without? The answer depends largely on the work you are doing, but most of us share the same basic test equipment needs. You can think of the essential items as the "core of four."
The core of four. Because of its versatility, the digital multimeter (DMM) is the one piece of test equipment no maintenance crew should go without. The infrared gun, ground testers (two types), and the insulation resistance (IR) test set make up the rest of the items you must have. This is not to say other test equipment is unnecessary. Switchgear work can require several other test instruments, including low-resistance ohmmeters, and current and voltage injection test sets. If you maintain a large number of motor drives, you'll need a meter that displays waveforms, and you'll probably need specialized motor drive test sets. On the other hand, if you install high-speed copper networks, you need a Cat. 5 test set.
Instead of rendering other equipment useless, these essential tools form the cornerstone of a good test equipment arsenal. Yet, most electrical workers don't use this cornerstone to its full potential. Most use a fraction of the features the typical mid-range industrial DMM offers — even though one of those unused features might solve the particular problem they face. And a lack of understanding about the equipment can exacerbate the problem. Many people use a three-point ground tester with the utility connected, which tests the utility neutral connection, not the ground system. To avoid problems like this, let's clear up some misconceptions surrounding these tools, starting with the DMM.
The DMM. This one device allows you to measure voltage, current, resistance, and other parameters. A typical mid-range-or-better DMM will permit peak sensing at 1-ms or 100-ms sampling — true RMS sensing. It can record minimum, maximum, and average values for the recording interval. You can expand the DMM's capabilities with a range of accessories, but even without them, it's a formidable instrument. A well-trained person equipped with a high-quality DMM and appropriate current and voltage probes can troubleshoot fairly sophisticated power problems.
Here's an example of how one manufacturer used a DMM to solve what appeared to be a difficult power system problem with loading their generators to full load. The site's four generators had been running for years without significant problems when one failed. Instead of rewinding it, the owners replaced it with a different manufacturer's machine. All of the problems that would follow could be traced back to that decision. Neutral cable failures, the loss of two more generators, and difficulties loading the generators to capacity plagued the site after the switch.
Engineers from PRIT Service connected a DMM to the neutral of each generator using a current probe and found the new generator was supplying substantial third harmonic current, which was circulating through the neutrals of the other generators and to the loads. With the DMM, they were able to pinpoint the magnitude and the 180-Hz frequency. Because the DMM offered the capabilities of several test instruments in one, the engineers could leave their harmonics analyzer behind.
The infrared gun. This device can tell you a lot, and using it can prevent certain kinds of preventive maintenance (PM) errors. Many PM procedures call for an annual tightening of screw terminals. This is the wrong approach, because if the screw is already tight, further tightening will damage the threads and weaken the connection. This will increase the resistance at the connection and cause the unit to eventually fail. Using an infrared gun to look for hot spots is an accurate way to check for connections that need repair or tightening -- as long as they are accessible to the line of sight required for the measurement.
The infrared gun can show clogs in the cooling duct filters of electronics, overheated breakers, damaged accessible conductors, and motor gearboxes in need of an oil change. It's useful for checking chiller input and output temperatures or troubleshooting plant air systems, where elevated temperatures can indicate certain problems. You can also use it for spot checks between annual thermographic camera scans of your bus and switchgear. In addition, while liquid-filled transformers are very predictive-maintenance friendly, dry type transformers are not. Yet, with the infrared gun, you can monitor vents and other areas for signs of potential trouble.
Ground testers. Power quality concerns like harmonics, voltage sag, and transient surges are usually symptoms of grounding system problems. With emphasis on power quality issues increasing, ground testers are essential tools for ensuring power quality. They are also essential to protecting people and equipment from undesired current, as well as ensuring proper operation of protection devices. The two common types are the clamp-on tester and the three-point tester.
The three-point ground tester measures the earth resistance of your grounding grid or grounding electrode -- provided you use it correctly. To get a correct and useful measurement, you must disconnect from the utility before taking the measurement. This means shutting the power off and disconnecting the utility neutral.
Because it's difficult to cut the power off, ground testing would appear to be impossible except during new construction. However, the clamp-on ground tester provides a way around this problem. In most situations, it will give an accurate and useful reading of the ground system resistance -- if you meet certain criteria when performing the test. For example, you must calibrate the unit before use, measure only at the final point of grounding, and not measure through any ground loops. Most models will indicate when you have failed to meet the criteria for a valid test. One tester, for example, displays 0.7 ohms when it measures a ground loop.
The IR test set. Insulation resistance testers come in many configurations. You are probably familiar with the traditional hand-crank set, which has been in use for over 100 years. IR testers also come in motor-operated, rectified, and battery-operated configurations. If you're shopping for one, compare the benefits of each model against the particular work you are doing.
Why is an IR test set so important? Insulation suffers from gradual breakdown. Trending this deterioration allows you to schedule cable replacement before catastrophic damage occurs. And by planning ahead you eliminate the expense of unnecessarily replacing good cables just because they've been in use for a predetermined amount of time. The benefits of this condition-based maintenance will usually pay for an IR tester the first time you use it.
You can also use the IR test to assess suspected cable damage. Suppose a transient surge from a lightning strike causes the windings in a large motor to burn open. Unless you knew the condition of the feeder or branch circuit supplying that motor, it would be risky to energize the repaired motor. It's also expensive to replace all that cabling. An IR test can tell you which -- if any -- cables are damaged. In this case, you might use the IR tester to determine the condition of the motor windings before sending it out for repair. IR tests you can perform include:
1-min insulation resistance
These test instruments enable you to handle most electrical measurement tasks, but as with all test equipment, they have one great limitation. This equipment is useful only in the hands of a trained user. As a result, it makes sense to employ NETA-certified test technicians -- who have extensive training on test equipment and test procedures -- as part of a comprehensive testing program.
However, all of us can benefit from thoroughly understanding the many features and applications of the core of four. Several vendors provide classes that explain how to use this equipment, but you can benefit immensely from reading the instruction manuals. Start by taking the time to read the instructions for your DMM, infrared gun, ground testers, and IR test set. If you haven't done this before, what you learn will amaze you.
Hageman is a former president of NETA and a principal with PRIT Services in Minooka, Ill.