An electrical system must have well-maintained insulation to operate properly and safely. Because insulation deteriorates with age, accurately testing its condition is essential.
The megohmmeter is the basic testing tool to measure the resistance of current flow between any two points to determine how effective insulation is in resisting flow of the electrical current. Tests are made on a deactivated system—most testers will not operate if voltage is present.
Jeff Jowett, senior application engineer at Dallas-based Megger, said insulation keeps people from being electrocuted and prevents fires. Beyond that, insulation specifications are critical in electrical design and a system must remain within spec throughout the equipment’s life.
Insulation deterioration, he said, allows increasing amounts of leaking current to flow through random channels throughout a device and can negatively affect operation and performance, especially with the more sensitive items.
Insulation resistance is relatively easy to measure and trend, and it can create a timeline for replacement or predictive preventive maintenance, which is invaluable information on the prospective remaining life of the item.
Jowett said there are three fundamental types of insulation testers:
- Most common are 1 kilovolt (kV) testers used for all building applications
- Medium-voltage testers (2.5, 5, 10 and 15 kV) for equipment higher than 1 kV, usually matched to rated voltage where it will be used
- Testers for telecom, datacom and other sensitive equipment that must be tested at lower voltages (500V and less) and special testers that only reach lower values
“There is one notable exception to the above, and that is the solar industry,” Jowett said. “Voltages are rising, and the demand on test equipment is rising with it. The traditional 1 kV ceiling for smaller, handheld models is being challenged, with 1,500V now a prime and sought-after test voltage. This can be expected to go higher in the future, and the testers will develop in step.”
Jowett said a major trend is testing voltages above 1 kV with handheld testers.
“This can be done anywhere, but is particularly advantageous in the solar industry where voltage and current characteristics are expanding and a small, easy-to-use tester is a must when moving around acres of solar array,” he said.
Another impressive improvement has been adjustable test voltage across the entire range of instruments.
“The operator no longer has to make a judicious decision among four or five available test voltages,” Jowett said. “Modern full-featured models include a blank dial position where the operator can set any voltage, even down to one-volt increments, across the entire range of the instrument. This enables better adaptation to the specs of the test item, precise conformance to standards, which are increasing in importance, and more diagnostic depth when analyzing or troubleshooting.
“Conformance to standards also is aided by a tightening of voltage accuracy. All field insulation testers will load up test voltage slightly with no harm done to the test or the test item. But limiting this to a narrow range enhances the quality of standards conformance and consistency in comparing test results,” Jowett said.
“LCDs are much improved in the amount and quality of information instruments provided. The main resistance measurement is enhanced with additional data: test voltage and current, test conditions and warnings, can all be seen on the display at once, which frees the operator from extra work and the possibility of costly oversight or error.”
Additional functions such as phase rotation, temperature, capacitance and inductance measurements save the operator time and error, which reduces the amount of equipment that needs to be carried to a job.
Measurement range has been extended all the way to tera-ohms for longer-term predictive maintenance. A built-in ramp test provides detailed and sophisticated information about insulation condition and developing problems.
“Remote operation adds another layer of safety to testing in potentially dangerous environments such as substations, and asset management software is a time-saving tool in predictive/preventive maintenance,” he said.
John Olobri, director of sales and marketing at AEMC Instruments, Dover, N.H., said insulation testers’ primary role is safety, followed by helping prevent costly equipment failures by predicting the possibility of pending failure. He added that dedicated insulation testers have the latest features for insulation testing that multipurpose testers lack.
Olobri said handheld, battery-powered testers are best used on small motors, pumps and short-length cables where a long test time is not necessary. AC-powered benchtop units are best used on large motors, pumps, transformers and long cables where higher test voltages and longer test times are required.
“More of the calculations by today’s tester models are performed by the instrument, such as temperature compensation,” Olobri said. “Today’s models also provide more access to the testing and results using wireless devices such as tablets and smartphones.”
Olobri suggests buyers of insulation testers should consider the products’ available test voltages, test current and available test modes such as spot testing, timed testing, ratio testing and step and ramp voltage testing.
David Kadonoff, field sales engineer for Sycamore, Ill.-based Ideal Industries, said insulation testers confirm whether the insulation is properly isolating the transmitted voltage and current within the specific conductors so it can provide efficient use of power, and to provide safety against possible electrocution.
He said multiuse testers cannot provide enough power to determine if a partial insulation fault (charred insulation, for example) is failing. The higher voltages that true insulation testers apply, such as 250V, 500V, 1,000V and even much higher, will cause issues to reveal themselves, which would easily be missed at 4V.
“Analog display, battery or hand-cranked testers can be used for general maintenance routines,” Kadonoff said. “Digital display units are slightly higher in cost, but are more commonly used and offer many more functions. Modern digital insulation testers actually are digital volt meters first. If they detect a live circuit above 30V, they prevent the unit from outputting a test voltage. DUT (device under test) must be de-energized and isolated so that the operator knows precisely which portion of the electrical distribution circuit is being tested. Larger digital units with internal memory and graphical displays are typically used at the utility and industrial maintenance levels where report generation is required.”
Insulation breaks down over time due to the heat and cold, exposure to UV and chemicals, carrying too much load, having more than the specified number of conductors in a conduit and more, he said. So if a piece of equipment is down for maintenance, an insulation test may make sense.
When evaluating insulation testers, Kadonoff said, test voltages are important.
“Some testers cannot go high enough to reach the proper test voltage, and many do not go low enough,” he said. “Avoid testing low-voltage cable at 250V if that is as low as the test will go. Timed tests, or at least a lock button, are recommended, and a UL listing is very important to many users.”
Because insulation testing uses more power than a multimeter, Kadonoff suggests taking an extra set of batteries to jobs. Batteries should be removed when testers are not in use.
Sabrina Kalsi, product development manager at Klein Tools, Lincolnshire, Ill., said insulation resistance testers are used for preventative maintenance to ensure an electrical system’s insulation is safe, and testing may occur at installation, during troubleshooting or at annual maintenance. An insulation resistance tester is used to confirm the electrical insulation is intact or warn that the electrical system has current leakage and needs maintenance.
Insulation is subject to mechanical damage, vibration, excessive heat or cold, moisture, chemical vapors and dirt or grease, which can cause systems to fail, Kalsi said. A pinpoint hole or nick in the insulation’s surface provides a path for unsafe leakage current.
“Insulation testers today are smaller, and less expensive options are available,” Kalsi said. “There are megohmmeters capable of producing 125V, 250V, 500V and 1,000V test voltages to test the insulation resistance measurements.”
“For new installation of wires, heated flooring or equipment, tests are made to ensure there were no cuts/damage during the installation,” she said. “Over time, a product’s insulation degrades from exposure to chemicals, repetitive movement or a breach in housing, and periodic insulation testing can confirm if the system meets standards or may need some maintenance.”
Sean Silvey, product specialist at Fluke Corp., Everett, Wash., said a best practice is to perform insulation tests during routine maintenance. This will allow a good picture of the assets with the organization. It will allow the customer to trend equipment to determine potential future failure. He added that software and logging capabilities for today’s testers are growing.
“Most insulation testers will do a quick check to determine if there is a problem in a motor or component,” Silvey said. “For the best test results, temperature and humidity will need to be calculated into this measurement. Some testers have a PI/DAR function, allowing a longer test based on a ratio of measurement. The DAR, or dielectric absorption ratio test, is a timed test that will measure for one minute. The PI, polarization index test, is a longer test that will measure for 10 minutes. The PI/DAR tests take temperature and humidity into consideration.”
Silvey said a key consideration when selecting testers is what level of voltage will be needed for making tests.
“Best practice is to double the value of the rated equipment when testing,” he said. “If the motor is a 120V motor, the test can be at 250V. Also, users will want to have a digital insulation tester that is self-discharging after they make a test. This will dissipate any charge after a test. Older crank versions of the past did not have this.”