Every home, business and industrial facility contains electrical wires that bring in and distribute power for lighting, heating and cooling systems; for computers and other equipment, such as motors and generators; and for multiple other purposes. These wires are encased in various types of insulation that provides basic protection against shock, limits the risk of fire, and contains the flow of power to its intended destination.


However, insulation is subject to harm and deterioration. Various environmental and physical factors can be detrimental to this insulation and cause potential damage to the electrical system and its components, increasing the risk of shock or fire. Therefore, regular testing of insulation is essential to detect insulation failures, keep the system operating as designed, and prolong its life.


Some multipurpose test meters include insulation-testing capabilities, and a variety of specialized instruments are available to detect failures in insulation. Four manufacturers share information on these products.


According to Jamie Cohen, Flir Systems/Extech product support manager, test and measurement: “There are more combination meters on the market that combine an insulation tester with DMM [digital multimeter] functions. While a DMM may be able to detect a dead-short, insulation testers stress the system under test with a high voltage and can identify failures or potential failures that a DMM may not be able to identify.


“While DMMs and insulation testers measure resistance, the two units perform the measurement differently. A DMM uses a small test current and looks for a voltage drop to make a resistance measurement. The insulation tester uses a high voltage to excite the equipment under test and looks at changes in the current—instantaneously and over set periods of time—to calculate the resistance. To make things more complicated, there are multiple current parameters to consider during an insulation test: capacitive charging current, absorption current and leakage current.


“There are insulation testers with either analog or digital displays, and there are units that provide the basic insulation test function, often with a ‘hold’ button to make testing easier. There also are units that provide the time-based dielectric absorption and polarization index tests as well as the basic test,” Cohen said.


Fluke Corp.’s Luis Silva, electrical test products manager, said: “While basic insulation-testing procedures and technology remain the same, new convenience and productivity enhancements have been introduced.


“For example, our Connect mobile app allows wireless communication with specific tester models. This app saves time and increases productivity by capturing readings up to 15 meters away on a smartphone, then using a live video call to share test results in real time with team members wherever they are. Additionally, readings over time can be viewed on a smartphone to establish baseline.


“The most important reason for testing insulation is to ensure public and personal safety. By performing a high DC [direct current] voltage test between de-energized current-carrying (hot), grounded, and grounding conductors, the possibility of having a life-­threatening short circuit or short-to-ground can be eliminated. This test usually is performed after the initial installation of the equipment. This process will protect the system against miswired and defective equipment, and it will ensure a high-quality installation, customer satisfaction and protect against fire or shock.


“The second most important reason for insulation testing is to protect and prolong the life of electrical systems and motors. Electrical systems are exposed to dirt, grease, temperature, stress and vibration that can lead to insulation failure, resulting in loss of production or even fires. Periodic maintenance tests can provide valuable information about the state of deterioration and will help in predicting possible failure of the system. Correcting problems will result in a trouble-free system and also extend the operating life of a variety of equipment.


“A combination tool is designed for people who work on a regular basis with motors and other insulation-test candidates as well as other general electrical maintenance tasks. For those who do not wish to carry multiple tools, the combination tool serves as an all-up, go-to tool. However, there are many ranges of insulation testing required, some of which outstrip the capacity of an all-in-one meter. For those tests, a higher range dedicated insulation tester is required. In other cases, such as winding shops, a single-function insulation tester may be all that is required.


“Multiple-use testers can be used for insulation tests, provided that the meter has been designed for the task at hand. Some standard-issue multi­meters measure ohms/resistance but do not test insulation. Combination function meters are specifically designed to provide the regular functions of a digital multimeter as well as insulation-­resistance testing,” Silva said.


Robert D’Amico, Hioki’s senior sales support engineer, said: “Performing insulation tests is an integral part of the preventive maintenance of a system or component. Testing voltage and insulation-resistance range are most important. Other items to consider are size, portability and pass/fail indications. The insulation test ensures that the resistance value between the supply voltage and ground of the system or component remains at a high level on the order of hundreds of mohms [megohms] to teraohms, preventing a short-circuit situation.


“The application for a specific voltage and resistance range will depend on the component or system being checked. Typically, one insulation tester will vary from another by its test voltage and resistance range. In most cases, multiple-use testers do not have the resistance ranges necessary to perform such tests. At this point, the only improvements in insulation testers are more features,” D’Amico said.


Jeffrey R. Jowett, Megger’s senior product engineer, said: “Good insulation delivers power to the load both safely and efficiently. Leakage losses are minimized, which enables the load to operate within spec and with maximum efficiency, while heat loss is minimized to prevent dangerous buildup in confined areas with subsequent combustion. Most important, personnel are protected from shock and can operate in a safe working environment.


“Principle variables are power source, range and movement, and test voltage. Power sources are battery/line or hand-cranked. Hand cranks do not provide a better test, but they do free the operator from human error—dead batteries—and also will operate in environmental extremes that may defeat batteries.


“Measurement range can be basic or high-range. Basic range to a gigohm or so is available in economic meters and is all that is needed for pass/fail applications.


“High-range models to teraohms are necessary for preventive/predictive maintenance and record-keeping. Analog movements are preferred by veteran operators who can draw conclusions from pointer travel. But mechanical analogs are easily damaged.


“Electronic displays add digital accuracy to the measurement, are easier to interpret and more rugged. Combinations are available that offer both digital reading plus an electronic pointer. Curved bar graphs do not lend to interpretation as well as logarithmically expanded scales. Finally, 50- and 100V tests are most employed in telecom/datacom applications; odd lower test voltages in some specialty applications like maritime and aeronautical; 220V to 1 kV for building-wiring applications; and 5, 10, 15 kV and higher for medium- and high-voltage test items.


“Multifunction testers can be used in some instances, but it is necessary to be careful to avoid multimeters that read into the megohm range because these are typically operating at only a few volts and do not provide a representation of the insulation’s capabilities under load. The tester must provide test voltages at operating level and above. 


“Also, multifunction testers, by definition, are more complicated to operate. They aren’t designed just for the job at hand. What may be gained in cost of instrumentation can be lost in work time. Finally, dedicating a tester to a single function or coherent regime of related functions facilitates maximum capability in the design process. It provides the best possible instrument for an application,” Jowett said.