Choosing An Insulation Tester

Choosing An Insulation Tester Reprinted with permission from Megger.

To paraphrase Jimmy Durante, there's a million of 'em! Insulation testers. With models available ranging from hand-held to barely portable, from a few hundred dollars to several thousand, the task of making a selection may appear daunting. But a little organization, getting the priorities in order and duly weighted, can simplify the process down to an agreeable level. Let's look at the various characteristics of insulation testers, and what they mean to the selection process.

Start with voltage selection. This is almost always the first determination to be made. Insulation testers are high-voltage DC instruments. Since the purpose of the tester is to evaluate the current-carrying capabilities of material that is expressly designed not to carry current, some special functions are required from the tester. High voltage is necessary in order to pull enough current (commonly referred to as "leakage") to be measurable, and even this is typically on a nanoamp level. Consequently, the measuring circuit must be highly sensitive, and the voltage source highly stabilized. A current-limited DC source provides the necessary voltage and stability without damaging the test item.

Insulation testers provide from one to several discrete test voltages, engaged by a selector switch. Applying the right instrument to the job involves, first, knowledge of the rated voltage(s) of the item(s) to be tested, then a determination as to whether an as-rated or an overvoltage test is desired. Testing at a voltage that approximates the operating voltage of the test item gives a measurement that is a reasonable evaluation of the insulation's capabilities when the equipment is running. Overvoltage tests, which are commonly performed at twice rated, can expose "borderline" insulation where multiple flaws will pass increasing currents at higher voltages, as well as evaluating the capability of the item to withstand line disturbances.

Residential electricians commonly use a tester with a single 500V output, and that's all. But if testing is to be done on a wide range of equipment, multiple voltages will be in order. Insulation testers typically divide between 1 kV and 5 kV models. Equipment operating from standard distribution voltages of 120, 240, and 480 can be serviced by 1 kV models, while utility and heavy industrial applications will call for a 5 kV unit. For convenience sake, equipment operating at such high voltages as 13.2 kV need not be tested at matching voltage. A 5 kV tester can still impart sufficient data to service and maintain equipment operating at the highest voltages. At the other extreme, telecom and data transmission functions typically require testing to be done at 50 and 100 V. These options will be available on a full-function 1 kV tester.

Multiple outputs are also required in order to perform Step Voltage tests. This industry-standard procedure calls for increasing the test voltage at regular intervals, to look for the drop-off in resistance that reveals mechanical flaws such as cracks, tracks, and pinholes. The more voltage that is applied, the more of these imperfections will conduct leakage current. Industry standard is five voltages applied in one-minute intervals, but the test can be adapted to fit the capabilities of the tester.

Possibly the next most important feature is the range that the tester can measure. Testing goals determine whether basic function is all that is needed, or enhanced range is recommended. Simple proofing applications, such as an electrician signing off a job, can be met with a basic range of a few thousand megohms (MW). New equipment, if not defective or damaged in installation, will over-range all but the most advanced testers. In such cases, the electrician is not looking for an actual measurement, but rather wants to see an "infinity" (¥) indication. [Note that "infinity" is not a measurement; it is an indication that the insulation being tested has a resistance that exceeds the measuring capabilities of the tester.] There is no need for a high range for such applications.

But for maintenance of capital equipment, a tester with only a limited range is short-changing the operator. For preventive / predictive maintenance, infinity readings don't represent genuine data. The operator knows that the test item is "good", but not much more. By the point in the equipment's life cycle at which the readings have drifted down into the measurable range, the maintenance person may be left with comparatively little time to schedule routine off-line maintenance. Testers with extended range, even into tera-ohms (TW), afford actual measurements right from the time of installation, thereby establishing a long time line that gives the maintenance personnel plenty of "breathing room". When selecting a tester, become familiar with what the actual readings are expected to be, even for brand new installations. That way, the model that is acquired won't be falling short of the goals of the test regime.

Next, examine the options for power source. On-board generators, operated by a hand crank, remain enormously popular, but not always for a consistent reason. Experienced personnel tend to prefer the "response" of a hand-crank, and although, like the "handling" of a car, this factor can't be objectively evaluated, it's real enough to its advocates. On the other hand, there is no quantifiable basis for a hand-crank giving a "better" test than a battery-powered unit. The true objective advantage to a hand-crank is that it will never let you down! Batteries can go dead, rechargeables can become exhausted, but a hand-crank can be relied upon to operate. If long timed tests are to be performed, however, a battery-powered unit is virtually a must. Batteries achieve the necessary voltage by limiting current, which is, after all, consistent with an insulation tester's function. The tester doesn't need a lot of current to measure a material that is specifically designed to retard current flow. If it passes more than a couple of milliamps, it's no longer doing it's job of insulating! If Polarization Index tests are in order, the operator does not want to have to crank for ten minutes! Battery-powered models are the units of choice for such applications.

A similar issue is that of digital versus analog displays. But in this case, the differences are readily identifiable. The pointer movement ("travel") of a mechanical analog display tells an experienced operator valuable information. Is the pointer traveling smoothly, or "stuttering"? Is it rising steadily, or intermittently dropping back? This kind of detail is difficult or impossible for the eye to extract from the scrolling digits on an electronic display. But whereas pointer travel is desirable, when it stops, the operator is left to interpolate the reading between the scale markings. This introduces an element of judgment which can be a source of error. Digital models present no such problem, as they inform the operator exactly (within the unit's accuracy specification) what measurement has been taken. The most modern testers may feature electronic combination displays that offer both digital accuracy and a moving indicator that travels along an arc scale just as a mechanical pointer would. This innovation combines the advantages of both traditional types of display. But be careful to examine the display closely before selecting an instrument. Less expensive versions may offer a curved bar graph in place of a genuine logarithmic arc, in which the low end of the scale is expanded relative to the high end. Bar graph simulations of pointer travel may not appear to the eye the same as the familiar pointer travel, and may not replicate a mechanical movement to the expected degree. A genuine logarithmic arc, in which the scale positions correspond to the markings on a mechanical scale, will afford complete satisfaction.

If accuracy is paramount, as in dealing with third-party situations, pay close attention to the model's accuracy statement. Don't accept a mere plus/minus percentage for digital units. The statement must also include plus/minus a number of digits, as no digital display can fix its last digit (least significant digit, or l.s.d.) to a single number. Accuracies specified as "percent of reading" indicate the same error at all points on the scale. Analog statements listed as "percent of scale" or "full scale" can be deceptive. Because the accuracy interval is based on the full scale length, it translates into an increasing percentage error as the readings rise against a logarithmic scale. In other words, the same number of pointer widths on the expanded low end of the scale will account for only a few megohms, while on the contracted upper end, this will be hundreds of megohms. Therefore, when meeting a desired or required accuracy spec, don't stop at the percentage statement, but also examine the terms.

Once these features have been decided, the number of models to consider should by then be reduced to a convenient choice. From this point, narrowing to a single unit is a function of two considerations: utility and preference. Some functions simply aren't necessary for a given set of applications, so why pay extra for a feature that will not be utilized? Other functions are a matter of the personal preference of the user, but that should not lessen their significance. Since the tester may be used for hours on end, an option which may seem trivial on purely objective review can be significant under repetitive field operating conditions.

Many insulation testers also act as voltmeters. This is more than just a convenience. While the electrician may appreciate the ability to perform quick voltage checks without having to resort to a second meter, the real purpose of this function is protection, both before and after a test. The voltmeter on an insulation tester should not have to be "asked" (by a selector position). It should engage automatically upon contact with any external voltage. At the initiation of a test, this primarily protects the tester. If the test item has not been properly de-energized, the voltmeter should sense this and warn the operator (some units may exhibit flashing symbols or audible tones). The operator then knows not to proceed with the test, which would damage the instrument. Some more advanced models carry this protection further, to include a function that actually inhibits testing if significant external voltage (that is to say, above mere "noise" level) is present. If an inexperienced operator doesn't realize the significance of the voltage indication and presses the "test" button anyway, no harm done!

But of far greater importance, at the conclusion of a test, the voltmeter will indicate any static voltage remaining on the test item. This could be lethal! Items with large windings or long runs of parallel conductors will become charged by the DC voltage during the test. The level of stored charge can be considerable, and present a hazard upon attempting to disconnect test leads. But the voltmeter function will immediately warn the operator of this condition, then monitor the decay as the discharge circuit "bleeds off" the charge.

Ohm (W) and kilohm (kW) ranges complete the measurement functions of many testers, imparting the ability to measure continuously from a fraction of an ohm to millions of ohms. Because of the lower resistances involved, these functions are performed at low voltage (typically 3 V) and pull larger currents. The ohm range (commonly called "continuity") is employed by the electrician or repair person to determine that a circuit has been fully connected, or that bonds, welds, solder joints and other connections are making sufficient contact. The kilohm range is useful for troubleshooting and repair, in order to positively identify faults. If a high voltage test yields a "zero" reading, it is helpful to be able to confirm this by switching to the kilohm range and observing an actual measurement that is below the resolution of the high-voltage range. In cleaning and drying operations of flooded equipment, the kilohm range is first utilized to gauge the progress of the drying operation. Applying a high voltage megohm test to badly deteriorated, moisture-soaked insulation can promote the development of mechanical flaws such as pinholes. If the moisture-influenced resistance is first measured at low voltage, then a drying operation applied and a second test taken, the effectiveness of the drying process can be readily evaluated and planned. When drying has raised the resistance into the insulation range, high voltage tests can be utilized. The kilohm range is also used to test components and subassemblies which, because they are parts of a larger apparatus, must only meet functional requirements rather than the high resistances of safe isolation from ground. In selecting a tester, be careful not to focus entirely on the high voltage capabilities, and inadvertently overlook these corollary functions.

Some insulation testers have two terminals, others have three. As these are DC testers, two of the terminals are the + and -. The third (if present) is a guard. It does not have to be used, and many persons use insulation testers satisfactorily without ever employing the guard. However, it affords the operator an extra function for diagnosis of equipment problems. The guard is a shunt circuit that diverts leakage current around the measurement function. If parallel leakage paths exist, a guard connection will eliminate those from the measurement, and give a more precise reading of the leakage between the remaining elements. As an example, dirt and moisture on a transformer bushing will promote surface leakage between the + and – connections, thereby bringing down the reading and possibly giving a false impression that the bushing is defective. Connecting the guard to a bare wire wrapped around the bushing will intercept this current, and yield a measurement based solely upon leakage through defects in the ceramic.

It is most important not to confuse the guard with a ground. Connecting the guard and return lead to the same element of the test item only shunts the current that is supposed to be measured, and thereby short-circuits the measurement function. When selecting a tester, consider the goals of testing (basic installation checks don't generally require a guard), the electrical composition of the items to be tested (motors and transformers can be tested for leakage between windings, with ground leakage eliminated), the possible effects of surface leakage (wire and cable can carry current across the surface, via dirt and moisture, as well as through the insulating material), and the degree to which results must be analyzed (are "bad" items merely to be replaced or discarded, or will it be necessary to localize faults for possible repair). Testers with guards generally cost a bit more than two-terminal models, so don't pay for it if it's never to be used. On the other hand, in many applications, a two-terminal model won't be imparting the full spectrum of information that can be accrued by insulation testing.

New models now include various capabilities for data storage and downloading. This isn't a core function of insulation testing but an enhanced convenience, like a stereo system in a car. Test records can be stored and organized, such as by panel board and circuit, recalled for comparison to present results, and printed out in graphics or as test reports. The ability to print test reports, aside from its obvious convenience, has the added advantage of eliminating the possibility of human error in transcription. This is especially useful in dealing with clients or authorities, as the test results and conditions are transposed directly from the tester to the report, and therefore are above questioning. Some testers offer only limited "real time" download, in which case the results are not stored by the tester, but must be downloaded in process to a laptop or printer. If on-board memory is desired, read the product specifications carefully. Some retain only the last test result, but literature that is only scanned may leave the impression that the unit has memory. Then compare different offerings for how many events they can store. Also examine closely the ability to recall data. If desired, be sure that stored results can be recalled to the display. Some models only offer recall through a printer or computer, which is fine for the office but not at all convenient for field work when a present reading must be evaluated for deviation from the previous result. And don't forget the software! With memory and appropriate software, the unit may be left on-line to double as a data logger. Some models will have it included, but others may offer options. In some cases, these are available at a nominal charge, but in others they may significantly extend the cost of insulation testing.

As if all this weren't plenty, microcircuitry has permitted the inclusion of so many functions that a small, hand-held model may now offer full multimeter capabilities as well. Look for a millivolt input. With this, appropriate transducers can be connected that will expand the tester's applications into hvac, telecommunications, and process control. Current measurement may be performed directly, to the milliamp level, or expanded indefinitely with optional current transducers. Some advanced units also display leakage current. Don't confuse this with current measurement. The display of leakage current is showing the operator the current that is flowing through the insulation, not that which is flowing in a circuit. It is the inverse of the megohm reading. Leakage is the standard measurement in certain applications, such as the isolation of faulty power cables prior to fault locating, so the capability of reading both leakage and resistance may be a useful feature for multple applications.

The benefits multiply with the features: in addition to having to acquire and carry only one unit, test time is saved when performing all tests without having to change testers, and regular calibration and maintenance responsibilities can be reduced to a single unit. But don't lose sight of the benefits of basic-function models, either. It's more than just a matter of less expense. While full-function models have reached new heights of development in recent years, simplified units remain indispensible. For new or not-fully-trained personnel, or those "borrowed" from other departments to meet contingencies, a basic-function model can reduce or eliminate the possibility of human error that can waste whole days of testing. It is also much more convenient to write the organization's Standard Operating Procedures (S.O.P.s) around models with few superfluous controls. Ergonomics should not be overlooked. How much does it weigh? Like lifting a 5-pound weight, once is nothing, but a thousand times can be another story. The operator may have to use the unit under trying conditions where small advantages telescope. Models are available with a variety of straps and attachments to make them easier to carry and manipulate. Special leads with just the right method of attachment or with test buttons that override the panel switches can free the operator's hands for difficult-to-reach situations. Backlights and locking test buttons can save time by cutting down the amount of retesting in poorly lighted areas.

Don't overlook the "fine print". There's lots of stuff at the bottom of product bulletins that commonly gets overlooked, but could be key information. These include IEC1010 ratings, IP ratings, and load graphs.

The first of these is a safety rating established by the International Electrotechnical Commission. It dictates various design considerations that insure that an instrument is adequate for the hazards of the environment in which it is to be used. Remember, the operator may be working just inches from thousands of volts. A poorly designed instrument, or one of an inadequate rating, can develop an "arc blast" that can be lethal to the operator. When evaluating an IEC1010 rating, put simply, the higher the number, the safer the instrument. The rating consists of two designations: a category (CAT) and a voltage rating. Make sure they're both specified. Some bulletins may list only the "CAT", and neglect the accompanying voltage. The category rating indicates how far "downstream" from the utility feed the instrument may be safely used. The voltage rating indicates the maximum phase-to-ground voltage that the test item can be rated. An instrument rated CAT II cannot safely be used in a CAT III application, even if its voltage rating is higher than that of the test item. Similarly, a CAT III 300 V model is not applicable to a CAT III 600 V environment. Determine the highest-rated environment in which the tester will be used, and select a model that meets or exceeds it. If no rating is available, don't buy it!

The IP rating doesn't relate to a condition that can be lethal to the operator, but certainly to the instrument. Defined in IEC 529, "IP" indicates "ingress protection". It specifies...reliably...the degree to which the protective casework of the instrument can keep out dirt and moisture. These won't kill the operator, but they can make the instrument short-lived indeed! Any advertising blurb can call a unit "water resistant" or the like, but the IP rating actually gives objective meaning. As with IEC1010, the higher the number, the better the rating. This consists of two digits. The first relates to ingress of solid objects, rated by diameter of the largest object that can penetrate. The second digit refers to moisture, classified by degree of exposure. The highest degree of particle protection is "dust tight", indicated by a rating of 6. The highest grade of moisture resistance is "submersible", rated as 8. When selecting a tester, consider the environment in which it will be operating, and evaluate that against the IP rating. Suppose the unit is rated at 42. That would indicate particle protection against objects no smaller than 1 mm. Suitable for a factory with flying dust? Hardly. A moisture rating of 2 indicates dripping water. Resistant to flying spray? Not so. Acquiring an instrument for an environment that exceeds its IP capabilities likely means that you'll need another very soon.

An insulation tester should also be provided with a "load graph" that indicates output voltage characteristics against load resistance. Remember, the tester does not need much current in order to test a material specifically designed to impede it. Therefore, insulation testers have limited output currents. The consequence of this is that they will load down and drop voltage when the test item imposes a resistance insufficient to be considered adequate insulation. As already discussed under the kilohm function, this characteristic helps protect deteriorated insulation from permanent breakdown, when it might be restored to "health" by basic front-line maintenance. A quality insulation tester will present a load graph that exhibits a steep drop in voltage at the low end, but when viewed the other way, a sharp rise in voltage up to a level of resistance commensurate with good insulation. Since the time-honored industry standard is that insulation should never be less than one megohm (the "One-Megohm Rule"), this is a good benchmark when evaluating a load graph. Voltage should rise sharply up to anywhere from one to five megohms, depending on the voltage selection, and maintain that voltage at all higher resistances. Graphs that climb slowly up to the selected output voltage may be giving readings at well below the selected voltage across critical parts of their range. This is not good for comparative record keeping, conformance to standards, or client specifications. Some testers merely specify the resistance range over which the model exhibits full output voltage. This is merely a difference in wording. But if no load graph or equivalent definition is available, that says something about the quality of the unit.

Make a checklist of what is critical, then one of what is desirable. By evaluating the available models in a manner that is orderly and thorough, you won't be disappointed or short-changed by the final selection.