Ask the right questions when selecting an insulation tester

With models available ranging from hand-held to barely portable, and from a few hundred dollars to several thousand dollars in cost, the task of selecting an insulation tester may appear daunting. By organizing and evaluating the priorities, the process can be simplified. Voltage range Voltage selection is almost always the first determination to be made when selecting an insulation tester.

04/01/2005


Key Concepts
 
  • If an insulator passes more than a couple of milliamps, it's no longer doing its job of insulating.

  • When meeting a desired or required accuracy spec, don't stop at the percentage statement, but also examine the terms.

  • The voltmeter on an insulation tester should engage automatically upon contact with any external voltage.

Sections:
Voltage range
Measurement range
Power source
Digital vs. analog displays
Accuracy
Voltmeter capability
Ohm and kilohm ranges
Guard terminal
Further considerations
More Info:

With models available ranging from hand-held to barely portable, and from a few hundred dollars to several thousand dollars in cost, the task of selecting an insulation tester may appear daunting. By organizing and evaluating the priorities, the process can be simplified.

Voltage range

Voltage selection is almost always the first determination to be made when selecting an insulation tester. Insulation testers are high-voltage dc instruments. Because 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 an nA 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.

Depending on the model, insulation testers provide either one or several discrete test voltages, which are engaged by a selector switch. Applying the right instrument to the job requires knowing the rated voltages of the items to be tested, and then a determination 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 %%LOOKSI%%%%SMALLA%% where multiple flaws will pass increasing currents at higher voltages %%LOOKSI%%%%SMALLA%% and evaluate the capability of the item to withstand line disturbances.

If testing is to be done on a wide range of equipment, multiple voltages are in order. Typically, insulation testers are available in 1 kV and 5-kV models. Generally, 1-kV models can service equipment operating from standard distribution voltages of 120, 240, and 480 Vac. However, utility and heavy industrial applications require a 5-kV unit. 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.

The step voltage test is an industry-standard procedure that requires increasing the test voltage at regular intervals to look for a decrease in resistance that reveals mechanical flaws such as cracks, tracks, and pinholes. The more voltage that is applied, the more these imperfections conduct leakage current. Industry standard is five voltage levels applied in one-minute intervals, but the test can be adapted to fit the capabilities of the tester.

Measurement range

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. 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 (%%LOOKSI%%Þ) indication. It is important to 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.

For maintenance of capital equipment, a tester with only a limited range is short-changing the operator. For preventive or predictive maintenance, infinity readings don't represent genuine data. The operator knows that the test item is "good," but not much more. The point in the equipment's life cycle at which the readings have drifted down into the measurable range may leave the maintenance person left with comparatively little time to schedule routine off-line maintenance. Testers with extended range afford actual measurements from the time of installation, thereby establishing a long timeline that gives maintenance personnel plenty of breathing room (Fig. 1). When selecting a tester, become familiar with what the actual readings are expected to be, even for new installations.

Power source

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! Standard batteries can go dead, rechargeable batteries can become exhausted, but a hand-crank can be relied upon to operate. If long timed tests are to be performed, a battery-powered unit is virtually a must.

Digital vs. analog displays

The pointer movement, or 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. SoSme testers 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 feature combines the advantages of both traditional display types.

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, is useful.

Accuracy

If accuracy is essential, 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 LSD) 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. Therefore, when meeting a desired or required accuracy spec, don't stop at the percentage statement, but also examine the terms.

Voltmeter capability

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 %%LOOKSI%%%%SMALLA%% both before and after a test.

The voltmeter on an insulation tester should not have to be "asked" by selecting a switch 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 deenergized, the voltmeter should sense this and warn the operator. The operator then knows not to proceed with the test, which would damage the instrument.

Advanced models include a function that actually inhibits testing if significant external voltage is present. If an inexperienced operator doesn't realize the significance of the voltage indication and presses the "test" button anyway, no harm is 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 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, and then monitor the decay as the discharge circuit bleeds off the charge.

Ohm and kilohm ranges

Ohm and kilohm 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 electrical 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 used 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 performed, 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 used. The kilohm range is also used to test components and subassemblies that, 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.

Guard terminal

Some insulation testers have two terminals; others have three. As these are dc testers, two of the terminals are the + and

Using the guard connection offers an extra function for diagnosing equipment problems. The guard is a shunt circuit that diverts leakage current around the measurement function. If parallel leakage paths exist, a guard connection eliminates them from the measurement, allowing a more precise reading of the leakage between the remaining elements (Fig. 2).

For example, dirt and moisture on a transformer bushing promotes surface leakage between the + and

It is very 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. In terms of guard connections, insulation tester selection considerations should include:

  • Testing goals (basic installation checks generally don't require a guard)

  • Electrical composition of the items to be tested (motors and transformers can be tested for leakage between windings, with ground leakage eliminated)

  • 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)

  • Degree to which results must be analyzed (Are "bad" items 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 this feature 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.

      Further considerations

      Data storage and download capability Insulation tester models now include various capabilities for data storage and downloading. 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.

      Multimeter functions Some small, handheld models offer full multimeter capabilities. Current measurement may be performed directly, to the milliamp level, or expanded indefinitely with optional current transducers. Some advanced units also display frequency and/or leakage current. Don't confuse this with current measurement. The display of leakage current shows the operator the current that is flowing through the insulation %%LOOKSI%%%%SMALLA%% not that which is flowing in a circuit.

      Safety rating The International Electrotechnical Commission established safety ratings. A safety rating dictates various design considerations that insure that an instrument is adequate for the hazards of the environment in which it is to be used. A poorly designed instrument, or one of an inadequate rating, can develop an arc flash or arc blast that can be lethal to the operator.

      When evaluating insulation testers for IEC1010 ratings, 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. 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. 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!

      IP rating

      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. Any advertising blurb can call a unit "water resistant," but the IP rating actually gives objective meaning. The higher the IP 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.

      Load graph

      An insulation tester should also be provided with a load graph that indicates output voltage characteristics against load resistance. 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.

      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. Voltage should rise sharply up to anywhere from 1

      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, and 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.

      More Info:

      More details on this subject are available at plantengineering.com . The author is also available to answer questions about insulation testing. Mr. Jowett can be reached at 610-676-8539 or jeff.jowett@megger.com . Article edited by Jack Smith, Senior Editor, Plant Engineering magazine, 630-288-8783, jsmith@reedbusiness.com .





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