Choosing a Megger

Choosing a Megger Insulation Tester

With over 30 models to choose from, selecting the proper Megger Insulation Tester can appear a bewildering task at first. Actually, all the process requires is a little organization. All Megger Insulation Testers perform essentially the same test in fundamentally the same manner, accurately and reliably. Refinements and added features, however, separate one model from another, in application and operator appeal.

Make a check list of important or essential features and specifications. This should automatically reduce the choices to a workable number, from which personal preferences can easily determine the final selection.

Test Voltage(s) – An electrician interested only in installation and proof testing may need only a single voltage. A repair or maintenance man, however, may want the diagnostic capabilities that derive from comparing tests at different voltages. Base your voltage requirement(s) on the rated voltage of the equipment to be tested, then decide if you want to test at rated, or perform stress tests at higher voltages. Do you want to carry out Step Voltage Tests?

Remember, pervasive insulation damage like moisture and oil soaks are revealed at any voltage, while mechanical damage like pinholes may require voltages high enough to arc an air gap in order to be detected. Test instrumentation commonly makes a quantum leap from 1 kV to 5 kV, so this may be your most critical voltage determination and most significant decision in selecting a tester.

Measurement Range – For an electrician or repairman interested only in proofing, infinity readings may be sufficient. For predictive maintenance, however, it is critical to be able to see the change in resistance between successive measurements, even though the actual values remain exceedingly high. Don’t limit your testing capabilities with a short-range model. The newest technologies permit resistance measurements to the tera ohm (T) range! Try to determine the insulation resistance values of your equipment when new, then select a tester that can actually measure to these values.

Power Source – The test is the same, irrespective of the power source. Not everyone believes you can get 1000 volts out of AA’s, but you can! Current limitation is the means. Batteries free the operator from the extra work of cranking, while hand-cranks relieve dependence on batteries and the possibility of human error. Remember, if you plan to do Polarization Index (PI) testing, you don’t want to crank for 10 minutes! Rechargeables are the most convenient, but throwaways are ready when you are without having to be charged over night.

Voltage Detection – Most models feature detection of unwanted voltage on the test item. Electricians may want an audible signal for rapid trouble-shooting that is not dependent on visually monitoring the display. Maintenance men for large equipment will want to be able to see high-voltage capacitive charges decay at the conclusion of a test.

Display – Digital or analog is largely a matter of preference, but newer models combine both capabilities in a single, convenient display.

W/kW Ranges - Generally referred to as “Continuity” and “Resistance” ranges, these are low-voltage, mid- to low-range functions that add greatly to the depth of testing capabilities that your tester offers. They can make the selective difference between several models that are similar in the more apparent functions, and should not be overlooked. Ohm (W) ranges can be used to verify integrity of circuits and connections, while kilohm (kW) ranges are useful in locating areas of insulation deterioration. The electrician will want an ê -range, the maintenance man will want kW, the repairman will want both!

Guard Terminal – This third terminal shunts the measurement function. It is useful in eliminating certain components of leakage from the measurement, and provides a valuable extra tool in analytical work. The electrician may not need it, but the maintenance man should, and the repairman will!

Price – Megger Insulation Testers range in price from a few hundred to several thousand dollars. Regardless of your budget, there is a model that will fit, and offer a surprising range of features as well.

“Extra” Features – Remember, Megger Insulation Testers are not arrayed from “good” to “better” tests; all perform accurate and reliable measurements. Rather, additional capabilities and greater flexibility are achieved from one model to the next. The newest versions offer such additional features as pre-programmed standard tests (Polarization Index, Step Voltage, Dielectric Discharge), calculation and storage of results, downloading to computer or printer, timed tests, measurement of leakage current and capacitance, and “burn” mode. These may not be necessary, but they certainly are convenient. If it fits your budget, go for it!

Insulation Testing / Affording the Best

The Megger name has traditionally been associated with the highest quality in Insulation Testers. But high quality need not impose a high price. Through years of leadership in the testing field, Megger has rightly become recognized as the highest standard in test instrumentation. But that fact need not be interpreted by those with tight budgets or limited applications as putting Megger quality beyond their reach.

Remember, there is no such thing as a “better” insulation test…not as long as a Megger instrument is employed. All of our models perform a reliable, high-quality measurement to rigorously specified standards. The more advanced models provide greater capabilities…more features, more flexibility…added to the basic concept that has defined Megger for a century: the performance of the best possible insulation measurement.

Megger Continuity Testing

Most models of Megger Insulation Testers have “continuity” ranges. It is not uncommon, however, for this function to be regarded much like the low gears on an automatic transmission; that is to say, by disuse. This practice isn’t too hard to rationalize. After all, insulation testers are thought of as high-voltage units capable of measuring enormous resistances. The opposite ends of the resistance and voltage spectra are easily overlooked, inadvertently consigned to the province of multimeters.

That “W ” symbol next to the selector switch indicates a low-voltage, low-resistance test. Specifications vary somewhat with the model, but in general (and for all of the new hand-helds) a fixed 5 V dc measures from 0.01 to 99.9 W . If nothing else, this convenient function frees the operator from obtaining, carrying, locating, and employing a second piece of instrumentation. You don’t have to pause to hook up a multimeter when a simple click of the switch on the insulation tester will do the job.

There’s more. A continuity range neatly complements the already considerable capabilities of the insulation resistance ranges, giving the tester an added dimensional quality. If the insulation test pegs to the low end, assessment of the problem doesn’t have to stop there. Unexpectedly low resistance? Your Megger Insulation Tester doesn’t just tell you it’s low; click to the continuity range, and you can determine how low.

Insulation looking just fine? What about proper circuit connections? Wherever there’s insulation, there’s also a current path, and this needs to be checked and maintained just as well as the insulation. The continuity range can determine if connections are tight, or indeed, if they have been made to the correct circuits! Old connections can be checked for corrosion, or for assurance that they haven’t loosened. New connections can be checked for a good weld or solder joint. The 200 mA current produced by the newest models is especially useful in this manner, making the Megger Insulation Tester not just the equivalent of a multimeter, but critically superior. The robust test current stresses bad joints and reveals weaknesses in marginal electrical connections that will pass the few milliamps produced by common multimeters with deceptive ease. In short, the continuity function not only determines that current flow is established across connections, but that it is also not present between isolated elements.

The electrician can perform his full function with a single instrument, assuring that isolated surfaces are properly insulated and that wires have not been skinned or damaged, and that connections have been made between the correct elements and are tight. Models with buzzers are particularly advantageous in permitting concentration on the work, not on the display. Maintenance men installing motors and other equipment can put the unit through its specifications test, then check for good connections. Motor repairmen can test that the insulation has been brought back up to spec, then check that all the proper reconnections have been made, in order and firmly. Technicians and design engineers can trouble shoot boards and circuits quickly and easily. Verify plate-through on sandwich boards with the touch of a probe. Continuity buzzers speed the process while applying a quantified pass/fail criterion of 5 W . Even heavy-duty applications can be enhanced with judicious use of the continuity function. Utilities can help characterize a cable fault by determining the amount of resistance, an important step in deciding on the proper locating equipment and procedure to reduce trial and error.

Even the simplest Megger Insulation Tester can provide a surprising wealth of added applications once all of its functions are fully utilized.