A frequently asked question by prospective instrument buyers is, “Why should I pay that price when I can get the same thing for $75?” Seemingly costly equipment is routinely countered by models that proclaim to do the “same thing” for half the price and less! To begin solving this conundrum, start by considering what really constitutes the “same thing”.

Models that offer comparatively little boldly trumpet what they do offer. These are the principle features and specifications, and often match those of more expensive models. The prospective buyer is confused, and may conclude that the inexpensive unit does what is needed. Indeed, it may. But a wrong decision can result in injury, lost time, incomplete testing, and recurrent down time. If the product literature stresses a few eye-catching features in glowing terms, but more detailed information is not provided or readily available, that should act as a flag. The concept of “quality” is easy to claim but tough to specify, without going into engineering language. But a willingness to fully “spec” the product indicates that the manufacturer isn’t banking on customer ignorance. An understanding of additional features makes an educated decision preferable to a “sell”.

Let’s take a look at applying this general concept to the specifics of purchasing an insulation tester. Such an instrument is going to be used around high voltage (but emphatically not on live equipment itself!), employs a high-voltage output of its own, may have to meet both demanding and various test environments, and must have a high degree of sensitivity. In short, it must be safe, rugged, and reliable. Anyone can say this, but what can the user look for to back it up?

Insulation testers are, by definition, high-voltage, high-range instruments. But nearly all operate at the lower end of the scale, as well. This is because they are operating on “both sides of the coin”, so to speak. Circuits must be isolated by good insulation, but they must also be continuous, through good and proper connections. The “continuity” range, therefore, measures low resistance at low voltage. But to do this, the input impedance must switch to a low-impedance state, to accommodate a comparatively large test current. The operator could now be at risk. Insulation testers are not supposed to be connected to live equipment. But through operator error, miscommunication, and system faults, they often are. If such a condition prevailed during a continuity test, and the second probe was not yet connected to the test item, it would become “live” at source voltage. Any metalwork to which the second probe were left connected would similarly become energized. A quality instrument averts this disaster by including a “contact detector” feature.

With this feature, the test circuit remains protected by the high input impedance associated with the high-voltage function until both probes are connected. The tester can sense whether or not a complete connection has been made, and if external voltage is present. It will not switch to low impedance until proper conditions are met. The contact detector works automatically, without the operator necessarily even being aware of the added protection.

Next, there is the importance of “ingress protection”. This doesn’t protect the life of the operator, but that of the instrument. Ingress protection is defined by the International Electrotechnical Commission in IEC Standard 529, and designated by an “IP rating”. This rating should appear in product literature. It is comprised of two digits, each signifying a separate characteristic. The designation indicates how well the item is sealed against invasion by foreign matter, both moisture and dust. The higher the number(s), the better the degree of protection. What would a typical rating of IP54 tell a buyer about the application capabilities of a model?

The first digit refers to particulate ingress. A level of “5” indicates “dust protected”, as well as protected from invasion with a wire down to 1.0 mm. The second digit refers to moisture. A rating of “4” means resistance to “splashing water, any direction.” Now, suppose the rating were IP43. Close enough? Hardly! Such a rating would indicate that the instrument is only sealed against “objects equal or greater than 1 mm”, and “spraying water, up to 60 ° angle from vertical.” For applications in trying environments, like quarries and many industrial processes where dust and flying water are omnipresent, the difference between these two ratings can mean the difference between effective use and excessive repair or embarrassing repurchase.

We’ve looked at increased protection of the operator and instrument, now let’s consider improved performance. The difference between how a model “looks on paper” and actually performs in the field is critical, and can be deceptive. Quality products don’t hesitate to specify details that might well seem almost ridiculously trivial…from the safety and ease of an office desk. Actually using the thing for an eight-hour shift can be a whole different world! It’s much like lifting a 5-pound weight; easy if just to show that it can be done, but numbing if repeated endlessly. Quality testers have subtle conveniences that can only be appreciated by using them on an extensive job.

One such example is the timing and tone of a continuity buzzer. The buzzer is purely a convenience feature that sounds an audible tone when a circuit or connection is continuous below an acceptable resistance value. When “proofing” a job, it frees the operator from having to look at the display, mentally take the reading, and decide if it passes, then move on. When working at a large panel or doing point-to-point testing around a complicated piece of equipment, it allows the operator to focus on making and changing connections, reducing mental and physical fatigue, aggravation and error. The operator doesn’t have to concentrate on the actual reading, only that it’s acceptable, and can work faster.

But evaluation of a beeper feature doesn’t stop with merely its absence or presence. There’s more! A little-known consideration is the timing of the indication. Lesser quality instruments can take up to a second to sound upon contact, whereas improved models will indicate in as little as a few milliseconds. Think you can’t tell the difference? Just think about having a phone conversation where there’s a delay. You think you’re not heard, and start to repeat yourself just as the other party answers. Before long, you’re asking to be called back on a “real” phone. The operator trying to get through a job quickly has a similar aggravation with a “low end” tester. Every little delay adds up until the cumulative effect is greater than the sum.

A related convenience is tone differentiation. More developed models offer distinct tones at different thresholds. For bond testing, no more than a token resistance is acceptable, so a typical threshold would be around 5 W . But for service testing of components, much higher values, into the k W range, are typical. It is convenient for the tester to have separate tones for “high” and “low” range, in order to readily distinguish these two areas of application. But don’t trust the instrument to get it right! Some have gone on the market with arbitrary values that don’t adequately distinguish either application, and can be more confusing than helpful. Check the actual value associated with each tone to be certain that it fits with the equipment that you’ll be testing. Also, don’t overlook the measurement range itself. Economy models may not provide enough range to cover the representative values of all the components and subassemblies that will be encountered. Having to settle for an overrange indication isn’t what measurement is all about. Models that reach to only a few k W fall short of many critical applications.

Just as a pickup truck or a Lexus will both get from one place to another, any insulation tester can be expected to test insulation. But don’t stop short with merely fulfilling the basics. Over its life, the tester will be called upon to meet a variety of situations that may not have been visualized at time of purchase. A minimalist unit will fall short and may require additional acquisitions, while a quality model will prove its worth.

Jeff Jowett is a Senior Applications Engineer with Megger, a manufacturer of electrical test and measurement equipment, including the Megger Ò brand of 1 kV, 5 kV and 10kV insulation testers. He has written extensively on the subject of insulation testing.