By Jeff Jowett

Take time to thoughtfully match test equipment to the requirements of a particular application. It’s an understandable tendency to try to grab whatever is at hand and get going! Some tools are so trusty that they are taken automatically, as if they had “universal” application. But on the job, they may be inefficient, taking too long or providing incomplete information. Without a knowledge of alternatives, such shortcomings might not even be recognized. But all the while, job efficiency could be suffering and the chances of having to do additional or repeat work at a later date increased. It’s worth the time to pause long enough to match the tool specifically to the job in order to accomplish every task to its maximum effectiveness while at the same time not wasting time with unnecessary functions. And what applies to time also applies to money. There is no sense paying for features that you will never use when less expensive equipment is available without them. On the other hand, you could be skipping some critical aspects of a job or taking too much time with some clumsy manipulation when features are available that will accomplish the same task(s) quickly and conveniently. We will take a look at insulation testers and examine how various features and functions apply...or perhaps do not.

Determining If the insulation Is No Longer Doing Its Job

Insulation testers are among the most fundamental of electrician’s tools. Basically, they are connected across insulation, as for instance “hot” and neutral to ground in a basic two-wire circuit, and apply a high test voltage to the insulation barrier. No insulation is perfect; apply enough voltage and current will flow. Air, as an example, is a pretty good insulator, but think of a lightning stroke! The high voltage pulls some current through the insulation, but it’s not going to be much. Insulation testers have to be able to accurately measure nanoamps. This is typically called “leakage” current. The test voltage and leakage current parameters are then combined through Ohm’s Law (R = V/I) to yield a resistance measurement. Insulation testers are typically limited in their output capabilities to around 2 mA. Do the math! At an industry standard 1000 V, a leakage of 2 mA means a half megohm of resistance. As an accepted “rule of thumb”, you never want to see less than a megohm of insulation resistance on a finished product (lower values are standard on components and sub-assemblies that will be further protected once installed within insulated equipment). So by the time the tester is measuring one half or one quarter of a “meg”, the insulation is no longer doing its job. The tester doesn’t need to supply any more current. The test item has deteriorated, and is in need of repair or replacement.

Understanding the Familiar “Infinity” Symbol

It is on the high end of resistance measurement that various models of tester are distinguished as to price and application. The higher the range of the tester, the smaller the current it must be able to measure. Once its limit of sensitivity is reached, the tester renders an “overrange” indication. On analog models, this is represented by the familiar “infinity” symbol. Many persons don’t really appreciate what this means. It is not truly a measurement! It is an indication that the actual measurement is greater than the tester can accurately read. For simple pass/fail testing, as in commissioning equipment or proofing a job, this is “good”. Basic testers tend to have a limited range, to a few thousand megohms, and many field operators just look for “infinity” while rarely bothering with the numbers. More expensive models, however, offer higher ranges, up into tera-ohms, and these requirements are getting higher as sophisticated electronic equipment becomes increasingly sensitive. Don’t just grab the insulation tester and head for the truck. Knowing how to fit the range to the job is critical.

Range evaluation breaks down into two broad areas of application: pass/fail, or “go/no-go” testing, and preventive/predictive maintenance. For the former, extended range is merely overkill. The most basic inexpensive tester will provide enough range to determine that the test item wasn’t miswired at the factory or had a nail run through it during installation. Indeed, many operators are used to looking for “infinity”, and if provided with a high range model, a posting to the display of something like “1.25 T W ” can cause confusion and unnecessary interruption of job flow.

The Cheapest Tester May Not Be the Best

For ongoing electrical maintenance, however, the acquisition of an economical tester can be a poor choice. For the sake of cost, or merely from familiarity with a particular model, a tester of standard range may end up being used in situations that call for extended range. The problem is compounded by the fact that the standard tester will work just fine, within its limitations, and give the operator no occasion to realize what is being missed. The basic issue is that overrange or “infinity” indications tend to provide a nice sense of security, which can also be to some extent false. They tell the operator that the equipment will operate just fine for now and for the limited future, but give no idea of a time frame. Insulation resistance values act much like the odometer on an automobile. Because insulation deteriorates gradually but steadily over time, resistance values, like mileage, give a pretty fair idea of where a piece of equipment is on its life cycle. Testing a piece of apparatus component by component is impractical, but in a few minutes, an insulation test can be performed. More expensive models that have high range will give readings where a standard model would merely overrange. The significance of this is that it affords the ability to do a commissioning test on new equipment and record an actual number for comparison later on, not just a “pass”. Regular maintenance can then show the rate at which change is occurring, rather than having to wait until the values drop below a ceiling. If the numbers while still high are falling sharply, it could be an indication that the apparatus is operating in a hostile environment where it is being impacted by dirt, moisture, heat, corrosive fumes, or some such factor. The maintenance person is then afforded sufficient time to take appropriate steps without disruption of production.

Don’t Forget Continuity

Nearly all insulation testers also test the opposite of insulation: continuity. The electrician wants to know that isolated circuits are in fact isolated and that connections are in fact continuous. In common expression, this comes down to looking for “shorts” and “opens”. The insulation function looks for the “shorts” and the continuity test looks for the “opens”. Continuity is checked at low voltage, usually about 5 V, but with more current than an insulation test. Available current output can vary quite a bit with the model, but an accepted standard in much of the world is 200 mA. Basically, this is a simple function, but there are some important subtleties. Reasonably good testers commonly have a “buzzer” function. This sounds a tone when continuity is achieved. To understand the real significance of this, think of lifting a five-pound weight. It’s easy once, but becomes prohibitive upon numerous repetitions. Many functions of electrical testers should be evaluated in the same manner. What seems quite adequate at the trade show demonstration can become an impediment in the intense environment of job completion.

Additional Features You Should Consider

Accordingly, many electricians don’t want to have to look at the meter when doing repetitive sign-off checks. They can move the probes from circuit to circuit as quickly as possible and listen for the tone. The actual number isn’t really an issue, and the environment may be dim and making it difficult to see the display. But this basic convenience and time saver can be improved. Imagine proofing a large commercial panel, anxious to sign off and move on. The time that it takes the buzzer to sound can be like the five-pound weight. Response time can vary from a few milliseconds to one or two seconds. On paper, or at the wholesaler’s counter, this seems like nothing. But for comparison, think of those televised live newscasts where the correspondent is halfway around the world. The anchorperson asks a question, and there’s a transmission delay while the viewer starts to wonder if the correspondent heard it. These pauses are next to nothing, yet they become annoying in short order. Trying to sign off a large panel and being held back though ever so slightly by the tester eventually becomes frustrating, and frustration leads to error.

Further, it can be enlightening to check the resistance value at which the buzzer sounds. Many testers are designed at the convenience of the design engineer, not necessarily what’s best for field operation. Some have thresholds that are arbitrary and don’t reflect conformance to any specific requirement. Chances are that other functions may be equally ill considered. It may be to the operator’s advantage to have a tester that has several selectable buzzer thresholds, matched to specific applications. It makes the job more thorough and professional, and reduces the chance of questions and callbacks later on. And it makes you better prepared for highly regulated industries, like nuclear generation.

Nothing saves like safety, both in terms of time and dollars. Extra safety features should not be overlooked when selecting equipment. A continuity test can be a hidden danger when accidentally connected to a live circuit. Insulation functions typically have a high input impedance, but this switches to a low impedance when the continuity test is engaged. If one lead has been accidentally connected to a live voltage and the other left dangling, or connected to metalwork, the operator could be at risk of becoming a path to ground. An added safety feature prevents this by keeping the high input impedance engaged until both leads have been properly connected and no external voltage is sensed.

As the above examples illustrate, sometimes less can be more. A higher price tag, more features, and greater “specs” don’t always translate into a “better” job. Many applications require only basic function, and indeed an “overqualified” instrument can be an impediment, especially where novice or relatively unskilled personnel are utilized. On the other hand, models with limited function can be a wasteful deception in giving the appearance of having performed the task, but having done so without providing all the information the operator may need. Don’t be in too big a hurry, over emphasize price, or be dazzled by multiple features. It’s well worth the time to maximize the tester against the particular job.