There are some low-end insulation resistance testers in existence that have features and specifications similar to those of more industrial models, so you may think your low-end unit is well-suited to the work you need to do. And maybe it is. But if it isn’t, you could endure injury, lost time, incomplete testing, and recurrent downtime. Like all test equipment, insulation resistance testers (photo) require you to do some research before deciding what model to purchase.
Insulation testers are high-voltage, high-range instruments. Nearly all operate at the lower end of the scale, as well. The continuity range measures low resistance at low voltage by switching the input impedance to a low-impedance state to accommodate a comparatively large test current, putting you at risk. Insulation testers should not be connected to live equipment, but through operator error, miscommunication, or system faults, many people do. If such a condition prevailed during a continuity test, and the second probe was not yet connected to the test item, that probe would become live at source voltage. Any metal the second probe connected to would similarly become energized.
A quality instrument prevents this situation with a contact detector feature, which allows the high-input impedance associated with the high-voltage function to protect the test circuit until both probes are connected. The tester, which works automatically, can sense whether there is a complete connection and if external voltage is present — it will not switch to low impedance until proper conditions exist.
Ingress protection protects the instrument, not you, but it’s still important. The International Electrotechnical Commission defines ingress protection in IEC Standard 529. Its existence is identified by an “IP rating” that should appear in product literature. It consists of two digits, each signifying a separate characteristic. The designation indicates how well the item is sealed against invasion by foreign matter, moisture, and dust. The higher the number, the better the degree of protection.
Consider the typical IP54 rating. The first digit refers to particulate ingress. Level 5 indicates dust protection, as well as protection from wire invasion down to 1.0 mm. The second digit refers to moisture. A rating of 4 means resistance to water splashed from any direction.
Now, suppose the rating were IP43. Although it may seem close to IP54, the protection level it represents is vastly different. Such a rating indicates the instrument is sealed only against “objects equal or greater than 1 mm,” and “spraying water, up to 60º from vertical.” For applications in environments like quarries and industrial processes where dust and flying water are always a problem, the difference between these two ratings can mean the difference between effective use and excessive repair or repurchase.
The difference between how a model looks on paper and performs in the field is critical and can be deceptive. Vendors of quality products don’t hesitate to specify details that might seem trivial, so review them carefully. Using a test instrument for an 8-hour shift can be very different from using it for a few minutes on a test bench. A quality tester has subtle conveniences that can only be appreciated after you’ve used the instrument on an extensive job.
Consider the timing and tone of a continuity buzzer. The buzzer is a convenience feature that sounds an audible tone when a circuit or connection is continuous or below an acceptable resistance upper limit. Without the buzzer, you would have to look at the display, take the reading mentally, decide if it passes, and then move on. When working at a large panel or doing point-to-point testing around a complicated piece of equipment, this process is too complex and time-consuming — you need to focus on making and changing connections. Because continuity testers allow you to focus on verifying continuity, rather than processing numbers mentally, you can work faster.
However, there’s more to a continuity buzzer than whether or not it goes off. A little-known consideration is the timing of the indication. Low-quality instruments can take as much as a second after contact to sound, whereas improved models indicate contact within milliseconds. That might not seem like a big difference, but dealing with a one-second delay when testing can create more problems than you might imagine — if you’re trying to work quickly you could likely to get some “false negatives” in the process.
A related convenience is tone differentiation. More advanced testers offer distinct tones at different thresholds. For bond testing, no more than a token resistance is acceptable, so 5 ohms would be a typical threshold. But for service testing of components, much higher values (into the kilohms range) are typical. It is convenient for the tester to have separate tones for high and low range, readily distinguishing these two areas of application, but don’t trust the instrument to get it right. Some instruments’ arbitrary values inadequately distinguish the applications, causing more harm than good. Check the value associated with each tone to be certain it fits with the equipment you’ll be testing, and 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 you will encounter. Settling for an over-range indication isn’t what measurement is all about. Models that reach to only a few kilohms fall short in many critical applications.
Look for IEC 1010-1 conformance. This standard establishes objective terms to evaluate your level of protection from electrocution, internal arcing, and arc blast. If the unit doesn’t specify an IEC 1010 rating, don’t buy it. Know the category of wiring and the voltage rating (maximum) to which you will apply the tester and use a unit rated for this category of use. Make sure the specs include a voltage rating for the category, such as CAT III 600V, but remember that aCAT rating alone is not useful information.
Look for voltage protection. At the start of a test, voltage protection functions primarily to protect the tester. If you connect the tester to a live circuit, you should get an alarm. This function normally engages automatically on all functions and ranges, but you might still push the test button and toast your tester. Automatic test inhibition can solve the problem: Pressing the test button does not engage the test circuit in the presence of external voltage. However, in telecom applications, voltages of less than 50V may be present as part of normal crosstalk. In these applications you must be able to limit the test inhibition feature.
Static voltage at the conclusion of a test can put you at risk, too. Therefore, the tester should have a resistive discharge circuit and an indication of how the discharge is progressing. You should be able to observe the display to know when it is safe to touch the test item.
Look at the test leads. A protective boot should cover the lead tips, exposing as little metal as possible. The tester should have deeply recessed connections so you can’t come in contact with metal jacks. Leads should be sleeved or shrouded where they plug in, keeping exposed metal out of reach of your fingers. Fused leads and switched probes add further safety and convenience.
Any insulation tester can test insulation, but don’t settle for the basics. It’s difficult to consider all of the situations you may want to use your tester for at the time of purchase. A minimalist unit will fall short and may require additional acquisitions, while a quality model will prove its worth.
Jowett is a senior applications engineer with AVO International, in Valley Forge, Pa.