Does Your Meter Safety Measure Up?

Sept. 1, 1998
Meters can, and do, blow up, causing personal injury and equipment damage. The good news is it doesn't have to happen to you. Knowledge, as the saying goes, is power. However, when electrical power poses a hazard, knowledge is survival. While multimeters have inherent safety features, you need to know the limitations of those features and how to apply what you know to your electrical work. Two most

Meters can, and do, blow up, causing personal injury and equipment damage. The good news is it doesn't have to happen to you.

Knowledge, as the saying goes, is power. However, when electrical power poses a hazard, knowledge is survival. While multimeters have inherent safety features, you need to know the limitations of those features and how to apply what you know to your electrical work.

Two most common misuses of multimeters. A potentially serious misuse is measuring voltage while the test leads are in the current terminals. Most people do this by accident. The meter fuse does offer protection, but it's not absolute. Let's consider the design of the current terminals. They make current measurements in low-energy circuits by opening the circuit and inserting the meter in series.

The ammeter function requires a low input impedance (typical values are 0.1 ohm and 10 ohms).

Otherwise, the meter may reduce the current in the circuit you are trying to measure. The low impedance of the current input circuit makes it vulnerable, especially in a power circuit. In effect, the meter is a branch circuit in the palm of your hand, and therefore requires protection with high-energy fuses. If those fuses blow, it is imperative you replace them only with the ones specified by the manufacturer. If you are using the meter in the wrong power category, the power can surge across the blown fuse. Looking at Photo 1 (original article), you can see what can, and does, happen when using the wrong fuse.

The other common misuse can occur when making a resistance measurement. What happens here is you make contact with a circuit you thought was dead; when it is actually live. Modern multimeters generally have good overload protection and recover automatically. Their designers usually get this overload protection by inserting a thermistor in the ohms circuit. This component increases its resistance rapidly with the heat from overload current. Such protection is so effective many users take it for granted. This protection, like the airbag of a car, is not a license for recklessness. Photo 2 (original article) shows what happened to a meter accidentally applied to the wrong voltage.

The hidden danger from overvoltage transients. There is a frequently misunderstood potential hazard that has nothing to do with misuse. The voltage inputs have a high-input impedance. You make voltage measurements in parallel. The higher the parallel resistance (i.e., the input impedance of the meter), the less the current. For example, a typical voltage input impedance of 10 megohms (10,000,000 ohms) across 480V draws only 48 nanoamps (.000048A).

You'd think with such high-input impedance, there is nothing to worry about. But, the culprits are transient overvoltages ("spikes"), which appear on the power circuits.

You can't avoid these transients. Most are not going to do any damage, but the combination of high-voltage spikes and high current can be dangerous. Where do such spikes come from? Events like lightning strikes or motor switching cause them. These spikes can be severe enough to cross the input circuitry inside the meter. In effect, they can create a short circuit inside the meter. We all know it's not a good idea to short-circuit a 480V power supply!

What happens in a worst-case scenario? The meter arcs inside. You hear what sounds like a gunshot. As a reflex action, you draw the probes away from the circuit. But, there are thousands of amps of fault current running through those test leads! The test probe tips act like circuit breaker contacts and draw an arc. This arc follows the particle trail of the tips that vaporize from the tremendous heat. If these arcs (one on each probe tip) happen to join, they create a phase-to-phase or phase-to-earth fault. The instantaneous result is an arc blast and fireball, hotter than the temperature of an oxyacetylene cutting torch.

Will this always happen? Not if the meter, in the act of blowing up, opens the circuit fast enough. Could this happen? Absolutely. That's why some multimeter manufacturers put such considerable effort into protecting the input circuitry from transients.

Test leads and probes. Before we discuss the safety design of the multimeter itself, let's consider an often-overlooked accessory. Have you ever used taped test leads? Or used a connector to hold the broken sections of leads together? Or used test leads and probes that have no voltage rating marked on them? Or how about using some No. 12 wire clipped to a probe as a test lead extension? That's exactly like having a new car, equipped with the latest safety gear, running on bald tires. The probes and leads are where the rubber meets the road.

A system is only as strong as its weakest link, and the weakest link is often not the meter but the test leads. Let's look at a set of low-cost probes with fingerprints burnt into the probes. Unknown to the electrician, these probes carried no rating for the power circuit on which he was using. Unfortunately, using them on that circuit left him with a burned hand and forearm. In this case, poor test leads compounded an already bad situation of improper fusing. When considering the safety ratings of meters, keep in mind the same ratings apply to test leads, probes, clips and even clamp-on accessories.

IEC 61010 and Overvoltage Installation Categories. How do you know you're getting a multimeter with the right kind of protection against transients? A good starting point is to understand the implications of IEC 61010, which is a recent international standard for low voltage test equipment. (The standard defines low voltage 1000V or less.) National standards setting bodies, such as ANSI/ISA have adopted this standard. Certifying agencies like Underwriters Laboratories (UL) have incorporated it into their test procedures. For example, UL bases UL3111 on IEC61010.

IEC 61010 delineates four "Overvoltage Installation Categories." The higher the category number, the greater the danger posed by transients. These categories depend more on the fault current available at that point in the distribution system than the voltage level. Within each category, there are voltage ratings at 1000V, 600V, 300V, etc., all of which apply to both AC and DC voltages.

Rating a meter by both category and voltage can be confusing, so here's how to sort that out. The category to focus on if you ever work on power circuits is CAT III. Use only a meter with the category and an associated voltage rating, such as CAT III-600V or CAT III-1000V, marked on the front of the meter, typically somewhere near the meter's input terminals.

When is 1000V less than 600V? IEC 61010 and the concept of categories are relatively new. The previous standard, IEC 348, did not directly address transient protection. Rather, it focused on steady-state voltage withstand levels. So, our training led us to consider a "1000V-rated" meter safer than a "750V-rated" meter. To some degree the same logic still applies, as long as you are talking about meters in the same category. For example, a CAT III-1000V meter has better protection than a CAT III-600V meter. That meter, in turn, has a higher level of transient protection than a CAT III-300V meter. But it does not follow that a CAT II-1000V meter is "safer" than a CAT III-600V meter.

When it comes to transient protection, the CAT III-600V meter passes a test against a transient with much more energy than the CAT II-1000V meter. Table 2, on page PQ26, shows the CAT III-600V transient, even though having the same peak value, it has one-sixth the test source impedance and therefore six times the current of the CAT II-1000V meter!

IEC 61010 includes transient test voltages for each CAT-voltage combination. Also, it has detailed design specs, including component spacing requirements. The higher the CAT-voltage combination, the greater the spacing requirements and the more powerful the test transient.

What conclusion should you draw? The voltage rating tells only part of the story. This is true whether it's an "old" meter rated at 750VAC/1000VDC (with no CAT rating) or a "new" IEC 61010 CAT II-1000V meter. For transient protection, neither one is as robust as a CAT III-600V meter! That's why it's critical you look first at the CAT rating, and then at the voltage rating.

The newer "dual-rated" meters have alleviated some of this confusion. As Table 3 on page PQ28 shows, the dual-rated CAT III-600V/CAT II-1000V meter must meet the most demanding specs of either rating. It must pass the 2 ohm test source/6000V transient of the CAT III meter, but it must also have the 1000V fuse and higher creepage (surface spacing) of the CAT II-1000V meter. Most importantly, if you've never heard of categories, and then select your meter based only on the higher CAT II voltage rating, you still get a meter with protection for a CAT III environment.

What CAT and voltage rating do you need? Do you work on three-phase motors? Do you work on motor controls or soft-starts or variable speed drives? Do you work on 277V single-phase lighting circuits? Do you work in distribution panel boards? If you answer yes to any of the above, you're in a CAT III environment. These kinds of circuits could have enough energy to cause the fireball discussed earlier. Do you want to use a CAT II or CAT I meter in a CAT III environment? Not if you want to be safe. How can you tell? The IEC 61010 meter should have CAT/voltage markings around the input terminals.

For now, the IEC does not define CAT IV specifications, except for current clamps. Thus, for multimeters, CAT III is the highest applicable standard. Clampmeters that measure both current and voltage are CAT III, even if the current clamp portion meets CAT IV.

What meter do you choose for making low-voltage measurements in a CAT IV outdoor environment? The best choice is a meter with the highest voltage rating in CAT III. A CAT III-1000V meter will get you as close as you can get to the CAT IV-600V rating. Until we actually have such a rating.

Price need not be an obstacle to safety. When making simple voltage measurements, a low-cost alternative to a full function meter is a voltage tester. Testers designed in the last two years are now available in both CAT III-600V and CAT III-1000V ratings at prices well under $100. No electrical worker should compromise personal safety for financial, or other, reasons.

Certification is important. Unfortunately, IEC 61010 does not require independent testing and/or certification for meters. Manufacturers may self-certify that their meters meet the standard. Some meters labeled CAT III-600V do not meet the design specs and have failed actual testing to the specifications required by IEC 61010. What should you do? Besides looking for the category and voltage you need, look for certification by an independent testing lab, such as UL or CSA (Canadian Standards Association). To get certified, the meter must pass the mandated tests. Look for independent certification, not just for words saying "designed to meet" or "conforms to IEC 61010." Those statements are no substitute for independent testing. Many engineering designs have not survived real-world testing.

Let's sum up the elements of multimeter transient protection:

  • Identify the overvoltage category,

  • verify the voltage rating, and

  • look for independent certification.

These simple rules will help give you knowledge for survival to work around electrical power for years to come.

About the Author

Edward Shen

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