Probe Safety Demystified: CAT Ratings

Figure 1. CAT ratings required to
safely test different electrical sources.
(Sourced from “Measurement Categories,”
Wikipedia, Oct. 28, 2019.)

Any voltage probe will have several published specifications that are meant to indicate under what circumstances that probe is safe to use. They answer questions such as “What is the maximum voltage I can safely run through this probe?” and “Can I safely hold this probe while using it, or does it need to be mounted somewhere far away?” In this post, we’ll explain what the CAT rating is telling you.

CAT ratings are standardized ratings used to categorize the suitability of a voltage measurement device based on the source impedance of what it is used to measure. They are issued by the International Electrotechnical Commission (IEC).

CAT ratings might be one of the things least understood by people using oscilloscope probes, mostly because they’re not highly relevant to people whose work typically involves using oscilloscopes. The vast majority of electronics laboratory conditions meet the requirements of CAT O, CAT I or CAT II, which is where most oscilloscope probes fall. However, if you were a field service technician and your job was to do maintenance on the electrical distribution system in a building, this would be a critically important thing for you to understand, because all the test equipment that you would use in the field—that you would connect to a motor, or crossbar, or breaker panel, or utility feed or anything at all like that—would have a CAT rating. And that CAT rating would tell you whether that device was safe to measure the voltage at the location and in the manner in which you were measuring it.

The difference between locations is really based on source impedance back to the utility supply. The closer you get to the actual utility supply, the lower the source impedance. We all know that low source impedance means that a lot more current flows during a short circuit condition; therefore, the likelihood of death is much higher with very low source impedance than with very high source impedance. You’re going to be in a lot more trouble if you touch your fingers to a 40-volt breaker panel than if you stick a wire into a 120-volt AC receptacle, although neither is recommended.

So, if you're a field technician and you're measuring voltages on internal building electrical supply lines, the CAT rating is pretty important for you, but it's also been used over the years for test equipment that is not generally used to measure voltage on utility electrical supply lines—things like high-voltage differential probes—because some people do use those to measure 120 V, 240 V or higher.

Figure 1 is taken from a Wikipedia page on this topic and explains the difference between CAT ratings. The closer you get to the actual power grid, the higher a measuring device’s CAT rating must be for it to be safe. If you’re measuring the big wires that bring the power into the building, it's got to be CAT IV. Laboratory test equipment, like oscilloscope probes, are usually CAT I or CAT II, because those ratings are indicative of the lower voltages typically encountered in an electronic circuit and a hand-held method of use.

Adding to the confusion is that the meaning of various CAT ratings has changed with the evolution of safety standards. Some probes that may have formerly been rated CAT I through IV are now rated CAT O, meaning "Other". The CAT O rating designates test equipment that is “not intended to be connected directly to supply mains”, even if your probe can safely isolate the voltage. Test equipment suppliers may describe something as CAT O, or they may do that and also use one of the other four CAT ratings, as well, especially if the product was formerly certified to a more familiar CAT rating. A probe may have more than one CAT rating depending on the voltage measured and where it is being probed (e.g., 600 V CAT I or 300 V CAT II).

So, just be aware that this is what CAT ratings are about. They’re all about safety and how well the test equipment will protect you should a short circuit occur based on the source impedance of the load you're connecting to. They have nothing whatsoever to say about how suitable a probe is for a particular type of application, or if it will give you good measurement results in certain cases. For that, you need to examine other probe ratings, which are described at more length in the on-demand webinar, Probing in Power Electronics, What to Use and Why: Part I.