Multimeter Safety:
What to look for in an industrial DMM
Published with permission from Extech Instruments
For anyone who works on electrical components, there are certain must-have, electricity-101 tools that you’ll find in every tool box. In addition to all the hand tools that help you complete every repair or replacement properly, a big part of the job is related to testing and diagnostics. Let’s face it, electrical troubleshooting, more often than not, involves finding subtle variations on any number of operating factors that can point to a not so subtle problem.
When it comes to diagnostic work, a key weapon in the toolbox arsenal is a digital multimeter, or DMM. Depending on the multimeter, it can provide a digital readout of voltage (VDC and VAC) and current (A). More sophisticated models may also measure duty cycle (%), capacitance (nF), frequency (Hz), offer diode and continuity testing, and temperature readings via Type K (thermistors) probes.
Make sure that when you opt for a new multimeter, that it’s working overtime to protect you and your components.
Did you know that electrocution ranks as one of the top five causes of death in construction sites? Even the most “Cool Hand Luke” electrical contractor cannot be too cool for electrical safety. To that end, your multimeter can often times be one of the few things separating you from a jolt that could be harmful or fatal. So, you’ll understand if we spend the most time on safety. Without it, you could be dead in the water—literally.
Third Party OK’s: The first thing to check is that it has the label of an independent verification lab such as UL, CSA, CE, ETL or TUV. Don’t touch it unless they say it’s okay. We also have the sage guidelines set forth by the IEC, the International Electrotechnical Commission. Their IEC 1010 overvoltage category definitions set explicit standards for what kinds of protection an electrician would receive from his multimeter.
The CAT’s Meow: You’ve seen these overvoltage categories before: CAT I CAT II, CAT III and CAT IV. But did you know what they really meant? The primary hazard we’re trying to protect ourselves from is an unexpected spike or transient of high voltage. (Well, aren’t they all unexpected?) A spike can come out of nowhere from a variety of electrical sources, whether it was a good old fashioned act of God—lightning—to the typical components you deal with every day: motors, variable speed drives, capacitors as well as power conditioning and conversion components.
If something is switching on and off up the line that you don’t know about, there could be a hazardous spike coming your way as well. There’s no radar for advance warning on these things; the best protection is whatever is standing between you and that spike—your multimeter and its test leads.
So, back to those categories. There are four categories defined by the IEC defined roughly as: - Category I—generally electronics and equipment that is protected to control spikes
- Category II – single-phase loads connected to an outlet or receptacle with a physical distance from a CAT III source of 30 feet or a CAT IV source 60 feet away.
- Category III – three-phase distribution covering most industrial plant assets and electrical connections and components, plus commercial lighting systems
- Category IV – generally outdoor installations with three-phase connections coming “from the pole.”
Bottom line is, the higher the CAT, the closer you are to a utility connection and the higher the risk for high power transients. So, when you are looking for a digital multimeter, you want to be sure that not only is it capable of measuring the max steady state voltage that you’ll typically encounter on the job, but also the ability to stop a spike in its tracks and prevent a potentially lethal arc flash.
Your safest bet is to purchase a durable CAT IV rated meter and only use that meter. This eliminates the need to constantly check what CAT environment you’re working in. If you’re surrounded by industrial motors and drives, you don’t want to thoughtlessly reach for the wrong meter. Go ahead, find a nice shelf in the workshop and start a multimeter museum for the retired ones.
Don’t Confuse the Fuses: Make sure that the DMM you purchase has ceramic, HRC (high rupturing capacity) fuses for all high voltage functions. These fuses can respond quickly giving you a time advantage for a safe reaction and they can absorb extant voltage before it gets to you. If a fuse ever blows, you might want to give it some respect—it may have just saved your life. Here’s an idea for another shelf for the workshop museum—a war story behind every blown fuse. More importantly, be sure you replace all fuses with one that matches the ratings specified by your meter maker.
Too Hot to Handle: Many newer multimeters include temperature measuring capabilities using thermocouple or thermistor probes. These Type K probes require contact and can deliver an accurate temperature reading on a number of surfaces. Sometimes, that very contact however, can be hazardous. To avoid burns or dangerous proximity to moving equipment, use a non-contact infrared thermometer or “pyrometer” to get a safe point-and-shoot temperature reading from a distance.
Look for “distance to spot” ratios that permit precise one-inch spot readings from a safe distance. For example, a 12:1 IR thermometer provides a reading of a one-inch area from 12 inches away. A 30:1 IR thermometer provides a safer reading of an identical one-inch spot from 30 inches away. Further is better when it comes to safety.
While you can purchase an IR thermometer separately, newer digital multimeters are available with an IR thermometer built right in, saving you money and helping you keep your tool box clutter-free. Many already offer a bead probe thermometer built in. You should expect the infrared thermometer as well for convenience and safety.
Leads by Example: Your CAT IV multimeter should come with CAT IV test leads. This should be a no-brainer. What’s the point in having all that protection built into your DMM if the leads will be turning to putty in your hands? Look for professional-grade, CAT IV-rated test leads. This is not a place to save a few bucks. You wouldn’t put soft, cushy touring tires on a sports car and expect it to take the exit without a lot of drama. Same thing here: always ask for manufacturer-specified test leads with matching CAT ratings.
Conclusion
In this economy, we all want to do more with less. We’re voting with our wallets in many cases, but we want to make sure it’s still a sound decision. Remember to look for safety when looking to buy your next multimeter.
