Recap of Part 2
Part 2 focused on the importance of ensuring the digital multimeter (DMM) and test leads carry a certification from a Nationally Recognized Testing Laboratory (NRTL) and the training requirements to be a qualified person before using test instruments such as DMMs.
Now we will transition to the importance of the voltage rating of DMM’s; their associated accessories, including test leads, personal protective equipment (PPE) when using DMMs; and the need to properly inspect DMMs and the leads prior to use.
Never Exceed the Maximum Voltage Rating of the DMM or Leads
The requirement to use suitable electrical test equipment is driven by OSHA regulations 1910.334(c)(3):
Test instruments and equipment and their accessories shall be rated for the circuits and equipment to which they will be connected and shall be designed for the environment in which they will be used. (emphasis added)
Like any other tools and instruments, DMMs have limitations consisting of ratings for the maximum voltage that can be safely applied to them. There is good reason why NFPA® 70E®, 110.6 also requires test instruments, test equipment, and their accessories to be “Rated for circuits and equipment where they are utilized.”
Most high quality DMMs have a maximum voltage of 1kV, but others are limited to 600 volts or even down to 300 volts. This important information should be visible somewhere on the DMM itself, but the only way to know for sure is to read the safety instructions.
Fluke now has a DMM rated up to 1.5kVdc specifically designed for industrial solar photovoltaic plants. This voltage range is only for direct current (dc) and does not apply to nor is it safe for use on alternating current (ac) greater than 1kVac.
Fig. 8 is pulled from the technical specifications for the Fluke model 282 FC. The red highlights show the maximum dc range of 1.5 kV while the yellow shows its maximum ac range is only 1kV. So, rather than throwing those pesky instructions in the trash, we should retain them and then take the time to carefully review the important safety information contained within!
In Fig. 9 below, we have an extremely damaged “Electrical Tester” meter with a maximum voltage of 1kV (ac or dc) that was inadvertently connected to a 4.16kV circuit. Thankfully, the technician was wearing adequate PPE and wasn’t harmed. Miraculously, the meter’s body didn’t explode! But the two leads instantly vaporized.
If it wasn’t for his PPE, this error would have resulted in severe third degree burns and other significant injuries to the worker. While PPE is “the last line of defense” and the least effective method of controlling risk, PPE is still absolutely necessary and can only protect you if you’re wearing it.
Lastly, DMMs must never be used on any circuits energized at greater than 1kVAC medium voltage equipment such as 2.4kV, 4.16kV, 6.9kV, 12.5kV, 13.8kV, etc. This seems like common sense, but many experienced electrical workers have made this simple mistake resulting in irreversible harm to their lives, as we will soon learn through the Eddie Adams story.
Are Electric Shock and Arc Flash PPE Necessary When Using DMMs?
Occasionally, in my electrical safety classes, I’m asked the question, “Am I required to wear electrical PPE when using a DMM?” The honest answer is, “It depends on the task and the hazard you’re exposed to.”
If you are taking resistance measurements of new resistors from a work bench to validate their accuracy, then the only PPE needed would be safety glasses. However, anytime you’re accessing energized electrical equipment in the field or plant, rubber insulating voltage-rated gloves with protectors must always be worn – at a minimum. This applies whenever taking readings of live circuits, even for circuits energized at “only 120 volts.”
If the equipment labeling indicates arc flash PPE is also necessary, then wear the correct level of arc-rated garments that exceed the incident energy, and set up appropriate barricades at either the Limited Approach Boundary (LAB) or Arc Flash Boundary (AFB), whichever is greatest, to keep unprotected workers from entering the danger zone.
The question of PPE often arises when testing “only 120V systems” in relationship to the Restricted Approach Boundary (RAB) as identified by NFPA 70E or the Minimum Approach Boundary (MAD) according to OSHA. For 120 volts, both 70E’s RAB and OSHA’s MAD is “Avoid Contact” rather than a physical distance established by inches or feet as found for higher voltages.
To the astonishment and surprise of many, “Avoid Contact” does not mean “Don’t Touch It.” To better understand the context of “Avoid Contact” within the parameters of the RAB/MAD, I previously authored an article series titled “Avoid Contact: The Electric Shock Boundary Without a Distance” to explain this often-misunderstood phrase.
While probes have a voltage rating much higher than 120 volts, hands can slip while gripping or adjusting the grip. If your hand or fingers inadvertently contact the probes’ metal tips or any adjacent energized part inside the equipment, you will be shocked. Wearing rubber gloves with protectors will significantly reduce the chances of making inadvertent contact with the hands, thus preventing an injury.
Furthermore, the manufacturer’s safety instructions of every high-quality DMM that I’m familiar with direct the user to wear rubber insulating gloves to avoid shock injury and arc flash PPE as necessary. Failure to adhere to the safety instructions is a foolish practice that unnecessarily increases your risks in an inherently dangerous occupation.
There’s a good reason why I selected the photo of a worker wearing all his shock and arc flash PPE as the starting image for every part of this article. Anything less is gambling with your life.
Back in 2001, an experienced electrician by the name of Eddie Adams “lost his life for nothing,” according to one of his coworkers, after he attempted to take voltage readings of a 2.3kV motor starter using a 600V rated DMM. In this tragic case, Eddie also failed to wear his arc flash PPE; consequently, he suffered massive 3rd degree burns over much of his body, resulting in his death one day after the accident.
While there were many human errors that contributed to this tragedy, the two related to this article were Eddie failing to wear his PPE when using a DMM and incorrectly using the DMM on a high voltage system.
Obviously, there’s no way to know for sure if Eddie would have suffered no injuries if he was wearing correct PPE, but there’s no doubt the severity of the injuries would have been significantly reduced, giving him a better chance of survival.
Click here to watch the Eddie Adams video so we can learn from this unfortunate accident.
Inspect Your DMM, Probes, and Leads - This is Also Important to Your Safety!
Even if you purchase the highest quality DMM and accessories available with valid NRTL ratings, they can become a liability if you don’t take care of them, store them correctly when not in use, and fail to perform a thorough pre-use inspection each time before contacting a live voltage source. At a minimum, the batteries inside your DMM should also be replaced annually as well.
Pre-use inspection of test equipment is required by OSHA 1910.334(c)(2):
“Test instruments and equipment and all associated test leads, cables, power cords, probes, and connectors shall be visually inspected for external defects and damage before the equipment is used. If there is a defect or evidence of damage that might expose an employee to injury, the defective or damaged item shall be removed from service, and no employee may use it until repairs and tests necessary to render the equipment safe have been made.”
Prior to every use, complete a detailed visual inspection of the meter and the entire length of the leads and the probes. Cuts, dry rot, cracks in the insulation, broken parts, missing shrouds, and visual damage can unknowingly expose you to electric shock.
At the same time, hidden unseen internal damage, such as loose or intermittent wire connections, can also increase your risk of injury.
A quick and effective test to validate the integrity of the internal connections is to place the DMM in continuity mode with the audio tone on. Short the probe tips together by pinching them between your thumb and index finger which should display a near zero ohmic reading accompanied by the steady audio tone.
While keeping the probe tips shorted, pick up the DMM and leads, then lightly but firmly shake the leads and meter. If the audible tone remains steady, then this is indicative of good internal connections. However, if the audible tone stops or is intermittent, then it is identifying a problem within the leads and/or DMM which requires further investigation.
If the problem resides in the DMM, remove it from service, tag it out (i.e., “DO NOT USE – FAULTY METER”), and return it to the manufacturer for factory repairs.
However, if the problem is found in the leads, it’s much better they be destroyed to prevent others from inadvertently using them.
Don’t try to repair probes or leads with electric tape, heat shrink, butt splices, and other shop fixes. Test leads are relatively inexpensive to replace and repairing them isn’t a safe or approved option.
Ensure You Are Using the Correct-Rated Leads for The Applied Voltage
The requirement to use electrical test equipment with their design limitations doesn’t just apply to the DMM itself but to the leads and any accessories according to 1910.334(c)(3), “Test instruments and equipment and their accessories shall be rated for the circuits and equipment to which they will be connected…” (emphasis added)
While most good quality test leads are rated up to 600V or 1kV, not all are, even those fabricated by reputable manufacturers.
Fig. 10 shows a set of popular style test leads with mini-hook grabbers on one end and exposed “stackable” male 4mm banana plugs on the other end, often referred to as “Pomona leads.” Since they are made by a well-known test equipment company, many mistakenly assume they can be used with power circuits such as 120, 208, 240, 277 and 480V. But when we review the safety instructions, as we should always do, we learn they can only be used on extremely low voltage systems with a maximum of 30Vac RMS and 60Vdc, as highlighted in yellow.
Such test leads are very useful but designed and rated for only very low voltage signal, audio, communications, and electronic circuits and must never be used on power circuits exceeding their voltage rating.
Another significant safety issue when using stackable banana plugs outside of their voltage limitation is the possibility of plugging the black banana male plug on top of the red banana female jack, as depicted by Fig. 11. When you look at Fig. 11, it should be obvious this configuration is nothing less than a short circuit through the stacked plugs. Very bad things happen when the red and black probes are then placed across a voltage source.
An Unfortunate Incident
For those who think this will never happen, I personally know of an incident where an experienced electrician at the nuclear power plant I retired from did this exact thing. For whatever reason, when his DMM was not in use, he developed a bad habit of storing the black banana plug inserted into the female end of the red banana plug that was plugged into the same voltage jack of his DMM.
When in the field to take voltage measurements, he would move the black male banana plug into the “COM” jack, restoring the leads into the correct configuration. However, one day he was assigned to troubleshoot a 120V control circuit, and, as Murphy’s Law would have it, he forgot to move the black male plug to the “COM” jack and left it still plugged into the back of the red plug which was in the DMM’s voltage jack just like what is shown in Fig 11.
When he connected the probes to the energized circuit to read voltage, a short circuit occurred followed by a small arc which blew the upstream fuse and negatively affected the circuit and brought several unexpected alarms into the control room. Thankfully, he was not injured. But the incident caused significant impacts to the plant operations. He likely would not have been so “lucky” if the circuit was energized at higher voltages such as 208, 240 or 480 volts.
During the investigation, overconfidence, complacency, an unsafe attitude during critical tasks, and inaccurate risk perceptions were just a few of the obvious contributing human performance errors that led to this incident. However, the investigation failed to identify the use of the wrong type of leads that were not designed for voltages greater than 30Vac or 60Vdc as a primary factor leading to this preventable incident.
Test leads rated for 600V or 1kV, according to UL/IEC 61010-31, have an inherit design that prevents this mistake, meaning you can’t physically stack them together. The lesson here is to use the right tool and test leads for the task.
If you wish to know more information regarding human errors, how they can affect your safety, the safety of your co-workers, and how to minimize their occurrences, please see NFPA 70E, Informative Annex Q – Human Performance and Workplace Electrical Safety.
Do Not Perform Unauthorized Modifications of Test Leads
Fig. 12 shows a set of factory-built test lead plugs with a voltage rating of 1kV that have had unauthorized modifications done by field workers so the leads would plug into older test equipment. As you can see, the protective shrouding sleeves covering the male banana plugs have been removed, which now compromises its 1kV rating.
Newer types of male test lead plugs compliant with UL/IEC 61010-31 are equipped with permanent shrouding sleeves that cover the entire length of the male banana prongs. The shrouds prevent accidental contact with the male’s metal prongs as they are inserted or removed or if they are not fully seated.
The mating female ports of newer test equipment are also designed to UL/IEC 61010-31 specifications and can accept male banana prongs with or without safety shrouds. However, the female banana jack ports of older test equipment manufactured before the UL/IEC standard required that this design not accept shrouded male banana prongs. To compensate for this incompatibility issue, a common, albeit unsafe practice, is to cut off shroud protective sleeves so they can be used with any generation of test equipment.
As previously discussed, factory-made test leads with fully exposed banana male plugs are limited for use with very low voltage circuits not to exceed 30Vac or 60Vdc.
Do not modify your test leads in any way because that action not only violates the “listed and labeled” safety instructions, but, more importantly, doing so unnecessarily increases the risk of injury.
