On EEVblog 1734 - Uni-T UT892 2000V High Voltage Multimeter TESTED (2026-02-09) flymypg commented:
Story time!Between leaving the US Navy and starting college, I was a tech at General Atomics working on radiation monitoring systems, which often included 2500VDC supplies (and occasionally a very rare 10KV supply) to power ion chamber and Geiger-Mueller detectors. On our lab benches, we used VTVMs (vacuum tube volt meter, with input impedance over 1 GIGA-ohm on some ranges), but they were a PITA to lug around the system test floor. Instead, we made adapters to let our portable multimeters safely do the job. The first was a simple 20:1 voltage divider, with the "hot side" designed to handle 25KV, with short 50KV-rated probe cables attached. The second was a pretty schmick opto-isolated 1:2 current mirror rated for 500 ma, where 100 ma was a massive signal for ion chamber detectors that normally have ambient active measurement currents in the picoamps. (I'll save the story of designing and assembling those adapters for another time. I think I've already shared the story of how we had to measure attoamps to certify system operation at single-digit picoamps. Measuring currents that low is nearly electron counting!)
All our HV (high voltage) supplies had arc (high rate of current rise and/or sudden voltage dip) shutdowns on them, but we never trusted them. There is no world where HV is your friend! Our HV training was extensive, as our radiation monitors were designed to be installed in heavy industrial areas within nuclear power plants, where the environment could include high humidity, puddles on the floor, wide temperature variations, and even oil mist in the air (from hydraulic leaks). During controlled environmental testing, I've seen HV equipment coated with dancing sparks when even a "mild" insulation breakdown allowed the HV gremlins to sneak into the wild.
Needless to say, we did LOTS of insulation testing, including verification, validation and certification, mainly to ensure our vendors' HV-rated products (such as coax cable) did not have their ratings degraded by our use. We also did lots of lifetime environmental testing, where the longer and more severe the test, the more likely we'd see failures, for which we always had to be prepared. Which in turn meant we were routinely working with high voltages in some peculiar places. For example, to do accelerated radiation exposure testing on some coax cable we needed to certify for use in our products, we placed several coils of it behind the beam-stop of a linear accelerator, where it would receive a intense exposure during each accelerator run, gaining a century of application exposure in weeks or months. (Radiation does very weird things to the plastics used in cables! Radiation-toleration cables often were bespoke.)
It turns out, the most important HV precaution is simple cleanliness. All of our tools and test cables would receive a bath in our Freon dunk-tank (back before we knew just how horrible Freon was for the ozone layer) before each use, and we always wore gloves not so much for voltage protection, but to keep our skin oils far away from the HV side of things. We also wore face shields for similar reasons, not to keep stuff away from our face, but to keep the moisture in our breath away from the HV system. We also wore anti-static lab coats for similar reasons, and brought insulating foam mats to stand or kneel on while working in industrial areas (away from the lab or factory).
Dragging all that gear behind a dusty beam-stop and using it to safely test cables at both high voltages and high currents (without moving them, as we were simulating a long-term static installation) was the worst white-knuckle work I can recall doing. We'd first send someone in to clean up the dusty area around the cables and put down my work mat. Then I'd go in and setup and connect the test equipment, without turning any of it on. Then a third tech would enter to check my work. After that, we'd power up the equipment with nobody inside, and when no magic smoke was released, we'd power it off and I'd prepare to go in to do the testing.
Part of the prep included putting me in a climbing harness and attaching a clean (freshly Freon-dipped and dried) non-conducting rope to a ring on the back, so I could be pulled clear if unexpected things happened (such as the output filter caps on an HV DC supply decided to explode - but that's yet another story). Of course it's hot back there, with minimal ventilation, and I was nervous and sweating. You do NOT want shaking hands when working with HV, which is why we did the setup separately from the actual testing.
Our most important activity was an "after action" report (most of us were military veterans), which we'd use to critique the test and particularly our test procedures, seeking always to improve them. Many of our tests require specific training certifications (such as to work with HV), after which the procedure would "assume" that knowledge as the context. Most of our procedure rewrites focused on making it very clear WHEN and HOW that expertise was to be applied during each and every step, complete with references back to the training materials and the standards and experience they were based on.
As I look back, I remember my HV and radiation work as a technician as key aspects of what helped shape my engineering education and career. College classes were never about the grades, but about how this knowledge would not only enable me to do new things, but also to do existing things better. Going to school with a mission and goals made it a "stressful delight" rather than a seemingly endless academic slog.
Oh, one last thing: Soon after I started as a tech at General Atomics, I noticed I had a collection of black dots on some of my fingertips, and even some on my knuckles, that wouldn't wash off. My first thought was I had somehow repeatedly poked myself with a fine-point Sharpie marker, which made no sense. When I mentioned it to my boss, he laughed as said, "Welcome to working with HV!". Sure, I recall getting some tiny shocks that I thought were due to static electricity and my not being properly grounded. Nope. It was tiny zaps from our HV supplies. Turned out each of those dark dots was the charred path of a zap. I also found each dot was numb, with full sensation not returning until weeks or months later, after all the affected layers of skin had regrown. I hadn't been working without gloves, but I had grabbed the WRONG gloves that looked similar but lacked any any electrical rating that wicked my sweat to the surface. Doh! Always verify your equipment. ALL your equipment!