A letter in the December 2017 QST caught my eye. It had a safety tip for dealing with high voltage circuits, and I thought I would pass it along. It’s reprinted with permission from the author, Joseph Birsa, N3TTE…..Dan
Reading the article on DMM’s in the October 2017 issue of QST reminded me of a safety practice I want to pass along. Always, always, ALWAYS check the meter leads using the resistance function of a meter BEFORE checking a hazardous voltage! (Or any voltage or current for that matter.)
To do this, BEFORE YOU DO ANYTHING ELSE, turn the meter to ohms and short the tips of the leads to verify continuity (zero or low reading). Then, switch the meter to the appropriate function and proceed.
When I was starting out as an electrical engineer, a field engineer, who was a ham by the way, told me a story about an electrician who opened an equipment cabinet powered by three-phase, 480 V. Before beginning, he used his meter to check the 480 V. Seeing no voltage, he proceeded and was subsequently electrocuted! When they checked his meter, they found one of the leads was opened. Or perhaps a fuse was blown.
Since then, the first thing I do when I start to use a meter is to use the resistance function to check lead continuity.
Kenneth Finnegan says
Working on industrial equipment, when we lock-out tag-out a piece of equipment with hazardous energy, using a meter is a three step process:
1. Check a near-by live circuit to verify that the meter is functional
2. Check the de-energized unit to verify it has been powered down.
3. Check the near-by live circuit again to confirm that the meter is still functional.
Just switching it to ohms mode to check the leads isn’t good enough if your life is at stake.
Bas PE4BAS says
It is good enough tip for the average radioamateur or hobbyist. I always carry 2 meters with me and always check high voltage circuits with both meters just to be shure. 73, Bas
Joshua | DC7IA| KK4RVI says
When I became electrician, one of the first rules I learned, is to test the meter at a known voltage source.
That’s one thing that can lead to failing the examination and not becoming electrician: Not testing the meter at a socket outlet to see if there’s voltage before working on the switchbox, which needs to be wired/rewired in the examination.
Steve C - KE8HXM says
I have two Fluke multimeters that I used when I was working as a field engineer for Unisys back in 1997 through 2002, which were certified, per ISO 9001 standards, in 1999. This was done by the company for all our test instruments. I’d like to have them re-certified but I have no idea what or who to approach about this. Anyone have an idea where in this area (SE Michigan) I might find a place that can perform this operation/certification?
I know they are in good condition and have never been abused or improperly used and that they are safe enough to use, so perhaps this is a moot question, but being a stickler for details, I feel better knowing they are verified accurate and safe.
Dan KB6NU says
I haven’t had direct experience with any calibration houses here in SE Michigan, but I just Googled “test equipment calibration Detroit” and found several companies. I would bet that, though, that the cost of having this done would be more than what the meters are worth.
Larry Koziel says
While I agree that checking the meter on a known live circuit is a good idea, I wouldn’t do that as the first step. I’d do the resistance check first.
The reason is that years ago, a colleague was testing a newly-built piece of resistance welding equipment. The electrical cabinet had a 3-phase 400 A circuit breaker and was supplied with 3-phase 480 V via a disconnect and 400 A slow-blow fuses.
When he closed the breaker, none of the equipment in the cabinet powered up. The then attempted to check for 480 V at the line side of the breaker, but didn’t realize that the meter had been used to measure current earlier. With me and another person looking over his shoulder, he in effect applied a short circuit across two phases of the 480 V.
In the resulting explosion, both his hands, which had been within a probe’s length of the short, were severely burned as were all three of our faces. My colleague was hospitalized for more than a month.
In post-incident investigation, it was found that the breaker operating mechanism was misadjusted, and despite the fact that the handle was in the ON position, the breaker contacts had never closed. The short started between two of the poles, but due to the resulting ionization, arcing continued between all three poles and also to the grounded cabinet. The fuses supplying the breaker never opened.
Testing on a known live circuit as a first step would not have prevented an accident like this. It would have caused an accident at the live-circuit-as-test site. Testing the meter on the ohms setting first would have safely discovered that the meter leads were not plugged in as assumed. Subsequently testing on a live circuit would then confirm that you could trust the voltage reading.
Larry Koziel says
I should add also, that the Fluke meter had an internal fuse in the ammeter circuit, but the interrupting rating of the fuse was lower than the available short circuit current available from the mains, so when the fusible link evaporated, the fuse was full of plasma and ionized gas which continued to conduct current and wasn’t capable to open the fault circuit. Later versions of the meter used substantially higher rated fuses. Keep that in mind when you buy cheap Chinese or other meters and if and when you have to replace a fuse in your meter, don’t just drop in any fuse that fits. Be sure to use a properly rated fuse.
73 and work safely,
Larry K8MU
Vance N3VEM says
Another good tip that is related, is to make sure that your meter is rated for the voltages you’ll encounter – 600 volts seems to be a standard rating for decent meters, but some cheap meters are rated for much lower, which can cause some of the same nasty consequences those above mentioned in some of their scenarios.