Meter Test lead length

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4-Phase

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Hello everybody,

Years ago when I was an electronics technician in the Navy, we had an older Chief Petty Officer who taught a 2 day class on multimeter use and safety. One of the things he used to talk about was test lead length- sometimes we had to make up odd ball leads in the field. He said a number of times that longer test leads can introduce errors on your multimeter. I have procrastinated on this subject for 30 years and am trying to find out if he was correct. Does anyone know of a technical paper or reference that proves (or disproves) what he was claiming? I took a look on the IEEE website- nothing there. Don't know if this matters, but keep in mind 30 years ago most of the military was using Simpson 280 analog meters, not digital.

Thanks!
 
4-Phase said:
Hello everybody,

Years ago when I was an electronics technician in the Navy, we had an older Chief Petty Officer who taught a 2 day class on multimeter use and safety. One of the things he used to talk about was test lead length- sometimes we had to make up odd ball leads in the field. He said a number of times that longer test leads can introduce errors on your multimeter. I have procrastinated on this subject for 30 years and am trying to find out if he was correct. Does anyone know of a technical paper or reference that proves (or disproves) what he was claiming? I took a look on the IEEE website- nothing there. Don't know if this matters, but keep in mind 30 years ago most of the military was using Simpson 280 analog meters, not digital.

Thanks!
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I have one of the 260's. It has a zero adjust, so I can't think of a reason that test lead length would be a problem. Just how long are you making your leads??
 
This is more of a general question as to how long a set of leads could be before they start impacting the meter readings. A couple of times we had to use test leads that were 40 or 50 feet long. Had to crawl on my back, arms extended above my head to take a measurement inside a missile launcher while someone else recorded the readings outside.
 
4-Phase said:
This is more of a general question as to how long a set of leads could be before they start impacting the meter readings. A couple of times we had to use test leads that were 40 or 50 feet long. Had to crawl on my back, arms extended above my head to take a measurement inside a missile launcher while someone else recorded the readings outside.
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I could see that being a problem if you were taking ohm readings, not sure how sensitive my old simpson is...
 
Thanks Hv&Lv. I guess I should clarify here what the old Chief was saying. We knew to zero out the meter when taking resistance checks, but he insisted that long test leads could impact voltage readings as well. The thing that I got thinking about a number of years ago was the military was just then starting to use the high impedance digital rms meters- I wonder if on those early handhelds they had the zero input behavior problems that were worse than today's models, and he was trying to explain that issue. Also, made a typo- that Simpson we used was the 260, not the 280.
 
4-Phase said:
This is more of a general question as to how long a set of leads could be before they start impacting the meter readings. A couple of times we had to use test leads that were 40 or 50 feet long. Had to crawl on my back, arms extended above my head to take a measurement inside a missile launcher while someone else recorded the readings outside.
Click to expand...

Fellow bubblehead (even if you are of the ceramic toilet seat branch) as a former Navy CPO instructir, we often made s*&t up. However, if you recall your basic drawing of a megger circuit connected to a test circuit (in some Neets module) you will see what the effect is, and should recall the formula for figuring it out, if not, drop and give me 20. :)
 
I'm not a test equipment expert, but I would say that nowadays it's probably a moot point. DMMs are sold with rated leads (CAT IV 600V or CAT III 1000V) and using homemade leads would invalidate all kinds of liabilities. The good news is that wireless technologies and datalogging are more and more available in DMMs so you can remotely monitor and/or record the data you need.
 
As little voltage and amps a tester uses, it's still there. So there'll be a voltage drop using longer leads.
So yes, the length of the leads will affect readings.
How much does it affect it to the point of reading being impractical? Not really sure, but it does affect it.

4-Phase said:
Hello everybody,

Years ago when I was an electronics technician in the Navy, we had an older Chief Petty Officer who taught a 2 day class on multimeter use and safety. One of the things he used to talk about was test lead length- sometimes we had to make up odd ball leads in the field. He said a number of times that longer test leads can introduce errors on your multimeter. I have procrastinated on this subject for 30 years and am trying to find out if he was correct. Does anyone know of a technical paper or reference that proves (or disproves) what he was claiming? I took a look on the IEEE website- nothing there. Don't know if this matters, but keep in mind 30 years ago most of the military was using Simpson 280 analog meters, not digital.

Thanks!
Click to expand...
 
Test instruments can give erroneous readings if the test leads are long when used near HV lines, especially if the leads run parallel to the towers. Also, meters with a very high input resistance are very sensitive meters and can pick up stray signals. Careful use and placement of long leads will reduce the error. In addition, microohm meters should have the leads claibrated with the meter to maintain its accuracy. If you use long leads on the microohm meter you must short the leads together, get a reading and subtract it from the reading taken on the test specimen.
 
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