Amp-probe question

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goldstar

goldstar

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Location
New Jersey
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Electrical Contractor
Can an Amp-probe measure amperage properly when clamped onto parallel conductors ?

The reason I ask this is because I had a power quality problem at an office complex where several 3-phase roof-top AC units were blowing (only 1) 30 amp fuse during the hot summer months. I checked every possible connection I could find right down to the service entrance and all were tight and properly terminated. While I was at one of the roof-top units in August I had my (analog) amp-probe clamped onto the leg that was giving us a problem and the AC guy had his (digital) probe clamped onto the same phase and all was well at around 18 amps. Suddenly and mysteriously my meter read 40 amps for about 30 seconds while the AC guy's did not change. I also checked the voltage (phase to phase) at that point in time and there was no significant change.

I contacted the POCO and they came out to the job to install a test meter at the service entrance. They believe (as I do) that there is probably a piece of equipment that is going bad somewhere inside the building. However, this is an 800 amp, 3-phase, 120/208 service with parallel 350 MCM cables. They clamped their meter leads across the parallel feeders and assured me that the readings would be accurate. I always thought that an amp-probe measured amperage based on induction of single conductors. Am I incorrect or is it done by heat or some other measuring method ?

If anyone has experienced this or knows the answers to either question would you please post and enlighten me.

Thanks,
Phil,
Gold Star Electric
New Jersey
 
Where both meters true RMS meters?

As long as your clamping around the same phase you will get good readings.

We have a flexible amp clamp and we often wrap it around four conductors of the same phase, this will produce a correct total reading of the current passing through the clamp
 
Yes, you do get a "total" reading when you're snapped around paralleled conductors. I did a lot of work in a plating plant, where there were many paralleled DC conductors for thousands of amps. We had special amprobes with long rectangular jaws to contain all the paralleled conductors.

You can troubleshoot a faulty termination of one conductor in the paralleled bundle by taking an amp reading of each conductor in the bundle. They should all be pretty close to each other. If you find one in the bundle low or high compared to the others, examine its terminations.
 
The meter works based on the magnetic field of what ever is in its jaws.

If you go around parallel conductors it will add them together. If you go around two different phases then the meter will add them vectorially and you will see the imbalance between them.
 
jim dungar said:
If you go around two different phases then the meter will add them vectorially and you will see the imbalance between them.
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Interesting. :)

If I went around 3 phases of a Wye would the meter indicate the current I should see on the neutral?
 
Yup.

And if you wrap the current sense around all the phases and the neutral, you should get zero current measured; if you get something other then zero current then you have circuit fault somewhere.

-Jon
 
winnie said:
And if you wrap the current sense around all the phases and the neutral, you should get zero current measured; if you get something other then zero current then you have circuit fault somewhere.
Click to expand...

It's odd how my mind works, I did know how a GFP main operates so I should have known that I could use an amp clamp in the same way.
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I also know if I wrap a conductor a few times through the clamp I can get a more sensitive reading which I have done when trying to measure 1 or 2 amps with a 200 amp clamp.
 
mdshunk said:
Yes, you do get a "total" reading when you're snapped around paralleled conductors. I did a lot of work in a plating plant, where there were many paralleled DC conductors for thousands of amps. We had special amprobes with long rectangular jaws to contain all the paralleled conductors.

You can troubleshoot a faulty termination of one conductor in the paralleled bundle by taking an amp reading of each conductor in the bundle. They should all be pretty close to each other. If you find one in the bundle low or high compared to the others, examine its terminations.
Click to expand...


How do you amprobe DC? Did the special unit use a Hall effect sensor?
 
infinity said:
How do you amprobe DC? Did the special unit use a Hall effect sensor?
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I didn't invent them. I only used them. Some had a switch for ac or dc, and some were one or the other. Amprobes are plentiful in plating factory maintenance shops.
 
iwire said:
Are they as plentiful as the rust and corrosion?
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No chit.... even stainless steel enclosures rust in plating plants. Freaking everything rusts. My eyeglasses started to get corroded just from working there. After you get over the stink, and the fact that there's 10's of thousands of gallons of crap that can dissolve your body all over the place, it's no too bad. :D
 
iwire said:
Where both meters true RMS meters?
Click to expand...


good question- maybe a drive or soft-start on the same transformer grunging things up periodically. Maybe rent a logging power quality monitor?
 
infinity said:
How do you amprobe DC? Did the special unit use a Hall effect sensor?
Click to expand...
You amprobe DC the same as AC. The magnetic field around a conductor carrying DC doesn't change rotation, only possibly magnitude if the DC current is varying in strength over time. Actually, you could determine that the load is DC by putting a compass over one of the conductors. The compass needle will align itself perpendicular to a conductor carrying DC.

Dan
 
iwire said:
Where both meters true RMS meters?
Click to expand...
I know my analog amp-probe was not but I can't speak for the digital one that was also used. I just found it interesting that my analog probe deflected to 40 amps while the digital probe did not change.

jim_dungar said:
The meter works based on the magnetic field of what ever is in its jaws.
Click to expand...
Thanks Jim. I was close with induction but no cigar I guess.


Anyone have any thoughts on what might be causing the increase in amperage on one phase only ? My original suspect was one particular roof-top AC unit (and a new one at that) but it was happening on two others also and in the same half of the building. The other half of the building is not experiencing any problems and is also fed from the same service.

Thanks,
Phil
Gold Star Electric
New Jersey
 
I agree with Ray, maybe your non-RMS analog cl/amp responds to a wider frequency range than the non-RMS digital, used by the AC guy.
RayS said:
good question- maybe a drive or soft-start on the same transformer grunging things up periodically. Maybe rent a logging power quality monitor?
Click to expand...
I believe, Ray's log idea is the best way to reliably rule out current surges associated with simultaneous, single-phasing v-drops. If the log rules out v-drops, then you can look for noise. Besides the obvious computer loads, some other common harmonic sources can include dampers SCR's, lighting ballasts, and even office copiers or laser printers.
 
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If you are getting an increase in the current, did the voltage drop on the service maybe? I can see having voltage drops on a phase causing problems, but would that correspond to the increase in amps you are reading. A drive or soft start should cause an intermittant voltage drop on a phase. Are there big exhaust fans being run on a single phase starter? Big refrigerator compressors, on single phase?

Tough one for sure.
 
For larger conductors, conductor groups, bus bars, and tight spaces, you can also get a flexible cable that can be routed around the conductors to be 'probed.
 
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