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#43414 10/12/04 10:43 PM
Joined: Jan 2004
Posts: 171
M
Merlin Offline OP
Member
I recently installed a panel meter on a single phase load of 10 hp, 230 volt, FLA 55. My local supply house sold me a 0-75 amp meter, and 75-5 amp donut transformer. I installed these as I have a few in the past. The running load on my Fluke is 22 amps. However the meter only reads about 5 amps. The only thing I can figure is either a defect. or possibly the donut is to close to another load. Is anyone familar with these things. Thanks.

#43415 10/13/04 02:51 AM
Joined: May 2003
Posts: 2,876
E
e57 Offline
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Emon-demon? Are the donuts and meter a match for eachother and calibrated? Also, the placement of the donut is crucial. If it is sitting on the side at an angle to the conductor you'll get false readings. They should be as centered and squared on the conductor, which can be hard to do in most panels.


Mark Heller
"Well - I oughta....." -Jackie Gleason
#43416 10/13/04 04:18 AM
Joined: Jul 2002
Posts: 8,407
Likes: 2
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Also,
Has the Ammeter got the correct shunt attached to it?.

#43417 10/13/04 08:35 AM
Joined: Jan 2004
Posts: 171
M
Merlin Offline OP
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e-57,

I don't know if they are matched, this is what my supply house sold me so I assume so. It is sitting pretty square although it is laying on top of other wires in a very crowded box.

Trumpy,

What do you mean by a shunt?

Thanks for the reply

#43418 10/13/04 02:04 PM
Joined: Jul 2002
Posts: 8,407
Likes: 2
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Merlin,
Quote
What do you mean by a shunt?
An ammeter only carries a very minute amount of current, compared to the amount that it may be measuring.
A shunt is a resistance that is lower than the resistance of the meter movement itself, this shunt is placed in Parallel with the meter movement.
In this way, when connected to the circuit, the majority of current is passed, or Shunted, through the Shunt resistance.
Thereby ensuring that the meter only recieves a small proportion of the total circuit current and prevents the movement being damaged.
Hope this helps. [Linked Image]

#43419 10/13/04 04:08 PM
Joined: Sep 2001
Posts: 806
N
Member
Quote
My local supply house sold me a 0-75 amp meter, and 75-5 amp donut transformer.

Check the meter carefully. Did they actually sell you a 0-75A meter, or a 0-5A meter with a 0-75A scale? You need a 5A meter for use with the CT.

Other than that, check the orientation/centering of the conductor in the CT window, and make sure all the secondary CT wiring is OK. Insert another ammeter in the CT loop, and verify how much current is being developed on the 5A side.


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