×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Anyone know why my "clip-around" AC
3

Anyone know why my "clip-around" AC

Anyone know why my "clip-around" AC

(OP)
Anyone know why my "clip-around" AC ammeter shows 2 slightly different readings depending on which of the 2 wires I clip-around?  I've seen as much as 10% difference.  

RE: Anyone know why my "clip-around" AC

By two wires, I assume you mean single phase, i.e. hot wire and neutral wire.  Does the meter show this anamoly on other machines?  Does moving the meter around; up and down the wire, side to side make any difference?  What is the machines's resistance to ground?  Some of the return may be flowing to ground.

Blacksmith

RE: Anyone know why my "clip-around" AC

(OP)
Blacksmith,

Yes, its a single-phase system.  There is no ground (yes, I know-dangerous).  I didn't notice any difference when moving the meter around.  BTW, this is not the first time I've observed this.

RE: Anyone know why my "clip-around" AC

You don't think there is a ground.  It sounds like there is an inadvertent leakage to ground.  Can you insulate the powered device from any contact with a grounded object (like yourself)?  Is there a large energized part that might have a significant capacitance to ground?  Current in has to equal current out, either through the return wire or somehow to ground, keeping in mind that the neutral and ground are connected at the service panel.

RE: Anyone know why my "clip-around" AC

(OP)
There is no leakage-the whole thing is isolated.  I am using a transformer to step down 220 v to 110v, and I checked the 110v side.

This is baffling.

RE: Anyone know why my "clip-around" AC

Metalguy,

Are the two wires you are measuring very close together?  Clamp on ammeters are very susceptible to stray flux causing erroneous readings.  If the wires are in different locations, this could possibly account for the discrepancy.  Otherwise, the return HAS to carry exactly the same current as the supply, again assuming that there is no leakage.

RE: Anyone know why my "clip-around" AC

10% difference in a clamp-on ammeter reading may not be all that significant.  

Try making sure the iron faces on the tongs are clean and that you have each conductor in the center of the opening perpendicular to the core of the clamp-on.

RE: Anyone know why my "clip-around" AC

Try clamping over BOTH wires in the circuit; this should read zero. If it doesn't, you have ground fault current or an inaccurate meter.

RE: Anyone know why my "clip-around" AC

What two wires are you clamping on to?  Hots?  Neutrals?  On the same circuit?  Are you clamping on to them at the same time, or individually?  What are they feeding?  Is this a 2-wire or 3-wire circuit?

Usually, you should only be clamping around one wire at a time.  On a 2-wire circuit, the measurement at each wire should be equal, unless there's a ground current.  On three-wire circuits, the reading taken on one wire should equal the reading taken by simultaneously clamping onto the other two wires, unless there's a ground fault.

Simultaneously clamping onto all wires in a circuit should read zero unless there is a ground fault.  Simultaneously clamping all wires including ground conductor should equal zero unless there's some other unintentional circuit connection.

RE: Anyone know why my "clip-around" AC


Good adivce by all.  The only comment I can add is to be sure the CT pole-face gap is as close to zero as possible.  
  

RE: Anyone know why my "clip-around" AC

(OP)
I went and played around with my clamp-on, and the question is solved.  I was clamping the wires very close to the transformer, and the mag. field from it REALLY screws things up.  It induces a current when the clamp-on "arms" are within about 4" of it.  The trans. is rated at 2500 watts, so it's not a baby-size.

Thanks to all; twosockets and the blacksmith win a star.

RE: Anyone know why my "clip-around" AC


Given a typical/readily available portable CT for use with a multimeter, consider an AEMC MN210 for line-frequency measurements. http://www.aemc.com/products/html/moreinfo.asp?id=204010201&dbname=products#specifications

Published accuracies…
Output Signal 1mA/A on 1 ohm [burden]
Influence of Conductor Position 0.5% of Reading @ 50/60Hz  
Influence of Load <3% of mA output 40Hz to 1kHz  
Influence of Adjacent Conductor < 15mA/A  @50Hz {!}

Maybe I’m misinterpreting published numbers, but it looks like adjacent-conductor influence could affect readings producing up to a 15x [15mA/A:1mA/A or 1500%] error.
  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources