Anyone know why my "clip-around" AC 3

[Metalguy]

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.

 

[TheBlacksmith]

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

 

[Metalguy]

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.

 

[jghrist]

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.

 

[Metalguy]

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.

 

[twosockets]

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.

 

[dpc]

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.

 

[DanDel]

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.

 

[peebee]

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.

 

[busbar]

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.

 

[Metalguy]

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.

 

[busbar]

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.