I think the title says it all, but I'll try to explain better:

I know a clamp meter measures current by the current induced (if AC) or the magnetic field created (if DC). But how does the measurement not get affected by wires arround the clamps (because they also create magnetic fields) and only the wires inside the clamps are measured?


An AC current sensor is basically just a transformer. If you just pass the load wire through the magnetic core, it is like having half a turn. Or you can coil the load wire around the core and make it resemble a traditional transformer more for higher sensitivity.

enter image description here https://www.electronics-tutorials.ws/transformer/current-transformer.html

The load wire produces a magnetic field that is concentrated in the core and is focused to pass through the secondary sense-winding. If the load wire has AC current in it, the magnetic field changes which induces current to flow in the sense-winding via Lenz's law.

But if you have an external load wire it kind of looks like this (did not draw the way the magnetic core influences the path of the magnetic field lines): enter image description here

A DC current sensor works as follows:

enter image description here Taken from: https://www.tamuracorp.com/electronics/en/currentsensors/circuit_system_description/

The DC current in the wire generates a static magnetic field that circulates around the axis of the wire (via the right-handle rule where the thumb points in the direction of the current flow and the other fingers curl around the wire and point in in the direction of the circulating magnetic flux.

The ring magnetic core concentrates the flux around the wire and focuses as much of it as possible to flow through the hall-sensor in the air gap. The hall-sensor detects the strength of the magnetic field which is indicative of the DC current flowing.

But if you have an external load wire it kind of looks like this (did not draw the way the magnetic core influences the path of the magnetic field lines):

enter image description here


My initial answer was:

The fact that it is a loop makes magnetic field generated outside of the loop pass through the loop in opposing directions on opposite sides of the loop as it passes through the loop from outside the loop, cancelling out.

Drawing it out, I can see the flux lines being distorted to follow the magnetic core as they pass through, but not in a way that induces two opposing circulating fluxes in the magnetic core. At the very least, it can be seen that external flux sources aren't concentrated very well through the sense-coil or hall-sensor. The ring also helps align the flux from the load wire properly through the sense coil or hall-sensor if the load-wire isn't perpendicular.

I know from experience that they are sensitive to external fields more than I would like. But not nearly as much as one might think. But more than I would like. You still have to zero out the sensor and even then you can see influences from surrounding magnetic fields that change during operation. I'll have to look into it a bit more later but right now, I have the conclusion it doesn't actually cancel out external fields as much as it just concentrates the internal field much more than external fields so that your "true zero" does not too much from your zeroing operation.

  • $\begingroup$ I'm sorry buy I still didn't understand how a wire on the outside is different from a wire inside. Because a wire on the outside would induce a stronger current on the outside winding and a weaker current on the inside winding, not cancelling each other... just as the wire inside but the other way around. $\endgroup$ – Dinis Jun 1 at 17:37
  • $\begingroup$ @Dinis What "outside" and "inside" winding are you talking about? $\endgroup$ – DKNguyen Jun 1 at 17:40
  • $\begingroup$ I mean "turn" and not "winding". The loop is around the ring, with the vector of the plane of the loop being tangent to the ring on each point, right? So each turn is 1/4 on the inside and 1/4 on the outside. $\endgroup$ – Dinis Jun 1 at 17:43
  • $\begingroup$ @Dinis Sorry, what are you referring to when you say loop and when you say ring? What type of setup did you have in mind? Because I'm thinking of the the clamp on meters where you clamp it around the wire (like a zero-turn transformer for AC). Are you asking about setup where you wrap multiple loops of wire around a current transformer the sensor ring to get higher sensitivity? $\endgroup$ – DKNguyen Jun 1 at 17:46
  • $\begingroup$ Yes, I was asking about the setup that you clamp around a wire but also that has ferrite core ring and a wire wrapped around a ferrite core and the ferrite core is split in two when you open the clamps. At least that was how I thought all clamp meters where made :) $\endgroup$ – Dinis Jun 1 at 17:56

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