# Can a time-varying magnetic field pass a metal sheet?

Hello.

I'm using CT (Current Transformer) to measure the current flowing on the power line. The frequency of the current is 13.56 MHz. CT appeared to be metal-shielded (This metal case of CT may be grounded when CT is used).

CT has a toroidal shape and the power line under the measurement passes through the center hole of the CT, so azimuthal magnetic field around the power line induces EMF (ElectroMotive Force) on the coil inside the CT. This EMF is measured so the current on the power line is measured. This is the basic story of how CT works.

If the current frequency is low like 10 Hz, then I fully accept that CT really works. But I'm now confused at a high-frequency operation. The current is the source of the magnetic field and when the current oscillates, there is not only the magnetic field but also the electric field. The combined field is what we call an EM (ElectroMagnetic) wave. My frequency is 13.56 MHz which is a rather high-frequency, so the power line should play as an EM wave emitting antenna. EM wave generated from the power line propagates to CT first. However, CT has a rather thick metal case (much thicker than the skin depth at this frequency) so EM wave will be reflected from it. It means the magnetic field as a part of the EM wave fails to reach inside the CT so CT should not work! I think the only way for CT to work is that the magnetic field alone pass through the metal so they get in touch with the coil. But..Is it really possible? The time-varying magnetic field can exist alone without the electric field?

Of course, a commercial CT works well even for higher frequency. Could you please give me some idea of breaking this confusion?

## Answers and Replies

tech99
Gold Member
Hello.

I'm using CT (Current Transformer) to measure the current flowing on the power line. The frequency of the current is 13.56 MHz. CT appeared to be metal-shielded (This metal case of CT may be grounded when CT is used).

CT has a toroidal shape and the power line under the measurement passes through the center hole of the CT, so azimuthal magnetic field around the power line induces EMF (ElectroMotive Force) on the coil inside the CT. This EMF is measured so the current on the power line is measured. This is the basic story of how CT works.

If the current frequency is low like 10 Hz, then I fully accept that CT really works. But I'm now confused at a high-frequency operation. The current is the source of the magnetic field and when the current oscillates, there is not only the magnetic field but also the electric field. The combined field is what we call an EM (ElectroMagnetic) wave. My frequency is 13.56 MHz which is a rather high-frequency, so the power line should play as an EM wave emitting antenna. EM wave generated from the power line propagates to CT first. However, CT has a rather thick metal case (much thicker than the skin depth at this frequency) so EM wave will be reflected from it. It means the magnetic field as a part of the EM wave fails to reach inside the CT so CT should not work! I think the only way for CT to work is that the magnetic field alone pass through the metal so they get in touch with the coil. But..Is it really possible? The time-varying magnetic field can exist alone without the electric field?

Of course, a commercial CT works well even for higher frequency. Could you please give me some idea of breaking this confusion?
The magnetic field of the conductor surrounds it circumferentially, and will induce a voltage in another wire which lies parallel to the first. Your CT conductor is wrapped round the wire, and no EMF is induced in this direction. In the longitudinal direction it is like a short parallel wire but does not form a compete circuit, so no current can flow. As no current flows, there is no shielding effect.

Hello.

I'm sorry but I don't understand your geometry.
The magnetic field of the conductor surrounds it circumferentially, and will induce a voltage in another wire which lies parallel to the first. Y
The circumferential magnetic field around the conductor will induce a voltage on another wire which is in parallel oto the first conductor? This EMF is very different than what I'm used to such as a solenoid coil so I don't get how EMF is induced on the latter wire.

In the longitudinal direction it is like a short parallel wire but does not form a compete circuit, so no current can flow. As no current flows, there is no shielding effect.
Longitudinal direction? short parallel wire? could you provide me a simple drawing so I can get a clearer view?