Magnetic fields outside a shielded power cable

[Dublin]

I was recently having a discussion about magnetic fields outside of a shielded power cable.

My understanding up to this point was shielded cables only shields electric fields and not magnetic fields which are associated with current flow inside a cable. For the shielding to shield magnetic fields it would need a counter current (Possibly Eddy Currents) equal in magnitude to cancel it out.;The line integral of the B-field along any closed loop is directly proportional to the sum of the currents.

His understanding was that the shielding helps contain the magnetic fields similar to how the core of a transformer contains 95% of the flux and all that escapes is leakage and is much smaller.

I was hoping for clarity on what the magnetic fields look like surrounding a shielded power cable.

Thanks

 

[berkeman]

I was recently having a discussion about magnetic fields outside of a shielded power cable.

My understanding up to this point was shielded cables only shields electric fields and not magnetic fields which are associated with current flow inside a cable. For the shielding to shield magnetic fields it would need a counter current (Possibly Eddy Currents) equal in magnitude to cancel it out.;The line integral of the B-field along any closed loop is directly proportional to the sum of the currents.

His understanding was that the shielding helps contain the magnetic fields similar to how the core of a transformer contains 95% of the flux and all that escapes is leakage and is much smaller.

I was hoping for clarity on what the magnetic fields look like surrounding a shielded power cable.

Thanks

You are correct, and the other person is misunderstanding the shielding in power cords.

In the transformer analogy, there is a lot of high-mu ferrous material (the core) that mostly contains and guides the magnetic flux.

In a shielded power cable, there is a thin conducting metal foil wrapped around the three wires in the cable assembly (and the foil is connected to the ground pin). This is used to keep external E-field noise from coupling onto the power conductors, but more importantly it is meant to keep the common-mode (CM) RF currents on the power cable (generated inside the device powered by the cable) from using the cable as a radiating antenna. This is sometimes needed to pass Radiated EMI regulations.

Hope that helps.

 

[Baluncore]

For the shielding to shield magnetic fields it would need a counter current (Possibly Eddy Currents) equal in magnitude to cancel it out.

The return circuit conductor is in the same cable and must have an equal and opposite current, so there is no need for eddy currents, the cancellation current is there in the cable. Like a twisted pair or a coaxial cable, three phase power currents and voltages also sum to zero. That is why all conductors of a transmission line must run in parallel together, passing through one conduit or one hole in a steel plate.

 

[Windadct]

mmm... I'll add that one of the purposes of the shield and complete construction of MV and HV shielded cable is to maintain a specific E field in the dielectric insulation. The PHYSICAL stresses are so high that this needs to be carefully maintained to ensure the cable is reliable over time.

In LV or MV Inverter feeds, where there is high EMI situation then that is the purpose, but this is a specific case. As these features add significant cost, they are applied on a case by case basis.

 

[tech99]

May I mention that a non magnetic, conductive screen, such as Aluminium, can contain a magnetic field provided the frequency is high. In such a case, the eddy currents are confined near the inner surface of the shielded by skin effect, and to do not penetrate through to the outside world where they would radiate.
Ther are sometimes problems with coaxial cables at low frequencies, such as for video, where magnetic shielding is ineffective.