First, let’s understand that the magnetic fields from a single conductor wire emanate from that wire in a pattern that could be described as concentric cylinders. The image at right represents a cross section view of a current carrying wire. Notice the concentric circles of magnetic field lines around the wire. Notice also, that the magnetic field lines are more concentrated near the wire, and less concentrated as the distance to the wire increases.

Now, understanding that magnetic shielding “works” because it is a better “conductor” of magnetic field lines than air or just about any other material, let’s see what happens with 2 different shield designs. First, let’s make a shielding cylinder around the wire.
In the cross section image at right, we see that the magnetic field lines that would have occurred at the radius of the shield will exist INSIDE the shield. However, magnetic field lines at all other radii will not be affected. Net effect: no shielding.

But what happens if we use a flat shield? As you can see from the image below, the magnetic field lines which intersect the flat shield will be compressed into the shield, leaving less magnetic field on either side of the flat shield.

But also, note the following:

• There is an area near the shield which enjoys LOWER field strength
• The areas near the edge of the shield show HIGHER field strength
• The magnetic fields of large radius are unaffected
• The wider the shield, the larger the shielded area, both in width and depth

If the edges of the shield are bent slightly TOWARDS the source, the high field area at the edge of the shield will move further away from the “shielded area”.
In conclusion, for net current, flat (or nearly flat) shielding is more effective for fields from wiring in the area adjacent to the shield. The wider the shield, the larger the shielded area. Contact us if you have specific questions about your shield design.
For situations where you have balanced current (that is equal current in the hot and neutral wires), a cylindrical shield can be effective. Take a look at this 13 minute video from Michael Neuert which demonstrates this phenomenon: