Effect of seams in magnetically shielded enclosures? 1

[teagan]

Greetings,

We are starting work on a steel enclosure/rack. Our client knows he needs some level of magnetic shielding but is not real clear on the strengh of the field or the level of susceptability. The client has backed off from the more exotic materials and agrees that starting with a more conventional carbon steel enclosure makes sense.

Given the lack of data points, the question is some what generic in nature.

What is the effect of attenuation to a magentic shielded enclosure if the enclosure continuity is disrupted by seams between covers that mount to the chassis/rack?
Does seam geometry make a difference?
Are there any "gasketing materials" developed for EMI suppression that may be suitable in a magnetic shielding application?

Thanks

 

[drdick]

Magnetic shielding is a misnomer. The object is to guide unwanted magnetic flux to a place where it can return to its source without doing harm. If the flux must jump a gap, it projects a fringing field which will at least alter the field shape/strength within the "shielded" volume. A rule of thumb is to overlap seams/joints by at least 10 times the largest gap in the overlap area.
To make the "shield" volume more effective, you might consider a box within a box structure. The box farthest from the magnetic source can be very thin so long as it is separated from the primary box by a thin layer of air (foam sheeting).