Our former . EMC articles reviewed the principal conduction and radiation coupling mechanisms, as they affect equipment/system susceptibility, and the last one (EMC Article #5, June Issue) was addressing Shielded Cables. The present article is focusing on the shielding of equipment boxes, from the smaller hand-held devices up to large cabinets or even entire rooms.
Someone may wonder why treating separately cables shielding and box shielding? Against an EM field a shield is a shield, no matter if it is a tube or a cube… In fact, there is a significant difference: in a shielded cable, the wires are closely coupled with their tubular envelope, such as it is the mutual inductance that does the cancelling effect. In a shielded box there is no such close coupling: it is the portion of the field that goes through the barrier that gives a measure of the shield effectiveness.
1. Don’t let shielding happen by change – Design for it
Once all the EMI reduction techniques described previously have been applied (equipotential grounding, PCB layout, ground loop isolation, loop area reduction, etc ..), a conductive box may be the ultimate barrier against radiated susceptibility or emissions. However, too often, EMC performance is not regarded as a key element by people designing cabinets or equipment housing.
Most of times, one or a combination of the following approaches are used instead, that we could call “receipes for EMC failure”:
- Make the enclosure similar to earlier versions that were deemed to be EMI-free. Then, confirm expectations by a prequalification test on a functional prototype of the product.
- Starting from the ground up, make a box according to mechanical, aesthetic, cost and accessibility criterias and test it as above.
- Do as above, but perform only the mandatory Radiated Emission tests. Do not test for susceptibility unless a purchasing specification calls for it.
Such regrettable hit-or-miss process means that it is the final test that governs the outcome of a design, resulting in one or more of the following…
This text is unfortunately locked for further reading, but you can read all Michel Mardiguian’s texts and course material about EMC in the new book “Everything you always wanted to know about EMC but were afraid to ask”. Click for more information!