Page 118 - Power Electronics Handbook

EM shicMing techniques 11 1

current - Shield
Shield

Circulating
'Seam Circulating - Seam

I
(a)

Join (gasket)

Shield

(C) (d)
-re 4.6 Seams in shields: (a) and (b) position of seam relative to circulating CMent;
(c) illustration of transfer impedance of a seam; (d) reducing transfer impedpnce by bing
contact area

Screened cables, brought into the screened enclosure, should have their
screens terminated in connectors which totally enclose their screens and
provide a solid connection from the screen to the shielded wall of the
enclosure. Both ends of the cable should be so connected, and the finish
used on the enclosure must not be such as to make it difficult to provide a
good cunnection.
The low-frequency performance of a shield is determined by the
ekctrical properties of its walls and joints, whereas the high-frequency
performance is much more Muenced by the gaps and apertures which
exist in its surface. For effective screening there should be no gaps which
are greater than one hundredth of the lowest wavelength of the interfering
radiation.