S9407-AB-HBK-010 Datasheet, PDF (35/276 Pages) Glenair, Inc. – HANDBOOK OF SHIPBOARD ELECTROMAGNETIC SHIELDING PRACTICES

S9407-AB-HBK-010, Rev. 2
The determination of the best grounding configuration of magnetic shielding conduit (both
rigid and flexible) for frequencies between 10 kHz and 100 kHz, normally requires a case-by-case
engineering analysis by NUWC, Newport, for example, as authorized by NAVSEA. With the absence
of this analysis, it is recommended the installation drawings be followed. If there is not direction on
the drawings concerning the grounding configuration, the following recommendation will provide the
highest probability of attaining EMC:
• For cable categories S1-1, S1-2, S1-3, S2-1, S2-2, S2-3, S4-1, S4-2 and S4-3, use a
single-point ground configuration. These are the most sensitive cable categories and
would be susceptible if there were sufficient currents flowing on the conduit.
• For cable categories other than S1-1, S1-2, S1-3, S2-1, S2-2, S2-3, S4-1, S4-2 and S4-
3, use a multiple-point ground configuration. These cable categories are less
susceptible to the current amplitudes which, experience has revealed, exist on the
conduit. For these cable categories, it is not technically cost-effective to implement the
single-point ground in an all-inclusive manner.
At frequencies above 100 kHz, where approximately 70 dB of attenuation is provided against
structure current, multiple-point grounding of the conduit is recommended for reasons of economy,
ease of installation, and to reduce coupling problems caused by standing waves. The term "multiple-
point grounding", as used in this document, includes inadvertent grounding of conduit as well as
intentional grounding of conduit installed in accordance with existing plans. Note that multiple-point
grounding allows the use of Type 1 (unjacketed) conduit installed in accordance with MIL-STD-1310.
3.4.3 Shielding Effectiveness of Flexible Conduit
The shielding effectiveness of flexible shielding conduit against magnetic fields is somewhat
greater than just the penetration loss provided against structure current as described in paragraph
3.4.2. In addition to the penetration loss, there exists another loss that is dependent upon the
direction of the magnetic field incident on the conduit. This loss is maximum for flux approaching
perpendicular to the conduit axis and minimum for that approaching parallel to the axis. In practice,
the field is neither uniform nor does it approach the conduit axis at precisely zero or 90 degrees.
Therefore, there is always a penetration loss plus some additional loss due to these boundary
conditions.
The total shielding effectiveness exhibited by the conduit was determined empirically and is
presented in figure 6-7 of this document. Provided that the grounding requirements of paragraph
3.4.2 are observed, this is the shielding effectiveness that can be expected in practice.
3.4.4 Partial Shielding of Cable
Flexible shielding conduit may be used to enclose a portion of a cable run as an alternative to
enclosing the entire cable run. The subject of partial shielding of cables is addressed in paragraph
6.7.1 of section 6.
3.4.5 Bonding and Grounding Methods
Shipboard bonding and grounding methods for flexible shielding conduit shall be in
accordance with MIL-STD-1310 which states that the method of bonding requires the use of a Type IV
bond strap. One end of the bond strap is attached to the conduit braid by means of a hose clamp
(with worm-gear adjustment), and the other end is attached, typically, to a stud at ground potential.
(See figures 3-1 and 3-2.)
For single-point grounding, the rubber jacket of Type 2 conduit shall be cut away to expose
the braid at the area to be grounded in order to facilitate attaching the bond strap.
3-3