S9407-AB-HBK-010 Datasheet, PDF (23/276 Pages) Glenair, Inc. – HANDBOOK OF SHIPBOARD ELECTROMAGNETIC SHIELDING PRACTICES

S9407-AB-HBK-010, Rev. 2
Section 2
RIGID SHIELDING CONDUIT REQUIREMENTS
2.1 INTRODUCTION
The rigid shielding conduit described in this section consists of carbon steel pipe (MIL-P-
24691/1) used as conduit for electromagnetic shielding of cables. The conduit can be used for the
purpose of either protecting enclosed cables from external radiation, or for attenuating
electromagnetic radiation emitted by enclosed cables.
While rigid conduit is less expensive to buy, installation is generally more expensive than
flexible shielding conduit specified in appendix D because it must be installed by a skilled pipe-fitter.
Also, rigid conduit is much heavier than flexible conduit. For example, the 3/4-inch rigid conduit, at
1.32 pounds per foot, is four times heavier than typical 3/4-inch ID nonjacketed conduit at 0.33 pound
per foot, and is over two-and-a-half times heavier than typical 3/4-inch ID jacketed flexible conduit at
0.50 pound per foot.
Although at frequencies above 200 Hz the shielding effectiveness of an infinite length of rigid
conduit is higher than that of an infinite length of flexible conduit (see figure 6-7), when actual
installations were examined, it was found that the flexible magnetic installation was less susceptible to
magnetic fields because it had fewer end fittings than the rigid conduit installation. The major problem
with rigid metal conduit is that it is much more difficult to run rigid metal conduit so that the areas of
reduced shielding caused by the connector, transition fittings, and adapters are located in low
magnetic flux regions. In addition, if the field levels are too high and the shield of the conduit is not
enough to protect the system cables, it is a lot easier and less expensive to move flexible conduit than
rigid conduit.
For the reasons mentioned, flexible conduit is preferred over rigid conduit except in those
cases where flexible conduit does not provide adequate shielding (see paragraph 6.7). For existing
installations of rigid conduit, the policy of NAVSEA is to let the conduit remain as installed. However,
in cases where repairs or modifications to the installed rigid conduit are required, the flexible conduit
(where adequate) shall be substituted for the rigid conduit.
2.2 SCOPE
This section describes specification requirements for the rigid shielding conduit, conduit
installation, preparation methods, and rigid conduit couplings.
2.3 RIGID CONDUIT SPECIFICATION
Rigid metal conduit used as an electromagnetic shield for cables shall be carbon steel pipe in
accordance with specification MIL-P-24691/1, "Pipe and Tube, Carbon Steel, Seamless." The pipe
shall have a nominal wall thickness of not less than 0.120 inch. See table 2-1 for sizes and types.
Reprinted without change
2-1