S9407-AB-HBK-010 Datasheet, PDF (119/276 Pages) Glenair, Inc. – HANDBOOK OF SHIPBOARD ELECTROMAGNETIC SHIELDING PRACTICES

S9407-AB-HBK-010, Rev. 2
6.5.3 Self-Compatibility of Equipment
It is assumed that equipment supplied to the installing activity by the manufacturer will be self-
compatible. In cases where cable-connector mounting on individual cabinets or equipment violates
the spacing requirements described in this section, the manufacturer's spacing will take precedence.
It must also be assumed that, although spacing between cable penetrations on Government
Furnished Equipment (GFE) cabinets may be less than that prescribed in this section, the system in
question passed all the tests required of GFE and was accepted for service use. Therefore, the cable
spacing was adequate for the conditions under which the equipment was tested. It is possible that
acceptance tests were conducted in a sterile electromagnetic environment such as a screen-room
area supplied with filtered power. It is also possible that the spacing may be inadequate when the
equipment is operated in the more stringent shipboard environment. For this reason, spacing of
external cables connected to such a cabinet or equipment shall be routed to meet the spacing
requirements of this section within as short a distance from the cabinet or equipment as practicable.
6.5.4 Difference-Mode Versus Common-Mode Interference Considerations
Common-mode interference (CMI) voltages are those voltages appearing in equal magnitude
and phase from each signal conductor to ground. They are developed as follows:
a. Electrostatic coupling - The electrostatic environment is capacitively coupled to both
signal wires.
b. Electromagnetic induction - The environmental magnetic field threads both signal line
areas.
Electrical noise can be capacitively coupled from adjacent cables and cause CMI in ship
installations which utilize long lengths of unshielded cable. CMI can be eliminated in most cases with
the use of shielded cables (i.e., shielded twisted pairs as opposed to unshielded twisted pairs).
To minimize the effects of CMI, differential signal circuits and properly terminated shielded
cable should be employed in the design of susceptible equipment. As this document assumes the
common-mode case has been properly addressed, the spacing requirements contained herein are
based only on the effects of differential-mode interference.
6.5.5 Structure Currents and Powerline Isolation Transformers
For purposes of this document, structure currents are defined as power-line currents flowing
line-to-line via the ship's hull and structure. The connections from line-to-ground are completed
through line-to-ground filter capacitors installed on powerline inputs to electrical and electronic
equipment.
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