Narrowband  EMI Narrowband  EMI  consists  of  a  single  frequency  or a  narrowband  of  interference  frequencies.  Narrowband EMI  usually  has  a  minor  effect  on  communications  or electronic equipment. It can be tuned out or faltered out. Broadband  EMI Broadband  EMI  is  not  a  discrete  frequency.  It occupies  a  relatively  large  part  of  the  electromagnetic spectrum. This type of EMI is usually caused by arcing or corona and causes most EMI problems in digital data equipment. It will be especially noticeable when you are receiving data on digital data links. It is caused by the worn  or  improperly  installed  brushes  of  motors  or generators,   defective   fluorescent   lights,   arcing   of contacts  in  electrical  controllers  or  stepping  switches, ignition  systems  of  motor  vehicles,  igniters  for  jet engines,   and   defective   power   lines   or   power transformers. Improperly  bonded  lifelines,  rigging,  jackstays, ladders,  and  stanchions  also  produce  a  significant amount of EMI in a shipboard environment. They act as nonlinear mixing devices and antennas. They receive a number of different transmitted frequencies, mix them, and reradiate them over a broad spectrum. CONTROL OF EMI EMI can be controlled or eliminated if some simple procedures  are  followed  and  good  installation  practices are adhered to. We will divide the discussion of EMI control and reduction into two categories: shipboard and shorebased.  Many  of  the  problems  and  procedures  for reduction are the same for both types of installations. Shipboard EMI Control Shipboard  EMI  control  is  greatly  simplified  for typical electronic and digital-data installation. Because of the ship’s steel hull and construction, much shielding and   isolation   are   provided   for   typical   shipboard equipment  spaces.  This  blocks  out  most  broadband interference  generated  both  internally  and  externally. Five major factors must be considered in shipboard computer and digital equipment installations. They are as  follows: 1. 2. 3. Equipment  location Equipment   shielding System and equipment grounding 4. Interconnection cabling 5. Source of power EQUIPMENT   LOCATION.–   Digital   and computer equipment should be located in spaces that are free of EMI sources. It should not be located in spaces that contain radars, radio transmitters, or generators or other  rotating  machinery.  Simple  attention  to  the location of digital equipment can reduce or eliminate many sources of EMI. EQUIPMENT   SHIELDING.–   Electronic   and digital  equipment  should  never  be  operated  with drawers extended, cover plates removed, or doors open. Modern equipment contains EMI reducing gaskets and shields  that  enclose  the  equipment.  Defeating  this shielding  can  lead  to  serious  problems.  Always  reinstall cover plates with all fasteners in place. If a cover plate or shield must be removed in the course of corrective maintenance, ensure that the EMI reducing contacts or wire gaskets on the equipment opening are in good condition before the cover or shield is replaced. If the contacts or gaskets are bad, replace them. SYSTEM  AND  EQUIPMENT  GROUNDS.– System and equipment grounds are extremely important in  equipment  installations.  All  cabinets  should  be grounded together on a common system-ground bus. Normally a main system-ground bus of about 70,000 circular mills (1.5 inches in diameter) or more is run through all spaces. Each equipment cabinet is connected to the system ground by a heavy ground cable. The system ground is securely attached to the hull of the ship and provides a good ground reference for the system. In addition, all equipment cabinets have ground straps bypassing the shock mounts attached to the metal decks or mounting racks. A poor electrical connection will result from paint on ground straps or on the metal decks where the ground straps are mechanicaly attached, All terminal  lugs  or  ground  straps  used  to  bond  the equipment to the hull or the system ground should be bright, clean, and free of any foreign material. This is also true of grounding studs and the system ground cable.  This  clean  surface  ensures  a  good  electrical connection.  The  grounded  cabinets  provide  a  shield  at ground  potential.  This  keeps  in  any  signal  that  might cause a problem somewhere else in the system. It also keeps out stray interference that might cause a problem in  a  particular  piece  of  equipment. INTERCONNECTING   CABLES.–   All  inter- connecting  cables  used  in  shipboard  electronic  and digital systems should be shielded cables. They should be   assembled   correctly   according   to   installation 8-4