Replacement junction boxes should be fabricated using  the same material as the original or from a fire-resistant,  nonabsorbent material, such as aluminum, or an acceptable  plastic material. Where fireproofing is necessary, a stainless  steel junction box is recommended. Rigid construction  prevents oil-canning of the box sides that could result in  internal short circuits. In all cases, drain holes should be  provided in the lowest portion of the box. Cases of electrical  power equipment must be insulated from metallic structure  to avoid ground fault related fires.  The junction box arrangement should permit easy access to  any installed items of equipment, terminals, and wires. Where  marginal clearances are unavoidable, an insulating material  should be inserted between current carrying parts and any  grounded surface. It is not good practice to mount equipment  on the covers or doors of junction boxes, since inspection  for internal clearance is impossible when the door or cover  is in the closed position. Junction boxes should be securely mounted to the aircraft  structure in such a manner that the contents are readily  accessible for inspection. When possible, the open side  should face downward or at an angle so that loose metallic  objects, such as washers or nuts, tend to fall out of the junction  box rather than wedge between terminals. Junction box layouts should take into consideration the  necessity for adequate wiring space and possible future  additions. Electrical wire bundles should be laced or clamped  inside the box so that cables do not touch other components,  prevent ready access, or obscure markings or labels. Cables  at entrance openings should be protected against chafing by  using grommets or other suitable means. Connectors (plugs and receptacles) facilitate maintenance  when frequent disconnection is required. There is a multitude  of types of connectors. The connector types that use crimped  contacts are generally used on aircraft. Some of the more  common types are the round cannon type, the rectangular,  and the module blocks. Environmentally resistant connectors  should be used in applications subject to fluids, vibration,  heat, mechanical shock, and/or corrosive elements. When HIRF/lightning protection is required, special  attention should be given to the terminations of individual  or overall shields. The number and complexity of wiring  systems have resulted in an increased use of electrical  connectors.  [Figure 9-158]  The proper choice and application  of connectors is a significant part of the aircraft wiring  system. Connectors must be kept to a minimum, selected,  and installed to provide the maximum degree of safety and Rectangular connectors are typically used in applications  where a very large number of circuits are accommodated in  a single mated pair.  [Figure 9-160]  They are available with a  great variety of contacts, which can include a mix of standard,  coaxial, and large power types. Coupling is accomplished by  various means. Smaller types are secured with screws which  hold their flanges together. Larger ones have integral guide  pins that ensure correct alignment, or jackscrews that both  align and lock the connectors. Rack and panel connectors  use integral or rack-mounted pins for alignment and box  mounting hardware for couplings. Environment-resistant connectors are used in applications  where they are probably subjected to fluids, vibration, heat,  mechanical shock, corrosive elements, etc. Firewall class  connectors incorporating these same features should, in  addition, be able to prevent the penetration of the fire through  the aircraft firewall connector opening and continue to  function without failure for a specified period of time when  exposed to fire. Hermetic connectors provide a pressure  seal for maintaining pressurized areas. When EMI/RFI  protection is required, special attention should be given to  the termination of individual and overall shields. Backshell  adapters designed for shield termination, connectors with  conductive finishes, and EMI grounding fingers are available  for this purpose. Connectors must be identified by an original identification  number derived from MIL Specification (MS) or OEM  specification.  Figure 9-159  provides information about MS  style connectors. reliability to the aircraft. For the installation of any particular  connector assembly, the specification of the manufacturer or  the appropriate governing agency must be followed. 9-93 AN/MS Connectors Types of Connector Figure 9-158.  Electrical connectors. 