Cessna 170, Service Manual

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Cessna 170 Service Manual
Electrical System
17
b. Solderless Terminals . The terminal manufacturer will normally provide a special crimping or swaging
tool for joining the solderless terminal to the electric wire. Aluminum wire presents special difficulty in that
each individual strand is insulated by an oxide coating. This oxide coating must be broken down in the
crimping process and some method employed
to prevent its reforming. In all cases, follow the terminal
manufacturer’s instructions when installing solderless terminals.
c. Terminal Strips . Wires are usually joined at terminal strips. Use a terminal strip fitted with barriers to
prevent the terminals on adjacent studs from contacting each other. Studs must be anchored against
rotation. When more than four terminals are to be connected together, use two or more adjacent studs and
mount a small metal bus across the studs. In all cases, the current is to be carried by the terminal contact
surfaces and not by the stud itself. Replace defective studs with studs of the same size and material as
terminal strip studs of the smaller sizes may shear due to over tightening the nut. Assure that the
replacement stud is securely mounted in the terminal strip and that the terminal securing nut is tight. Mount
terminal strips in such a manner that loose metallic objects cannot fall across the terminals or studs. It is
good practice to provide at least one spare stud for future circuit expansion, or in case a stud is broken.
Inspect terminal strips which provide connection of radio and electronic systems to the aircraft electrical
system for loose connections, metallic objects which may have fallen across the terminal strip, dirt and
grease accumulation, etc. Such condition can cause arcing that may result in a fire.
d. Terminal lugs. Wire terminal lugs shall be used to connect wiring to terminal block studs or equipment
terminal studs. No more than four terminal lugs or three terminal lugs and a bus shall be connected to any
one stud (total number of terminal lugs per stud includes a common bus bar joining adjacent studs. Four
terminal lugs plus a common bus bar thus are not permitted on one stud). When the terminal lugs attached
to a stud vary in diameter, the greatest diameter shall be placed o n the bottom and smallest diameter on top.
Terminal lugs shall be selected with a stud hole diameter that matches the diameter of the stud. Tightening
terminal connections shall not deform the terminal lugs of the studs. Terminal lugs shall be so positioned
that bending of the terminal lug is not required to remove the fastening screw or nut, and movement of the
terminal lugs will tend to tighten the connection.
e. Copper terminal lugs . Solderless crimp style copper wire terminal lugs shall be used. Terminal lugs
shall conform to MIL-T-7928. Spacers or washers are not permitted between the
tongues of terminal lugs.
f. Aluminum terminal lugs . Aluminum terminal lugs conforming to MIL-T-7099 (MS-25435, MS-25436,
MS-25437 and MS-25438) shall be crimped to aluminum wire only . The tongue of the aluminum
terminal lugs or the total number of tongues of aluminum terminal lugs when stacked, shall be
sandwiched between two MS-25440 flat washers when terminated on terminal studs. Spacers or
washers are not permitted between the tongues of terminal lugs. Special attention shall be given to
aluminum wire and cable installation to guard against conditions that would result in excessive
voltage drop and high resistance at junctions that may ultimately lead to failure of the junction.
Examples of such conditions are improper installation of terminals and washers, improper torsion "torquing"
of nuts, and inadequate terminal contact areas.
g. Class 2 terminal lugs. Class 2 terminal lugs conforming to MIL-T-7928 may be used for installation by
contractors, provided that in such installations Class 1 terminal lugs are adequate for replacement without
rework of installation or terminal lugs. Class 2 terminal lugs shall be the insulated type unless the conductor
temperature exceeds 105 degrees C in which case un-insulated terminal lugs shall be used. Parts lists shall
indicate the appropriate Class 1 terminal lugs to be used for service replacement of any Class 2 terminal
lugs installed.
ATTACHMENT OF TERMINALS TO STUDS. Electrical equipment malfunction has frequently been
traced to poor terminal connections at terminal boards. Loose, dirty, or corroded contact surfaces
will produce localized heating which may ignite nearby combustible materials or overheat adjacent
wire insulation to the smoking point.
BONDING JUMPER INSTALLATIONS. Make bonding jumpers as short as practicable, and install in such
a manner that the resistance of each connection does not exceed 0.003 ohm. The jumper must not interfere
with the operation of movable aircraft elements, such as surface controls, nor should normal movement of
these elements result in damage to the bonding jumper.