Open Wiring; Heat Precautions - Cessna 170 Service Manual

Cessna 170 Service Manual
Electrical System
OPEN WIRING. Electric wiring is often installed in aircraft without special enclosing means. This practice is
known as open wiring and offers the advantages of ease of maintenance and reduced weight.
a. Wire Bundles. To simplify maintenance and to minimize the damage that may result from a single fault,
limit the number of wires in the run. Shielded wire, ignition wire, and wires which are not protected by a
circuit breaker or fuse are usually routed separately. Avoid bending radii less than 10 times the outer
diameter of the bundle to prevent excessive stresses on the wire insulation.
b. Insulating Tubing. Soft insulating tubing (spaghetti) cannot be considered as mechanical protection
against external abrasion of wire, since at best it provides only a delaying action. Use conduit or ducting
when mechanical protection is needed.
c. Clamping of Wire Bundles. Use clamps lined with nonmetallic material to support the wire
bundle along the run. Tying may be used between clamps, but should not be considered as
a substitute for adequate clamping. Adhesive tapes are subject to age deterioration and, therefore are not
acceptable as a clamping means.
Authors Note: Do not use automotive electrical tapes, wires, or terminals in aircraft due to lack of proper
tinning and use of PVC insulation materials.
d. Separation from Flammable Fluid Lines. An arcing fault between an electric wire and a metallic
flammable fluid line may puncture the line and result in a serious fire. Consequently, make every effort to
avoid this hazard by physical separation of the wire from lines or equipment containing oil, fuel, hydraulic
fluid, or alcohol. When separation is impractical, locate the electric wire above the flammable fluid line and
securely clamp to the structure. In no case, should a wire be supported by a flammable fluid line.
HEAT PRECAUTIONS. Separate wires from high temperature equipment, such as resistors, exhaust
stacks, heating ducts, etc., to prevent insulation breakdown. Insulate wires that must run through hot areas
with a high temperature insulation material such as fire sleeve meeting TSO C53a or C75, fiberglass, or
Teflon. Avoid high temperature areas when using cables having soft plastic insulation such as polyethylene
because these materials are subject to deterioration and deformation at elevated temperatures. Many
coaxial cables have this type of insulation.
PROTECTION AGAINST CHAFING. Protect wire and wire groups against chafing or abrasion as damaged
insulation may result in short circuits, malfunctions, or inoperative equipment. Support wire bundles using
MS-21919 cable clamps. When clamped in position, if there is less than ¼ inch clearance between a
bulkhead cutout and the wire bundle, install a suitable grommet. The grommet may be cut at 45-degree
angle to facilitate installation provided it is cemented in place and the slot is located at the top of the cutout.
STRIPPING INSULATION. Attachment of wire to connectors or terminals requires the removal of insulation
to expose the conductors. This practice is commonly known as stripping. When performing the stripping
operation, remove no more insulation than is necessary. Stripping may be accomplished in many ways;
however, the following basic principles should be practiced: Make sure all cutting tools used for stripping
are sharp. When using special wire stripping tools, adjust the tool to avoid nicking, cutting, or otherwise
damaging the strands.
TERMINALS. Terminals are attached to the ends of electric wires to facilitate connection of the wires to
terminal strips or items of equipment. The tensile strength of the wire to terminal joint should be at least
equivalent to the tensile strength of the wire itself, and its resistance negligible relative to the normal
resistance of the wire. Terminals specifically designed for use with the standard sizes of aircraft wire are
available through normal supply channels. Haphazard choice of commercial terminals may lead to
overheated joints, vibration failures, and corrosion difficulties.
a. Solder Terminals. For most applications, soldered terminals have been replaced by solderless
terminals. The solder process has disadvantages that have been overcome by use of the solderless
terminals. A few of these disadvantages are listed as follows:
(1) A more skilled operator is required.
(2) A corrosive flux may be used causing the joint to deteriorate.
(3) Maintenance is extremely difficult.
(4) The wire strands are stiffened by the solder and become more susceptible to breakage due
to vibration.
(5) The wire insulation may be charred during the soldering process.
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