ELECTRICAL CABLES AND

CONNECTORS

1.WIRES AND CABLES A wire is - a single solid or filament of drawn metal- enclosed in a suitable insulating material

and protecting cover A cable is- usually made up of a conductor composed

of a group of single solid wires stranded together

- provides greater flexibility- enclosed in a suitable insulating material

and protecting cover

2.TYPES OF WIRES AND CABLES The names adopted for various types are

derived from insulating material used- NYVIN (Nylon and polyVINylchloride)- TERSIL (polyesTER and SILicone)- TRINYVIN (cable made up of 3 single NYVIN cables)

- METSHEATH (cable enclosed in a metal braided sheath)

Two metals are used for conductors ; Copper & Al Both Cu & Al has same resistance but Al has 2/3

of the weight and 2 times cross sectional area of the copper conductor.

3.PROPERTY OF AN INSULATOR Toughness Flexibility Temperature range Resistance to fuel/hydraulic and

lubricant Ease of stripping Non-flammability Minimum weight

4.CIRCUIT TESTING The correct sequence of circuit checks

- Visual inspection- Bonding check- Continuity check- Insulation check- Functional check

4.CIRCUIT TESTING

a) Visual inspection

This may save damage, injury, time and money- Circuit wiring- Plugs and sockets- Terminal blocks- Damage- Deterioration- Poor routing(workmanship)- Security of attachment & connections- Corrosion check under clips & nuts

4.CIRCUIT TESTINGb)Bonding check

- The parts or airframe where bonding provided, bonding check need to be carried out for wear and tear or loosening.

- Bonding resistance should be .05 ohm (primary bonding ; major components, engines, external surfaces) and 1 ohm (secondary bonding ; metal conduits, Junction Box, pipeline containing flammable fluids)

4.CIRCUIT TESTINGc) Continuity check

The preconditions for continuity check are

- All manually operated switches must be placed ON position

- The circuit at the time of testing must be dead except for current originating in the test equipment

- The portion of circuit under test must constitute a simple series circuit

4.CIRCUIT TESTINGc) Continuity check(cont.)

Test apparatus requires- A low voltage supply- A lamp- A multimeter- A testmeter (if necessary for special circuit)

ALWAYS RECTIFY FAULTY PORTION BEFORE PROCEEDING FURTHER TEST SEQUENCE

4.CIRCUIT TESTINGc) Continuity check(cont.)

Methodical planning is the first step towards effective continuity testing.

- Through study of wiring diagram(1st stage)- Planned sequence of individual test that are

needed to check the whole of wiring- The use of low voltage supply

4.CIRCUIT TESTINGd)Insulation check

Two fundamental properties of insulation materials are

- Insulation resistance (measured with a meggar)

- Dielectric strength (measured raising voltage of sample until insulation breaks down)

4.CIRCUIT TESTINGd)Insulation check(cont.)

The steps toward effective insulation testing are

- Isolate the circuit required to be tested- All switches (fuse/breaker) ON of the circuit

under test- Disconnect all circuit negatives from the

earthing point- Link up all isolated sections to give

electrical continuity throughout the circuit or groups of circuit under test.

4.CIRCUIT TESTINGe) Functioning test:- Functioning test should be carried out using an

external supply coupled to ground supply connector

- Test must ensure proper functioning of individual and integrated sections of circuits

- Test should be in accordance with schedules established by reference of MM,WDM or instruction relating to incorporation of a modification or any substantial rewiring.

- After test carried out, switch OFF the circuit, fuse should be removed or breaker tripped and the circuit again switched on to check the isolation.

5.ELECTRICAL BONDINGDuring flight build up of electrical energy occurs in two ways

- Precipitation of static charge(by surface friction)- Charges due to electrostatic induction ( by

cloud)

To eliminate the electrical energy bonding system is introduced which

- Limit potential difference between all parts- Eliminate spark discharges and fire risks- Reduce interference with radio and navigational

aid signals

5.ELECTRICAL BONDING(Cont.)

To eliminate Charges due to electrostatic induction ( created by cloud) and lightning ; a leakage path is provided by the following means

- Making Nose wheel or tail wheel tire rubber electrical conductive

- A leakage path via short flexible steel wires secured to nose wheel or main wheel axel members making physical contact with the ground.

6.EARTHING or GROUNDING In the literal sense earthing or grounding

refers to the return of current to the conducting mass of the earth or ground.

In aircraft its fuselage is considered as earthing provider as in most aircraft the structure is of metal.

Earthing in aircraft is constitute of- Earth return cables & earthing bolts- Fuselage earthing strips (aircraft having

nonmetallic primary structure)- Lightning strike plates

7.SCREENING Screening provides a low resistance path for

voltages producing unwanted radio frequency interference.

The voltage conducted by screening system are those stray ones due o the coupling of external fields originating certain equipment & circuits.

The methods adopted for screening are- suppressor(provides a low resistance path

across interference source)- Metal screening(cables with metal braided

sheath)

8.STATIC DISCHARGE WICKS Corona discharge:- Occurs at wing tips, trailing edge, horizontal &

vertical stabilizer, radio antennae ,pitot tubes.- Creates poor visibility (glow)- RF signal interference Remedy:- Static discharger can discharge static charge

which consists of nichrome wires formed in the manner of brush.

- Sharp tungsten needles extend at right angles to the discharger tips to keep corona voltage low.

9.REFUELLING PRECAUTION:

During refueling risk of fire and explosion persists. Charge may be carried by- Aircraft- The fuel flowing the hose- The tanker The equalizing of potential is achieved by

providing bonding connection between the aircraft and the tanker which themselves are bonded to the ground. The hose is bonded to aircraft special point.

10.CRIMPING TOOLS

10.CRIMPING TOOLS (contd.)

Crimp tools mist be carefully inspected :

- Ensure that full cycle ratchet mechanism is tamper-proof

- If the tools does not function or faults are found, reject the tool

- Suitable gages of the Go/No Go type are available and shall be used prior to any crimping operation to ensure crimp dimension

10.CRIMPING TOOLS (contd.)

11.TESTING OF CRIMPING JOINTS: The terminal ends, pin and socket contacts, cable

splices and tools shall be inspected for compliance with approved drawing.

All crimp joints shall be visually inspected for:- Correct combination of cable , tool and terminal

end- Correctness of form and location of crimp- Freedom from fracture, rough or sharp edges- Adequate insertion of all conductor strands in the

barrel- Absence of damage to the conductor or insulation

12. PIN SOCKET SEQUENCING:

13.CONEECTOR PIN REMOVAL & INSTALLATION

Fig: Installing a contact

Fig : Removing a contact

13.CONEECTOR PIN REMOVAL & INSTALLATION (contd.) Installing a contact:

- Select correct insertion tool- Fit tool to contact/wire- Push fully into connector, DO NOT

ROTATE- Fully seat the contact until

snapping/clicking is beard/felt- Remove the tool in a straight line DO

NOT ROTATE

13.CONEECTOR PIN REMOVAL & INSTALLATION (contd.)

Removing a contact:

- Identify the connector as front or rear release- Remove any back-shell or strain relief clamp- Remove or loosen any compression ring- Insert the tool over the contact in a straight

line- Push plunger forward, the contact should

emerge from the rear of the connector

14.WIRING PROTECTION TECHNIQUES The cables are required to be protected from

abrasion, mechanical strain and excessive heat and against harmful effects of fuel, oil and other aircraft fluids.

Conduits, ducts and trunking used for carrying cables should have smooth internal surfaces

The cabling must be adequately supported throughout its length by clamps to reduce vibration

Bends in cable groups or bundles should not be less than eight(8) times the outside diameter of the group cable or bundle.

14.WIRING PROTECTION TECHNIQUES(contd.)

Cables should be so fitted and clamped that no tension will be applied in any circumstance of flight

Cables should be supported independently of and with maximum practicable separation from all fluid.

Routing of wires and cables- Open loom- Ducted loom- conduit

14.WIRING PROTECTION TECHNIQUES(contd.) The composition of a cable loom is dictated by

such factor as- Overall diameter- Temperature condition, i.e. temperature rise in

cables when operating at their maximum current carrying capacity

- Type of current, i.e. whether alternating, direct, heavy duty or light duty.

- Interference resulting from inductive or magnetic effects

- Type of circuit with which cables are associated; cables prone to short circuit with adjoining cables must be safeguarded.