Connecting Electrical Wire/Cables and Connectors

Connection of wires is performed, in the majority, by two methods either by the use of plugs which insert in to a mating plug with a special external barrel with a screw thread which is turned to lock the connecting plugs together or by a form of terminal block. When using terminal blocks it is important to know the proper installation practice to prevent corrosion and damage to the block and terminals.

If you are connecting copper wire terminals to the block then you should

  • Connect the wire terminal directly onto the nut securing the terminal block stud.
  • Place a plain washer over the terminal.
  • Followed by a self locking nut or, if there is not a self locking nut available, you may use a spring steel locking washer followed by a standard nut.

If the terminal you are connecting is for an aluminium wire then you should

  • Place a plain brass washer over the terminal block stud securing nut
  • Then place the wire terminal onto the stud.
  • Followed by another plain brass washer
  • Then either a self locking nut or spring steel lock washer followed by a plain nut.

If you must connect an aluminium wire terminal and copper wire terminal to the same stud then the following order must be achieved:-

  • Place the plain brass washer onto the terminal stud followed by the aluminium wire terminal.
  • Place another brass washer over the aluminium wire terminal followed by the copper wire terminal
  • Then place a plain washer followed either by a self locking nut or a spring steel lockwasher and plain nut.

Please note that unless the wire terminals are of different materials i.e. copper wire terminals and aluminium wire terminals, then no washer should ever be placed between them.

Electrical Testing – Continuity, Insulation – Aircraft

Continuity testing

What is the purpose of continuity testing?

The purpose of this task is to test for a break in the circuit.

There are two methods of performing this task. Either by a continuity tester ( a 3 volt battery and test bulb or L.E.D.) or by performing the milli-volt drop test using a volt meter. The latter is the preferred method but you must know both and always use the method as defined in the maintenance manual.

Before testing any circuit, The following should be performed:

  • All power removed
  • All circuit breakers, fuses and switches are in there necessary positions with respect to the maintenance manual. Note, Circuit breakers must be closed to test that circuit but some circuit breakers may need to be tripped to isolate that circuit to prevent damage to other sensitive circuits that may be connected Directly or in-directly,

SO ALWAYS FOLLOW THE MAINTENANCE MANUAL.

If performing the milli-volt drop test, you may now apply power as required to put that circuit into its usual working condition but

  • Always work progressively and systematically
  • Always try to start nearest the supply end of the circuit if the circuit must be tested in sections.

Continuity tester method

To perform a continuity test using this method you must simply connect the positive side of the battery to one end of the circuit, connect the bulb or L.E.D. to the negative side of the battery and then connect the end of the circuit, that is not connected to anything, to the bulb. If there is an unbroken circuit, the bulb will light up and vise versa. This must always be performed on a circuit isolated from other power sources.

Millivolt drop test

This test is more preferred because not only will it tell you if you have a circuit, but it will also signify the condition of the circuit. This test is accomplished by referring to the maintenance manuals for the allowable voltage drop and test connection points. Once you have consulted with the maintenance manual and put the aircraft circuit breakers, fuses and switches in the desired configuration, Then connect your voltmeter to the test connection points and take your volt reading. Now compare your results with the allowable limits in the aircraft manual.

Please remember the following:-

  • The milli-volt test requires the circuit to be powered up to its normal operational load so there is a hazard of Electric Shock so you must be aware of electrical safety and how to protect and insulate yourself.
  • If the circuit fails the test, check the circuit for corrosion, condition and that the terminals are clean and clamped correctly.

Insulation testing

Insulation testing is performed to check that the insulation of the wires within a circuit has not degraded or failed. This task is performed by following the directions in the maintenance manual of which normally follow the steps laid out below:-

  • Disconnect all power sources from the aircraft which includes the aircraft’s own batteries as well as external power.
  • Main power switches should be in the normal stand alone position. i.e. External power off, battery and generators switched on.
  • Circuit breakers should be in the position as required by the maintenance manual. Remember that although you must have the circuit to be tested with no circuit breakers pulled.
  • Some circuits which are voltage sensitive might not appreciate 300 volts being put up them and must be isolated as per the maintenance manual. This may be by means of physically unplugging them or just by pulling their circuit breakers.
  • All ancillary equipment that might be affected by the circuit(s), must be isolated or disconnected prior to the test.

The tests now differ for the two types of systems in use, The Double pole and the single pole, The latter being the system commonly in use today, but because there are still old aircraft flying and you must know everything, as well as have a maintenance manual, both methods are shown below:

Single pole system

Place one lead connector on to the earth ( normaly the casing or aircraft structure and then the other lead connector to the wires or leads of the circuit to be tested. If the resistance is less than allowed in reference to the aircraft manual, then the exact wire which has failed must be traced and the fault located and rectified.

Please note that although this method allows a bunch of wires to be tested together to save time, It is preferred that each wire be tested individually.

Double pole system

To test the insulation on a double pole system, you should follow the following process and order. First measure the resistance between the positive and negative battery leads ( leads must be disconnected from the battery),then the resistance between earth and the positive battery lead, and then do the same for the negative battery lead.

Post Testing

After the insulation tests have been performed, then it is necessary that the wiring is connected back up to its ancillary equipment and that the circuit breakers are reset to the required positions. Once the system has been restored, then a function test must be performed in accordance with the maintenance manual before release to service.

CHAINS AND THEIR ACCESSORIES – Aircraft

This article was written according to aircraft standards but the principles of Chains remain the same but if applying the knowledge to low power chains on Bicycles, Then you can make your own risk economic based reduced standards. If working on aircraft, always refer to the Maintenance manual.

The purpose of chains is to transfer power from one sprocket to another transfer kinetic(motion) energy. To accomplish this task you need at least an assembly of chain(s)(manufactured to SBAC standards like BS228:1994 or ISO606-1982), sprockets(toothed wheels) and possibly other accessories like interplaner blocks for changing the direction of the chains.

We will start with the main unit called the chain. This is an assembly of:-

  • outer plates
  • inner plates
  • Rollers
  • Bearing pins
  • Bushes

The pitch of the chain is taken from centre of one roller to the next one i.e. the distance between the bearing pins.

Maintenance

Chains riveted links are not allowed to be broken down and re-riveted and only accepted from an approved manufacturer, with the correct packaging and transportation precautions taken.

It is permitted that a bolted joint may be disassembled and re-assembled, but, it is worth while noting these next few points.

1) SBAC states that all nut and bolted chain assemblies must be peened with the exception of the 8mm variety which must be split pinned

2) All nuts used on the chains must be locknuts(this nut is normally part of the outer plate)

3) All attachments must be either riveted or bolted

4) SBAC have standardised four sizes of chains by pitch size

BUT

British Standards have laid down the proof load as one third of the minimum breaking load (mbl)

5) Continuous(endless) chains must consist of an even amount of gaps between rollers(pitches)

6) The following table is the four classifications of chains and their statistics.

PITCH — MBL(lbs) — PROOF LOAD(lbs) — BS
8mm ——— 800 ————- 267 ——————— 1
0.375″ ——- 1900 ———— 634 ——————— 2
0.5″ ———– 1800 ———— 600 ——————— 4
0.5″ ———– 3500 ———— 1166 ——————- 6

7) A non-reversible chain is a chain that may only fit a certain way round which can be achieved by correct unsymmetrical positioning by the outer plates fitted to the chain.

8) Irreversibility can be achieved by

  • Non-reversible chains
  • Non-interchangeable end fittings
  • Correct positioning of sprockets
  • Guards and shrouds

9) When storing a chain it must be well soaked in the approved oil, laid flat on its side on top of greaseproof paper and coiled firmly (not tight but not too loose).

GENERAL INSPECTION

This unit is used as a high strength power transfer device. When inspecting the chain make sure to check the proceedure in the maintenance manual which should at minimum account for the Following

  • Wear on the sprocket ( See spec in aircraft manual)
  • Wear on the rollers ( See spec. in aircraft manual)
  • Twisting (This is cause for rejection)
  • Stiffness ( This can be determined by drawing the chain around the fingers on your hand or maybe a substitute of the same sort of shape and texture [ You do not want to damage the chain by scraping the links on an object used in inspection] and checking for smooth articulation of the links. If stiffness is detected, the chain may first be cleaned and re-checked but if this is not a solution, the rollers may be lightly TAPPED preferably with a small pin hammer. if this does not fix it the chain is the scrapped.
  • Deformities, Cracks or corrosion ( These is cause for rejection)
  • Overheating (Bluing)
  • Elongation (Maximum of 2%)( To check for this the chain must be cleaned and dried with compressed air. Lay the chain flat and straight and apply a tensile load/ force [see table below]. Now measure the distance between all the pin centres and apply the following formula:

Percentage of Elongation (Method 1)

Length Of Chain with Force applied x (No. Of Pitches x Pitch measurement)
———————————————————————
No. Of Pitches x Pitch Measurement

Note: All measurements are in inches

Tensile load required on Chain (Size is British Standard)
Size – Load (lbs)
1 —- 12
2 —- 16
4 —- 28
6 —- 28

Percentage of Elongation (Method 2)

Measured Length
——————————— x100
Original Length