Electrical Bonding is a method used to equalize electrical charges throughout all components on an aircraft or system by creating conductive paths for any charges to travel through with a minimal resistance. Maybe when you were younger, you rubbed a balloon in your hair and watched your hair stand on end towards the balloon or rubbed your slippers on the carpet and touch a radiator to see that static spark ( not recommended of course) which is similar to when an aircraft or vehicle passes through air or storm clouds. This is only one issue regarding static but there are many reasons why we bond aircraft and there components, these include:-
- To prevent high electrical voltage potential differences.
- To route this high electrical voltage in a way that would not malfunction the aircraft or its components.
- To minimise damage or injury to the aircraft and its occupants.
- To minimise radio operation problems
- To prevent static charge build up
- To earth the aircraft to the ground via the special conductive nose or tailwheel or boom extended to prevent harm to occupants and personnel when leaving or entering the aircraft.
These reasons determine what type of protection is used and are then placed into two categories which are titled Primary Conductors and secondary conductors.
Primary conductors are the conductors that carry the high voltages i.e Lightning strikes
Secondary conductors are the conductors that carry the lower voltages like static build up.
Because there are diferent types of problems that bonding deals with, the bonding leads and bonding path must be suitable for all the tasks that they must perform.
When bonding between components, panels and structures, it is necessary to remember the following points:-
- Braided conductors made from aluminium or copper wires, may be used for either primary or secondary conductors, but always remember that if the conductor needs to be replaced, you must always refer to the IPC but for the exams sake, you should select a conductor of the same length and material as the original but with a greater cross sectional area.
- Terminals should be clean and and the bonding lead secured, it is required that the terminal and bonding lead connector be treated to prevent corrosion, this is normally achieved by a form of a sealant.
- The routing of bonding leads should be as straight as can be with all sharp bends avoided and routed in a manner that does not interfere with the operation of any equipment.
- The end terminal lugs on the cables. leads or wires should be either soldered or crimped, these processes will be discussed later.
- Always refer to the wiring manual to ensure the correct routing and quantity and specifications of the bonding lead.
- There is one more point in regards to the installation of bonding leads that you must be aware of, and that is in regard to magnesium based alloy structure or parts. Due to the corrosive nature of magnesium, the component manufacturer may make a special lead for the bonding of the component, but in the absence of this, you may use a aluminium alloy bonding lead connected indirectly to the magnesium structure i.e. via a mounting bolt or bolted connection that does not move.
Testing
Bonding leads and jumpers are tested using a 250 or 500 volt ohm meter. Before use the tester should be inspected for the following:-
- – The ohmmeter unit has not been damaged or tampered with
- – The leads have not been damaged or repaired ( no repairs or modifications allowed.). If the leads have been damaged then the meter must be fitted with a new set of leads and recalibrated with those leads.
- – The probes are not damaged or corroded and have a good point on the end to break through the coatings on the metal.
- – After inpecting the condition of the meter, the unit should be switched on and the probes shorted out to give a full deflection of the scale so the needle registers 0 ohms. There is usually a fine tune knob located on the unit to set up the scale before use.
When testing bonding leads or jumpers it is required that all bonding lead connections not be any greater in resistance than 0.05 ohms.In many installs, the manufacturer may require a lower resistance figure and therefore the lower figure should always take precedent.