Bonding, Grounding, And Continuity Flashcards
Why would bonding be done?
To minimise radio and radar interference To minimise lightning damage Minimise fire hazard from sparks To provide a low resistance return path in single wire systems To aid in shielding effectiveness
What else can the return path from bonding be used for?
Aid shielding
Provide means to ground the aircraft
What can be done to keep resistance low on bonding wires?
Remove anodising films and non-conductive paint at terminal points
How low could the bonding resistance get and where would required values be dictated?
0.025 ohms
Values can be found in the AMM
What is grounding?
Connecting the single wire systems and shields to the aircrafts structure at certain points
What are the three conditions for grounding?
Grounds must be separate (AC, DC, shields)
4 terminals max on each stud
Dual grounds only at fuel vapour areas
Why does the wire have to be long enough for grounding?
To allow for fault current, without generating heat
How is a ground terminal measured, and what does the resistance have to be lower than?
A bonding meter is used to measure the resistance which must be less than 0.0025 ohms
When are grounding measurements usually taken?
After:
Modification of ground stud or wires
Removal of ground stud nut
How is the insulation resistance measured?
Wire to wire
Wire to ground
What is the typical voltages for the insulation resistance check?
240 or 500 volts
Can an insulation resistance check be carried out when disconnected from the system?
Yes, this is the only time it should be done
When is the ideal time to carry out an insulation resistance check?
Before installation
What is the resistance values for wires for accessories?
2 miliohms (cowlings, landing gear, etc.) 5 miliohms (galleys, lighting, service systems) 10 miliohms (other service systems)
What is the resistance values for wires without contacts/connectors?
100 miliohms (between terminals) 200 miliohms (between terminal and ground)
What is the resistance value for systems such as instruments and computers?
5 miliohms
What is continuity testing?
The use of a ohmmeter or multimeter to determine if a circuit has a complete current path.
Done when power is off
What is bonding?
The electrical interconnection of metallic parts for safe distribution of charge and current
Is an aircraft earthed when on the ground? If so, then how?
It’s is through the landing gear and the electrically conductive compound impregnated into the tyres
What is bonding used to protect against?
Charges from build up of static, electrostatic induction, etc.
Electric shocks
Can also reduce interference on essential services
What could be used to bond moving parts?
Braided copper or aluminium cords with connecting tags or lug on the ends.
What is bonding?
The electrical interconnection of metallic parts for safe distribution of charge and current
Is an aircraft earthed when on the ground? If so, then how?
It’s is through the landing gear and the electrically conductive compound impregnated into the tyres
What is bonding used to protect against?
Charges from build up of static, electrostatic induction, etc.
Electric shocks
Can also reduce interference on essential services
What could be used to bond moving/vibrating parts?
Braided copper or aluminium cords with connecting tags or lug on the ends.
What could happen if bonding jumpers aren’t attached properly and why is it a problem?
The contact between parts could become intermittent due to the vibration.
Intermittent contact is worse than none at all (need context)
What should be made sure when installing bonding cables/wire?
Does not interfere with nearby/associate equipment
Not too tight or slack
Insulating materials removed before installation
What joins are self bonding?
Metal to metal held by threaded devices or rivets
Most cowling fasteners, locking and latching mechanisms
Metal to metal door and hinges
Metal to metal bearings
What are the two classes of bonding?
Primary and secondary
When would the manufacturer dictate the maximum resistance of a primary bond?
Between extremities of fixed non-metal/composite construction
At what times would the max resistance of a bond be 0.05 ohms as dictated by the CAA
Between fixed extremities of metallic construction
Between bonded components and portions of main earth system they are connected to
What bonding scenario would have a max resistance 0.5 megaohms or 100,000 ohms per square foot of surface area?
Between isolated conducting parts which may be subject to electrostatic charging and the main earth system
What would the max resistance be for metallic parts in contact with flammable fluids?
1 ohms
What is the maximum resistance for any other secondary bonds, and what are they?
Equipment supplied from the unearthed system- 1 ohm
Between equipment with circuits of 50 volts or more - 1 ohm
What are the lengths and number of spikes on the cables used for bond testing?
60 feet, single spike
6 feet, double spike
What does a commonly used bond test equipment consist of?
Ohmmeter operating on current ratio principle, single 1.2 volt nickel-alkaline cell in wooden carry case
What should be checked before doing any bonding tests?
Full scale deflection of meter when twin spikes are shorted
Meter reads zero when both cable spike are used to short each other
How is the first bond test method carried out?
60 foot cable connected to earth
6 foot cable used to check resistance between selected points
Bond resistance will be shown in ohms on meter
What should be done for the first bond testing method on large aircraft with multiple main earth points?
Check the resistance values between main earth points before any testing begins
How would the four terminal bond test method be carried out?
Use appropriate milliohmmeter
Test current supplied by internal batteries through resistance via cables c1 and c2
The voltage drop along the resistance is measured, compared with the current flow
What is the other form for the four terminal methods test equipment?
Duplex spikes
What should be done for excessive resistance across bonding jumpers?
Remove connecting lugs or tags
Clean the contacts with slight abrasive, removal all abrasive traces afterwards
Seal connecting area and treat with anti-oxidant as specified in manuals
Do the same as above for holding bolts
What should be done in cases of excessive resistance when required bond value cannot be obtained at the aircrafts structure?
Seek manufacturers advice
What is continuity testing used to find?
Concealed breaks in cable cores
What does a typical continuity tester consist of?
Low-voltage battery
Test lamp
Low reading volt meter
What preparations should be carried out before testing continuity?
Disconnect or switch off power
Check fuses are intact
Check appropriate cbs are closed to complete the circuit
What methodology should be used when continuity testing a circuit?
Work progressively through the circuit, from fuse or CB to the test equipment
What should be done when testing large circuits with multiple paths?
Work down each path systematically, breaking down at plugs as little as possible
When is the millivolt drop test recommended?
Excessive resistance in high current carrying circuits- faults such as loose terminals and poorly swaged lead ends
What current is best to use for continuously rated circuits?
Normal operating current with power derived form external source
What current should be used for short rated circuits?
The current should be scaled down with a dummy load or suitable resistance instead of normal load.
How is the millivolt drop test carried out?
The meter is connected to either side of the suspected joint and a note of the volt drop is indicated
