40 Questions
What is the primary purpose of insulation testing?
To detect electrical resistance of an insulator
What is the minimum insulation resistance value for a single conductor?
5 MΩ
When is an insulation resistance test typically carried out?
After visual, bonding, and continuity tests
What type of insulation tester is commonly used on aircraft?
Megger
What happens when the test resistance is a short circuit in a Megger?
The majority of the current flows in the deflection coil
What is the purpose of leaving the leads apart when testing a Megger?
To test the Megger before use
What happens when the test resistance is an open circuit in a Megger?
The majority of the current flows in the control coil
What is the result of insulation failure in an electrical circuit?
Different outcomes, including a short circuit condition
Where is the measurement of a bare conductor taken?
In the slot
What is the purpose of a wire gauge?
To determine the size of a wire
What is the primary concern for electrical wires used in aircraft?
Safety
What type of specifications do wire manufacturers provide?
Manufacturer's specifications
What is the purpose of Military Specifications for wiring in aircraft?
To cover the installation of wiring in aircraft
What is specified in Military Specifications for approved aircraft wire?
The type of conductor, the conductor plating (if any), and the type of insulation
What is the name of the industry standards body that provides specifications for electrical wires used in aircraft?
All of the above
What is the typical specification number for an approved aircraft wire?
All of the above
What is the purpose of specifying operating temperature ranges for aircraft wires?
To reduce the risk of fires caused by improper wire selection
What should be done when replacing copper wire with aluminum wire?
Use a wire two sizes larger
What is the specification number for aluminum aircraft wire?
MIL-W-7072
What is the minimum size of aluminum wire allowed in aircraft?
No. 6
What is the voltage rating of MIL-W-22759/1 wire?
600 V
Why is it important to inspect aircraft wires and cables?
To ensure adequate support and protection
What should be done before starting an inspection of the aircraft electrical system?
Turn off all electrical power
What happens when the current flowing through a wire exceeds the wire's current-carrying capacity?
The wire generates excess heat
Why should wires and cables be positioned carefully?
To prevent their use as handholds or support for personal belongings
How should wires and cables be routed to minimize damage?
In a way that minimizes exposure to damage from moving personnel
Why should wires and cables be protected in wheel wells?
To protect them from impact by rocks, ice, mud, etc.
What should be done with unused wires?
Individual dead-end them, tie into a bundle, and secure to a structure
How often should wires and cables be identified?
At intervals of 15 in.
Why should tape, tie straps, or cord not be used for primary support?
Because they can dry out in service or produce chemical reactions
What is recommended for coaxial cables?
Identify them at both equipment ends
What is the purpose of continuous lacing in panels and junction boxes?
Optional, but only in panels and junction boxes
What is the minimum bend radius for thermocouple wire?
20 times the diameter
What is the minimum bend radius for RF cables such as coaxial and triaxial?
6 times the outside diameter
Why is it important to ensure sufficient slack in wiring?
To prevent mechanical strain on wires
What is the maximum deflection allowed between support points for wire groups or bundles?
1.2 cm (0.5 inch)
What is the purpose of providing slack at each end of a wire group or bundle?
To permit replacement of terminals and prevent mechanical strain on wires
Why is it important to ensure that wire groups or bundles do not touch surfaces that may cause abrasion?
To prevent damage to the wires
What is the purpose of a loop in the wiring in some cases?
To provide adequate slack for equipment removal and replacement
Why is it important to ensure that wiring at terminal lugs or connectors has sufficient slack?
To allow two re-terminations without replacing wires
Study Notes
Insulation Testing
- Insulation testing measures the electrical resistance of an insulator separating conductors.
- A fault caused by insulation failure may trip the circuit breaker, but may also result in different outcomes.
- Minimum values of insulation resistance for electrical wiring are: • 5 MΩ for a single conductor • 2 MΩ for bundled wiring
Insulation Resistance Tests
- Insulation resistance tests are carried out after visual, bonding, and continuity tests.
- Tests are necessary when: • New circuits have been installed • Existing circuits have been disturbed • Evidence exists that insulation requires checking • As specified in the aircraft maintenance schedule
Megger Testers
- Megger is a type of insulation tester commonly used on aircraft.
- There are 250-V, 500-V, and 1000-V Meggers.
- The basic circuit of a Megger consists of a test resistance, deflection coil, and control coil.
Wire Specifications
- To ensure safety, all electrical wires used in aircraft must meet one or more of the applicable specifications.
- Specifications include: • Major Original Aircraft Manufacturers (OAMs) standards • Military Specifications (Mil-Spec) or Defence Standard (Def-Stan) • Wire manufacturers’ specifications • Industry standards bodies (e.g. SAE, NEMA, UL, ASTM, AECMA) • Governing bodies’ standards
Wire Military Specifications
- Military Specifications cover the installation of wiring in aircraft.
- Typical specification numbers for approved aircraft wire include: • MIL-W-5086, MIL-C-7078, MIL-W-22759, MIL-W-81381, MIL-C-27500, and MIL-W-81044
- Specifications cover conductor type, plating, insulation, operating temperature ranges, voltage levels, and general application guidelines.
Aircraft Wire Characteristics
- Characteristics of typical aircraft wires include: • MIL-W-22759/1: silver-coated copper conductor, TFE fluorocarbon and glass insulator, 600 V, 260 °C • MIL-W-22759/41: nickel-coated copper conductor, ETFE dual insulator, 600 V, 105 °C • MIL-W-22759/16: tin-coated copper conductor, ETFE insulator, 600 V, 150 °C • MIL-W-22759/19: silver-plated high-strength copper alloy conductor, ETFE insulation, 600 V, 150 °C • MIL-W-81044/9: tin-coated copper conductor, polyalkene-reinforced PVF insulation, 600 V, 150 °C
Wiring Inspection Requirements
- Wires and cables should be inspected for: • Adequacy of support • Protection • General condition
- Ensure that wires and cables are: • Not used as handholds or support for personal belongings and equipment • Not exposed to damage by personnel moving within the aircraft • Not susceptible to damage by the storage or shifting of cargo • Not exposed to damage from battery electrolytes or other corrosive fluids • Adequately protected in wheel wells and other areas where they may be exposed to damage
Other Conditions
- Ensure that: • Unused wires are individually dead-ended, tied into a bundle, and secured to a permanent structure • Each wire has strands cut even with the insulation and a pre-insulated closed end connector or a 1-in. piece of insulating tubing placed over the wire • Wires and cables are identified properly at intervals of not more than 15 in. • Coaxial cables are identified at both equipment ends • Primary support is not provided by tape, tie straps, or cord • Moisture-absorbent material is not used as ‘fill’ for clamps or adapters
This quiz covers the method of measuring electrical resistance of an insulator, fault causes, and minimum values of insulation resistance for electrical wiring.
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