Electrical Insulation Testing

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Questions and Answers

What is the primary purpose of insulation testing?

To test electrical currents

To identify electrical wiring faults

To measure electrical conductivity

To detect electrical resistance of an insulator (correct)

What is the minimum insulation resistance value for a single conductor?

10 MΩ

5 MΩ (correct)

1 MΩ

2 MΩ

When is an insulation resistance test typically carried out?

Before visual and bonding tests

During aircraft maintenance schedules

After visual, bonding, and continuity tests (correct)

Only when new circuits are installed

What type of insulation tester is commonly used on aircraft?

Megger Signup and view all the answers

What happens when the test resistance is a short circuit in a Megger?

The majority of the current flows in the deflection coil Signup and view all the answers

What is the purpose of leaving the leads apart when testing a Megger?

To test the Megger before use Signup and view all the answers

What happens when the test resistance is an open circuit in a Megger?

The majority of the current flows in the control coil Signup and view all the answers

What is the result of insulation failure in an electrical circuit?

Different outcomes, including a short circuit condition Signup and view all the answers

Where is the measurement of a bare conductor taken?

In the slot Signup and view all the answers

What is the purpose of a wire gauge?

To determine the size of a wire Signup and view all the answers

What is the primary concern for electrical wires used in aircraft?

Safety Signup and view all the answers

What type of specifications do wire manufacturers provide?

Manufacturer's specifications Signup and view all the answers

What is the purpose of Military Specifications for wiring in aircraft?

To cover the installation of wiring in aircraft Signup and view all the answers

What is specified in Military Specifications for approved aircraft wire?

The type of conductor, the conductor plating (if any), and the type of insulation Signup and view all the answers

What is the name of the industry standards body that provides specifications for electrical wires used in aircraft?

All of the above Signup and view all the answers

What is the typical specification number for an approved aircraft wire?

All of the above Signup and view all the answers

What is the purpose of specifying operating temperature ranges for aircraft wires?

To reduce the risk of fires caused by improper wire selection Signup and view all the answers

What should be done when replacing copper wire with aluminum wire?

Use a wire two sizes larger Signup and view all the answers

What is the specification number for aluminum aircraft wire?

MIL-W-7072 Signup and view all the answers

What is the minimum size of aluminum wire allowed in aircraft?

No. 6 Signup and view all the answers

What is the voltage rating of MIL-W-22759/1 wire?

600 V Signup and view all the answers

Why is it important to inspect aircraft wires and cables?

To ensure adequate support and protection Signup and view all the answers

What should be done before starting an inspection of the aircraft electrical system?

Turn off all electrical power Signup and view all the answers

What happens when the current flowing through a wire exceeds the wire's current-carrying capacity?

The wire generates excess heat Signup and view all the answers

Why should wires and cables be positioned carefully?

To prevent their use as handholds or support for personal belongings Signup and view all the answers

How should wires and cables be routed to minimize damage?

In a way that minimizes exposure to damage from moving personnel Signup and view all the answers

Why should wires and cables be protected in wheel wells?

To protect them from impact by rocks, ice, mud, etc. Signup and view all the answers

What should be done with unused wires?

Individual dead-end them, tie into a bundle, and secure to a structure Signup and view all the answers

How often should wires and cables be identified?

At intervals of 15 in. Signup and view all the answers

Why should tape, tie straps, or cord not be used for primary support?

Because they can dry out in service or produce chemical reactions Signup and view all the answers

What is recommended for coaxial cables?

Identify them at both equipment ends Signup and view all the answers

What is the purpose of continuous lacing in panels and junction boxes?

Optional, but only in panels and junction boxes Signup and view all the answers

What is the minimum bend radius for thermocouple wire?

20 times the diameter Signup and view all the answers

What is the minimum bend radius for RF cables such as coaxial and triaxial?

6 times the outside diameter Signup and view all the answers

Why is it important to ensure sufficient slack in wiring?

To prevent mechanical strain on wires Signup and view all the answers

What is the maximum deflection allowed between support points for wire groups or bundles?

1.2 cm (0.5 inch) Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

What is the purpose of a loop in the wiring in some cases?

To provide adequate slack for equipment removal and replacement Signup and view all the answers

Why is it important to ensure that wiring at terminal lugs or connectors has sufficient slack?

To allow two re-terminations without replacing wires Signup and view all the answers

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

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Description

This quiz covers the method of measuring electrical resistance of an insulator, fault causes, and minimum values of insulation resistance for electrical wiring.