Electrical Insulation Testing

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.