Plating be installed on older aircraft. Insulation materials for new  aircraft designs are made of Tefzel$^{®}$, Teflon$^{®}$/Kapton$^{®}$/ Teflon$^{®}$ and PTFE/Polyimide/PTFE. The development of  better and safer insulation materials is ongoing. Bare copper develops a surface oxide coating at a rate  dependent on temperature. This oxide film is a poor conductor  of electricity and inhibits determination of wire. Therefore,  all aircraft wiring has a coating of tin, silver, or nickel that  has far slower oxidation rates. Since electrical wire may be installed in areas where  inspection is infrequent over extended periods of time, it  is necessary to give special consideration to heat-aging  characteristics in the selection of wire. Resistance to heat is  of primary importance in the selection of wire for aircraft  use, as it is the basic factor in wire rating. Where wire may  be required to operate at higher temperatures due either  to high ambient temperatures, high current loading, or a  combination of the two, selection should be made on the  basis of satisfactory performance under the most severe  operating conditions. 1.  Tin-coated copper is a very common plating material.  Its ability to be successfully soldered without  highly active fluxes diminishes rapidly with time  after manufacture. It can be used up to the limiting  temperature of 150 °C. 2.  Silver-coated wire is used where temperatures do not  exceed 200 °C (392 °F). 3.  Nickel-coated wire retains its properties beyond  260 °C, but most aircraft wire using such coated  strands has insulation systems that cannot exceed  that temperature on long-term exposure. Soldered  terminations of nickel-plated conductor require the  use of different solder sleeves or flux than those used  with tin- or silver-plated conductor. Wire Shielding Insulation With the increase in number of highly sensitive electronic  devices found on modern aircraft, it has become very  important to ensure proper shielding for many electric circuits.  Shielding is the process of applying a metallic covering  to wiring and equipment to eliminate electromagnetic  interference (EMI). EMI is caused when electromagnetic  fields (radio waves) induce high frequency (HF) voltages in  a wire or component. The induced voltage can cause system  inaccuracies or even failure. Two fundamental properties of insulation materials are  insulation resistance and dielectric strength. These are  entirely different and distinct properties. Insulation resistance is the resistance to current leakage  through and over the surface of insulation materials.  Insulation resistance can be measured with a megohmmeter/ insulation tester without damaging the insulation, and data so  obtained serves as a useful guide in determining the general  condition of the insulation. However, the data obtained in  this manner may not give a true picture of the condition of  the insulation. Clean, dry insulation having cracks or other  faults might show a high value of insulation resistance but  would not be suitable for use. Use of shielding with 85 percent coverage or greater is  recommended. Coaxial, triaxial, twinaxial, or quadraxial  cables should be used, wherever appropriate, with their  shields connected to ground at a single point or multiple  points, depending upon the purpose of the shielding.  [Figure 9-113]  The airframe grounded structure may also  be used as an EMI shield. Dielectric strength is the ability of the insulator to withstand  potential difference and is usually expressed in terms of  the voltage at which the insulation fails because of the  electrostatic stress. Maximum dielectric strength values can  be measured by raising the voltage of a test sample until the  insulation breaks down. The type of conductor insulation material varies with the  type of installation. Characteristics should be chosen based  on environment, such as abrasion resistance, arc resistance,  corrosion resistance, cut-through strength, dielectric strength,  flame resistant, mechanical strength, smoke emission, fluid  resistance, and heat distortion. Such types of insulation  materials (e.g., PVC/nylon, Kapton®, and Teflon®) are  no longer used for new aircraft designs, but might still  9-68 Figure 9-113.  Shielded wire harness for flight control.