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Asbestos electrical insulation: history, manufacture, visual identification: how to recognize asbestos-based or asbestos containing electrical insulation materials in products & buildings - a visual guide to identifying asbestos in buildings. Page top photograph: asbestos insulation on electrical wiring in a theater.
This articles series about the manufacture & use of asbestos-containing products includes detailed information on the production methods, asbestos content, and the identity and use of asbestos-containing materials. Page top photo shows asbestos used in an electrical motor.
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The following text is Adapted from Rosato (1959) p. 106-112  © 2013 InspectApedia.com
[Click to enlarge any image]
The main reasons for the use of asbestos are its fire and heat resistance, electrical insulation properties, acid resistance, and durability. Electrical insulation is also used to protect the surfaces of conductors from such adverse conditions as moisture and chemicals, and to fill spaces where corona discharge is liable to occur.
In electrical applications, asbestos insulation is used with low iron content. All grades of crude and milled chrysotile fiber are predominately used.
The lowest iron content fibers for use in the electrical field are available from ore deposits in the North American Continent. ASTM specifications list the total iron content for electrical grade products. Details are at ASBESTOS IRON CONTENT.
List of products using asbestos electrical insulation
Large quantities of asbestos are used to cover electrical wire in the traction type of electric motors and coils where there is a possibility of rising temperature damaging ordinary cotton coverings. The general use of asbestos electrical insulation includes ordnance equipment (aircraft, missiles, etc.) oil burners, heating and cooking apparatus, lighting equipment, agricultural machinery, construction and mining machinery, hoists, oil field machinery, machine tools, portable tools, bakery machinery, food products 'machinery, textile machinery, paper industry equipment, pumps and 106 compressors, conveying equipment, blowers and fans, trucks and tractors, industrial ovens and furnaces, mechanical stokers, computers and cash registers, vending machines, laundry equipment, vacuum cleaners and units, refrigeration and air conditioning units, motors and generators, power transformers, switchgear controls, welding units, insulated wire and cable, lamps, radio, TV and radar units, x-ray units, railroad units, dental units, and signs and advertising displays.
Lightweight asbestos insulation is of primary importance on Naval and Maritime ships. Navy cable insulation is of particular importance; it is a combination of asbestos paper and textile products (lap, tape, etc.)
In the manufacture of multi-conductor cables, the single or paired conductors are cabled into required multiples, together with cushioning fillers in the valleys. These fillers are generally made with asbestos roving.
Specifications for Asbestos-Based Electrical Insulation Materials
Classification of electrical insulation by the American Institute of Electrical Engineers (AIEE) follows:
Another classification which includes asbestos-electrical insulated products is given by the Underwriter's Laboratories, Inc. This laboratory classifies asbestos electrical insulating material as asbestos-insulated wire (460 12) and as asbestos-varnished cloth wire (460 13).
Asbestos-Insulated Wire (460 12) Labels & Classifications
Asbestos-Varnished Cloth Wire (460 13)
Data pertaining to electric shipboard cable which includes asbestos is given in the "Cable Comparison Guide," NAV- SHIPS 250-660-23 (1956) published by Bureau of Ships, Navy Department, Washington 25, D. C. and available through Government Printing Office.
The data pertains to Government procurement specifications MIL-C-915A (Ships), MIL-C-2194B (Navy), MIL-C-2681 (Ships), and MIL-W-16878B (Navy). All cables are identified by types. The letters listed under types identify the first letters of the words used in describing the cable. Other letters identify construction of the cable. Some of these cables which incorporate asbestos are identified as follows:
Asbestos Paper Products as Electrical Insulation
Asbestos papers are very popular for use in the manufacture of miniature electrical component units. Miniature transformers can use asbestos papers. A 30 per cent reduction in weight has been obtained in Class-B transformers.
World War II initiated many development projects specifically for developing inorganic paper products. Prior to 1940, no inorganic papers were commercially available. At present, there are four basically different types of inorganic papers commercially available; i.e., asbestos, mica, glass, and ceramic. Asbestos papers are manufactured by Johns- Manville, Inc. under the trade name "Quinterra" or "Quinorgo"; by Raybestos-Manhattan, Inc. under the trade name "Novabestos"; and by General Electric Co. under the trade name "Terratex."
These basic paper products are available in many different forms which include combinations of asbestos-glass, asbestos-mica, papers backed with or sandwiched between other types of papers or fabrics and held together by an oleoresinous varnish, shellac, silicone, rubber, or other composite insulation structures. These combinations provide for increased tensile strength which is desirable in many of the cable or wire taping operations.
The chief function of asbestos [in electrical applications] is that of a separator or as a wrapping for wire insulation. Untreated paper is roughly equivalent to air as an insulator; when it is completely dry its resistivity is good. Untreated paper can contain moisture since it has the characteristic of absorbing moisture from the air—a characteristic typical of other paper products. It is usually treated with a varnish or lacquer in order to eliminate the moisture problem. Where high temperature requirements are of prime interest, the paper is treated with silicone varnishes or fluorocarbons.
In the manufacture of asbestos paper, there is always continued interest and development in producing open asbestos fibers. Various methods of manufacture and various equipment are used in order to fiberize bundles of asbestos fiber. By opening the fibers, entrapped undesirable magnetite can be removed.
Manufacturing processes have also been developed which permit uniform blending of asbestos fiber with such synthetic fibers as glass. These types of products are used by electrical insulation manufactures and by the manufacturers of reinforced plastics and filters.
"Quinterra" is a pyrolysis-resistant electrical insulation made of highly purified asbestos. These electrical insulations greatly extend the life of electrical equipment. By permitting magnetic units to operate at higher temperatures, they promote savings in space, weight and materials.
The asbestos products enable electrical apparatus to withstand higher overloads; thus, they reduce the necessity for standby equipment. Their thin layers require less time and labor to apply and occupy less space than the extra thicknesses formerly employed to compensate for the loss of dielectric strength caused by heat. (Courtesy Johns-Manville Corp.)
Figure 6.1. Installation of Quinterra wrapped coils in electric motor armatur e[click to enlarge]
In transformers, their uses include barriers, core tubes, spacer sticks, end packings, interlaminates, interlayers, and wrappers. They have replaced metal cores in some resistors and conserve insulation in others. In magnets, they insulate the ribbon coils, and form liners, spacers, and wrappers. Both wires and cables are wrapped with "Quinterra" and "Quinorgo" Typical motor wire insulation is shown in Figure 6.1.
Patent 2,626,213 describes a unique method of dispersing and forming asbestos papers. The Novabestos paper products produced are characterized by their ability to stretch and distort, both dry and wet, to a much greater extent than ordinary paper. The smooth asbestos fibers in the paper allow more flow and better packing properties than the commercial organic paper products.
The Novabestos paper machine developed specifically for producing asbestos paper is similar in many respects to the conventional Fourdrinier machine. The asbestos pulp is picked up by a wire screen and carried over flat boxes operating under a vacuum of 2 to 8 in. mm. The flat boxes remove 60 to 70 per cent of the water from the slurry. The next step in drying is a suction transfer roll operating under a vacuum of 12 to 15 in. mm. In this operation, another 5 to 10 per cent of water is removed. From the suction transfer roll, the paper goes over a steam-jacketed drying roll and then over a windup drum. The finished paper product has the appearance of snowy white paper.
Asbestos, Its Industrial Applications - Rosato: Text & Chapter Index 
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Frequently Asked Questions (FAQs)
Question: did house wiring ever contain asbestos?
I was wondering if house wiring ever contained asbestos. The wiring in my home appears to have an asphalt impregnated type covering almost looks like a snake skin. Could this contain asbestos? - D.T., Canada 10/4/2013
A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer about wiring (noting any markings, exact composition of its insulation, and perhaps data about its age) than we can give by email alone. You will find additional depth and detail in articles at our website.
That said I offer these comments:
Asbestos was indeed used in electrical wiring insulation, as discussed in the article above (ASBESTOS ELECTRICAL INSULATION) and appears in our lists of asbestos-containing materials found at ASBESTOS LIST of PRODUCTS and at ASBESTOS PHOTO GUIDE to Materials
Asbestos was in particular, more widely used in a more pure form in certain electrical wires exposed to high risk (inside of heaters, toasters, electric stoves, theatre wiring).
I would like to see sharp photos of your wiring, its external jacket, cross section of the insulation, and if occasion permits, an actual sample - those details would permit further comment or even testing of the material.
Depending on the age and appearance of the wiring about which you ask, it may be appropriate to treat it as PACM - presumed asbestos containing material. But don't panic. Usually asbestos hazards are minimized by leaving PACM alone unless it is friable, already damaged and / or subject to damage and is in an occupied space.
I suspect that unless there is a demolition project in progress, the asbestos exposure hazard from asbestos-containing residential electrical wiring is very low, perhaps below the limits of detection, both because of low friability of the typical (excluding my list above) application and insulation materials, and because wiring is normally enclosed in wall, floor, or ceiling cavities.
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