U.S. patent number 3,576,940 [Application Number 04/780,644] was granted by the patent office on 1971-05-04 for flame-retardant wire and cable.
This patent grant is currently assigned to Cerro Corporation. Invention is credited to Christian A. Alm, George S. Buettner, John G. Stone.
United States Patent |
3,576,940 |
Stone , et al. |
May 4, 1971 |
FLAME-RETARDANT WIRE AND CABLE
Abstract
An electrical conductor or cable, capable of maintaining
electrical integrity when exposed to open flame temperatures of
1000.degree. F. without propagating a fire or resulting in falling
burning particles or generating large volumes of smoke. The
conductor or cable of this invention is comprised of a metallic
conductor or plurality of conductors covered by a layer of primary
insulation. Such conductors are then covered by a layer of silicone
rubber which is, in turn, covered by a layer of glass fiber. The
layer of glass fiber may then be covered by another layer of
silicone rubber which is, in turn, covered by a layer of braided
asbestos impregnated with an intumescent material. In the
alternative, the second layer of silicone rubber may be covered by
an asbestos tape which is then covered by a layer of elastomeric
material having fire-retardant properties.
Inventors: |
Stone; John G. (North Haven,
CT), Buettner; George S. (Cheshire, CT), Alm; Christian
A. (Cheshire, CT) |
Assignee: |
Cerro Corporation (New York,
NY)
|
Family
ID: |
25120214 |
Appl.
No.: |
04/780,644 |
Filed: |
December 3, 1968 |
Current U.S.
Class: |
174/113R;
174/121A; 427/118; 427/119; 427/120 |
Current CPC
Class: |
H01B
7/295 (20130101) |
Current International
Class: |
H01B
7/17 (20060101); H01B 7/295 (20060101); H01b
007/28 () |
Field of
Search: |
;174/110.7,113--116,121,121.1,121.4 ;117/218 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Goldberg; E. A.
Claims
We claim:
1. An electrical insulated conductor, capable of maintaining
electrical integrity when exposed to open flame temperatures of at
least 1000.degree. F. without propagating a fire or resulting in
falling burning particles or generating dense smoke,
comprising:
a. a metallic conductor,
b. a layer of primary insulation which covers said metallic
conductor along its length,
c. a layer of glass fiber over the layer of primary insulation,
d. a layer of silicone rubber on one side of said layer of glass
fiber; and
e. a layer of asbestos which covers said layers of glass fiber and
silicone rubber, said asbestos fibers impregnated with an
intumescent material said layer of asbestos being the outer layer
of said insulated conductor.
2. The electrical conductor of claim 1, wherein the lay of the
layers of glass fiber and asbestos is in the same direction.
3. The electrical conductor of claim 1, wherein the electrical
conductor is selected from the group consisting of a single solid
conductor and a plurality of solid conductors.
4. The electrical conductor of claim 1, wherein the glass fiber
layer is covered on both sides with a layer of silicone rubber.
5. The electrical conductor of claim 1, wherein the primary
insulation is selected from the class of compounds consisting of
chemically cross-linked polyethylene compounds, silicone rubbers,
heat-sealed polyester tapes such as polyethylene terephthalate,
ethylene propylene rubber, nylon, butyl rubber, polysulfone,
polyvinylchloride, polytetrafluoroethylene and fluorinated ethylene
propylene.
6. The electrical conductor of claim 1, wherein the intumescent
material is selected from the class consisting of raw isano oil,
polyamide resins, amine formaldehyde resins, water insoluble metal
metaphosphates, polypentaerythritols, sodium silicate and
combinations thereof.
7. The electrical conductor of claim 1, wherein the glass fiber is
selected from the group consisting of glass cloth, glass braid,
glass mat and other glass fibers and combinations thereof.
8. An electrical insulated cable, capable of maintaining electrical
integrity when exposed to open flame temperatures of at least
1000.degree. F. without propagating a fire or resulting in falling
burning particles or generating dense smoke, comprising:
a. a plurality of metallic conductors,
b. a layer of primary insulation which covers and separates the
plurality of metallic conductors along their lengths such that each
metallic conductor is insulated from the other conductors,
c. a layer of glass fiber over said primary insulation,
d. a layer of silicone rubber on one side of said layer of glass
fiber; and
e. a layer of asbestos which covers said layers of glass fiber and
silicone rubber.
9. The electrical cable of claim 8, wherein the layer of asbestos
comprises asbestos selected from the group consisting of asbestos
braid, asbestos mat and asbestos felt, which asbestos material is
impregnated with an intumescent material.
10. The electrical cable of claim 8, wherein the layer of asbestos
comprises asbestos tape which is then covered by a layer of an
elastomeric material selected from the group consisting of
neoprene, butadiene acrylonitrile, chlorosulfonated polyethylene
and butadiene styrene.
11. The electrical insulated cable of claim 8, wherein a layer of
silicone rubber covers both sides of said layer of glass fiber.
12. The electrical insulated cable of claim 8, further including an
intermediate layer of insulating material which covers the
plurality of metallic conductors insulated with the primary
insulation and is immediately beneath the layer of glass fiber,
wherein said material is selected from the group consisting of
neoprene and polyvinylchloride.
13. The electrical cable of claim 10, wherein the lay of the wires,
the glass fiber layer and the asbestos tape layer is in the same
direction.
14. The electrical cable of claim 8, wherein the primary insulation
is selected from the class of compounds consisting of chemically
cross-linked polyethylene compounds, silicone rubber, heat-sealed
polyester tapes such as polyethylene terephthalate, ethylene
propylene rubber, nylon, butyl rubber, polysulfane,
polyvinylchloride, polytetrafluoroethylene and fluorinated ethylene
propylene.
15. The electrical cable of claim 9, wherein the intumescent
material is selected from the class consisting of raw isano oil,
polyamide resins, amine formaldehyde resins, water insoluble metal
metaphosphates, polypentaerythritols, sodium silicate and
combinations thereof.
16. The electrical cable of claim 8, wherein the glass fiber is
selected from the class consisting of glass cloth, glass braid,
glass mat and other glass fibers and combinations thereof.
17. The electrical conductor of claim 10, wherein the elastomeric
material has compounded therein a chlorinated parafin.
18. The electrical conductor of claim 10, wherein the elastomeric
material has additionally compounded therein halogenated phenols
selected from the group consisting of tetrabromobisphenol,
hexachlorophene and dichlorophene.
19. The electrical conductor of claim 10, wherein the elastomeric
material has additionally compounded therein fire-retardant
inorganic compounds selected from the group consisting of antimony
trioxide, zinc borate, tricresylphosphate, trioctylphosphate and
triphenylphosphate.
Description
This invention relates to electrical wire and cable and more
particularly to electrical conducting wire and cable possessing
relatively flameproof properties.
During the past several decades, the tray or ladder method of
wiring industrial plants has come more and more into widespread
use. The tray system involves the use of suspended metal work, in
which wire and cable is laid in running the cable from a source of
motive power to service panels or devices, as the case may be. This
ladder or tray system is to be contrasted with the system of
pulling wires through specified diameter conduits which, of course,
limits the size and number of cables which can eventually be
connected from one place to another within a fixed or prescribed
conduit.
Tray systems are used in utility generating stations, steel mills
and other industrial plants where the ease of running new cables in
trays in a stacked fashion is common practice. The trays eventually
end up in stacked rows with layers of cable side-by-side to various
heights. In such plants, debris and foreign matter have a tendency
to collect in the trays and, if there is any fire hazard presented
by other operations such as welding and the like, the trays with
their multitudinous number of cables set side-by-side and one upon
another, pose a serious problem as to propagation of fire, smoke
generation and the falling of burning particles. Since the tray
system is used in many utility and industrial installations, fires
of electrical nature can result in severe damage to expensive and
critical equipment and devices and may propagate throughout the
entire plant via the tray route.
At present, the only electrical conductors or cables which, when
subjected to high temperatures of open flames, do not form burning
falling particles, propagate flame or generate smoke, are
electrical conductors or cables which have coverings commonly made
of metal, metal alloys or the like. These conductors or cables are
relatively expensive because of the difficulty involved in
producing the metal covering and, further, they are unwieldy, bulky
and cumbersome and difficult and expensive to repair and
replace.
In view of the foregoing, applicants have devised an electrical
conductor or cable capable of maintaining electrical integrity for
sustained periods of time beyond 5 minutes and up to as much as 20
minutes when subjected to open flame temperatures in excess of
1000.degree. F. The electrical integrity will be maintained without
producing any falling burning particles which can deposit
themselves on other devices or materials, causing separate and
distinct fires from the original fire. In addition, the conductor
or cable of this invention will not propagate a fire along its
length when subjected to open flame conditions at a particular
source or region, nor will it generate large quantities of
smoke.
Accordingly, it is a principal object of this invention to provide
a new and improved flameproof electrical wire and cable.
Another object of this invention is to provide a new and improved
flameproof electrical wire and cable which will not produce
falling, burning particles when subjected to open flame and which
will not generate smoke.
Still another object of this invention is to provide a new and
improved flameproof electrical wire and cable which will not
propagate a fire along its length when subjected to open flame at a
particular point or region.
A further object of this invention is to provide a new and improved
flameproof electrical wire and cable which will maintain electrical
integrity while subjected to an open flame, which electrical
integrity will be maintained after the open flame hazard is
removed.
A still further object of this invention is to provide a new and
improved flameproof electrical wire and cable which is
substantially flexible and relatively inexpensive.
Still other objects and advantages of the invention will in part be
obvious and will in part appear hereinafter.
The invention accordingly comprises the several steps and the
relation of one or more of such steps with respect to each of the
others and the article possessing the features, properties and the
relation of elements which are exemplified in the following
detailed disclosure and the scope of the invention will be
indicated in the claims.
For a fuller understanding of the nature and objects of the
invention, reference should be had to the following description,
taken in connection with the accompanying drawings, in which:
FIG. 1 is a cross-sectional view of the insulated wire of this
invention;
FIG. 2 is an isometric exploded view of the insulated wire of FIG.
1;
FIG. 3 is a cross-sectional view of an alternate embodiment of the
invention; and
FIG. 4 is a cross-sectional view of a cable insulated according to
this invention.
In FIG. 1 there is illustrated a metal conductor 10 which may be of
stranded or solid metals, such as copper or copper coated tin, as
the case may be. Other metals, such as aluminum, silver plated
copper, or other conductive alloys such as stainless steel, may
also be utilized. The metal conductor may be a single solid metal
conductor or a plurality of metallic conductors. A layer of a
primary insulation 11 is applied over the conductor, generally in
thicknesses of from about 5 mils to 2 inches and, preferably, in
thicknesses of from 10 mils to about one-half inch.
In the preferred embodiment of the invention, the primary
insulation 11 may be Rockbestos X Link 90 (a blend of polyethylene
and ethylene propylene rubber which has been cross-linked) or other
chemically cross-linked polyethylene compounds, silicone rubber,
heat-sealed polyester tapes such as polyethylene terephthalates and
ethylene propylene rubber. Other insulating materials which may be
used are polyvinyl-chloride, polyethylene, nylon polysulfone,
extruded polyester, butyl rubber, polytetrafluoroethylene and
fluorinated ethylene propylene. Moreover, it is to be understood
that other, equivalent thermosetting materials may be used as
primary insulation 11. Where superior electrical properties are
desired, silicone rubber is preferable although the cost of
silicone rubber is extremely high. Therefore, Rockbestos X Link 90
is preferred as the primary insulation since it has good dielectric
properties, does not readily melt upon exposure to very high
temperatures and is relatively inexpensive. In applying the
insulation over the metallic conductor, the insulation layer may be
extruded or applied as a tape or film. In the preferred embodiment,
an extruded layer of Rockbestos X Link 90 is applied, preferably in
thicknesses of 10 mils to about 500 mils.
Immediately adjacent to the primary insulation layer 11, there is a
layer of silicone rubber 12, generally in thicknesses of 1 mil to
10 mils and, preferably, in thicknesses of 2 mils to 5 mils. The
layer 12 may be methyl silicone rubber, vinyl methyl silicone
rubber, phenyl silicone rubber, fluoro silicone rubber, or other
types of silicone rubber. Although methyl silicone rubber is
preferred, it should be understood that any of the other silicone
rubbers may be utilized. The silicone rubber may be applied to the
primary insulation layer as a tape or film or as part of a silicone
rubber glass tape or film.
The layer of silicone rubber is a very effective insulation since,
on exposure to high open flame temperatures, the layer reverts to a
nonconductive inert silica layer. This silica layer performs two
functions: (1) it acts as a fireproof media and prevents oxygen
from passing through to the primary insulator and, thus, it
prevents the primary insulation from decomposing and losing its
effectiveness, and (2) the silicone rubber layer does not lose its
own effectiveness as an insulation even though it is present as a
silica layer.
Adjacent to and covering the silicone rubber layer 12, there is
applied a glass fiber layer 13, generally in thicknesses of 3 mils
to 10 mils and, preferably, in thicknesses of 4 mils to 7 mils.
Glass cloth, glass braid or any other glass fiber such as glass
mat, may be used to provide the required support and reinforcement
for the silicone rubber layers. The glass cloth acts as an
insulation and, as before, the glass cloth may be applied as a tape
or film.
Directly over the glass fiber layer 13, there is applied another
layer of silicone rubber 14, generally in thicknesses of 1 mil to
10 mils and, preferably, in thicknesses of 2 mils to 5 mils. It
should be understood that the second layer of silicone rubber is
preferred, but is not necessary to this invention. This second
silicone rubber layer prevents oxygen from passing into the
underlying layers. Thus, the combination of layers 12, 13 and 14
provides a very effective insulation barrier against high open
flame temperatures and prevents oxygen from passing into and
reacting with the primary insulation. The combination of the
silicone rubber layers 12, 14 and glass fiber layer 13 may take the
form of a unitary layer, wherein the glass fiber has been coated or
impregnated on one or both sides with the silicone rubber.
Adjacent to and immediately over the silicone rubber layer 14 there
is applied a layer of asbestos 15, generally in thicknesses of 20
mils to 90 mils and, preferably, in thicknesses of 25 mils to 50
mils, impregnated with an intumescent material. The asbestos layer
15 may be an asbestos braid or any loose asbestos fiber such as an
asbestos felt. The asbestos provides mechanical protection for the
wire or cable at all service temperatures in wet or dry locations
and is readily available and inexpensive. In addition, asbestos has
good bulk characteristics, in that it readily absorbs intumescent
material while glass and other inorganic fibers are not very
absorbent. Further, the asbestos has a very high melting point, is
noncombustible and has the desirable property of expanding slightly
at high temperatures. In actual operation, the asbestos acts as a
noncombustible, inorganic high-strength support for the intumescent
material at the elevated open flame temperatures to which it may be
exposed. Moreover, the asbestos layer serves the additional purpose
of supporting the silicone rubber layer 14 upon its reversion to
silica when the system is exposed to an open flame. The intumescent
material in the asbestos, when subjected to open flame
temperatures, swells and forms a thermal insulating fire-retardant
barrier between the flame and the layers of insulating material
underneath. As a result, the cable or wire does not propagate a
fire and there are no falling burning particles nor generation of
dense smoke. Additionally, the flameproof electrical wire and cable
of the present invention can be used in wet or dry locations. As a
result, the electrical conductor or cable of this invention need
not have an additional or ancillary protective coating such as
metal braid, armor, conduit or tubing. Furthermore, a protective
coating other than metal armor would not have the fireproof
characteristics of the intumescent impregnated asbestos.
Intumescent materials or compounds which may be used to impregnate
or coat the asbestos layer are raw isano oil, polyamide resins and
amine formaldehyde resins. Examples of amine formaldehyde resins
that have been found to be satisfactory in the present invention
are melamine formaldehyde resin, urea formaldehyde resin and
triazine formaldehyde resins. The polyamide resins used in the
present invention are the products obtained by reacting a
polymerized unsaturated vegetable acid with an amine. For example,
the polyamide resin may be obtained by reacting dimerized and
trimerized linoleic acid or linoleic acid of soy bean oil with
ethylene diamide. It has been found that a superior intumescent
composition is obtained when a mixture of raw isano oil with a
polyamide resin or an amine formaldehyde resin is prepared where
the amount of raw isano oil is 33--70 percent of the total
composition. The use of the above composition in intumescent paints
is set forth in U.S. Pat. No. 2,754,217.
Other intumescent compounds which may be used as intumescent
material to impregnate the asbestos layer of the present invention
are water insoluble metal metaphosphates, water insoluble polyols
and water insoluble aminoplasts. Examples of water insoluble metal
metaphosphates are insoluble potassium metaphosphate, insoluble
sodium metaphosphate, zinc metaphosphate and calcium metaphosphate.
Any of the water insoluble metaphosphate salts are applicable to
this invention.
Various water insoluble polyhydric compounds may be used in various
polymeric forms such as dimers or trimers, to impart intumescent
properties to the asbestos layer. In particular,
polypentaerythritols such as dipentaerythritol and
tripentaerythritol are preferred.
The aminoplasts or amino aldehyde condensation products are
prepared from aldehydes and organic compounds containing at least
one amino group which has at least two replaceable hydrogens. The
preferred compounds are water insoluble urea formaldehyde
condensation products. In general, the aminoplasts are prepared by
reacting an aldehyde such as formaldehyde, urea aldehyde,
propionaldehyde and the like with amino compounds containing one to
nine carbon atoms and having the grouping of the formula
where N is a member selected from the group consisting of a
nitrogen atom having two single valences attached to separate atoms
selected from the group consisting of hydrogen and carbon atoms,
and a nitrogen atom having two free valences representing a double
bond attached to a carbon atom, and where Y is a member selected
from the group consisting of O, S, and a nitrogen atom with one
free valence which is attached to an atom selected from the group
consisting of hydrogen and carbon atoms.
In order to obtain superior intumescent properties, it is preferred
to use the above intumescent materials in combination in a
composition containing 27--66 percent by weight of
polypentaerythritol with a water insoluble metal metaphosphate.
Further, an aminoplast may be added to the composition to increase
the intumescent properties of the composition, such that the
composition contains 15--70 percent by weight of urea formaldehyde
resins. The uses of the above intumescent compositions in paint is
illustrated in U.S. Pat. No. 3,037,951.
Another compound suitable for use as an intumescent material, is
water-glass or sodium silicate. However, because of the solubility
of sodium silicate in water, this compound has limited utility as
an intumescent compound to impregnate the asbestos according to
this invention.
Other publications which describe the uses of intumescent materials
in paints, which intumescent materials or compositions are suitable
for use in the present invention, are: J. Amer. Oil Chemists' Soc.,
"Water-resistant, Oil-based, Intumescing Fire-retardant Coatings,"
41(10), 670-4 (1964), and Off. Dig., J. Paint Tech.,
"Water-resistant, Oil-based, Intumescing Fire-retardant Coatings,"
, 38 (793), 105-12 (1966) (Eng.).
The above intumescent compounds and compositions may be mixed or
dissolved with a common carrier such as linseed oil, xylol, toluol
and the like, so that the resulting mixture may be applied to the
asbestos layer. Intumescent compositions premixed and suitable for
their application to the asbestos layer are sold under the
tradenames of No. 144639, Fire-retardant Coating, and Series 180,
Coatings for Cross-linked Polyethylenes, sold by the Standard T
Chemical Company, Inc. The intumescent material may be added to the
asbestos before or after the asbestos has been placed on the
silicone rubber layer 14, although it is preferred that the
intumescent material be applied to the asbestos after the asbestos
has been applied over the silicone rubber layer. While it may be
understood that the wire or cable dimensions of the present
invention may vary according to the design for any particular
application, the following is a typical range for the electrical
conducting system of the instant invention, applied in the given
order: ##SPC1##
It is to be understood that the above dimensions are typical and
not intended to limit the invention in any way.
A wire of the present invention was subjected to the flame of a
bunsen burner (in excess of 1500.degree. F. ) for 10 minutes. The
exposed wire was found to have substantial insulation left and was
capable of passing the dielectric test. Thus, electric integrity
was maintained, there were no falling, burning particles and fire
was not propagated along the wire nor was excessive smoke
generated.
An alternate embodiment of the invention is illustrated in FIG. 3.
Insulation layers 11--14 are the same as those disclosed with
reference to FIGS. 1 and 2. As in the embodiment of FIG. 1, the
second silicone rubber layer 14 may or may not be present. In the
preferred form of the alternate embodiment, there is utilized a
second silicone rubber layer. Silicone rubber layer 14 is covered
by a layer of asbestos tape 16 or other form of asbestos wherein
the fibers are closely woven. Since asbestos tape is rather closely
woven, it is difficult to properly impregnate with intumescent
material. Therefore, in place of the intumescent material, asbestos
tape layer 16 is covered by a layer of an elastomeric material 17
having suitable fireproofing properties. The elastomeric material
may be any elastomeric material to which the necessary
fire-retardant compounds may be added and which can be extruded
about asbestos tape layer 16. Some examples of such elastomers are
butyl rubber, silicone rubber, polyurethene rubber, neoprene,
butadiene acrylonitrile, chlorosulfonated polyethylene and
butadiene styrene. Before the elastomeric material is extruded over
the asbestos, chlorinated parafins and halogenated phenols are
compounded into it to impart additional fire-retardant properties
to the material. Some examples of chlorinated parafins which were
found suitable for the present invention are Chlorofin (trademark)
manufactured by Hercules Chemical Company, Halowax (trademark)
manufactured by The Union Carbide Corporation, and Chlorowax
(trademark) manufactured by The Diamond Alkali Company. Halogenated
phenols which were found suitable for imparting fire-retardant
properties to the elastomeric material are tetrabromobisphenol,
hexachlorophene and dichlorophene. Although, in the preferred
embodiment, both chlorinated parafins and halogenated phenols are
compounded into the elastomeric material, it should be understood
that only one of the above two classes of compounds need be added.
Further, to impart additional fire-retardant properties to the
elastomeric material, fire-retardant inorganic compounds such as
phosphates and borates, can be compounded into it. Specifically,
compounds which were found to produce excellent results are
antimony trioxide, zinc borate, tricresylphosphate,
trioctylphosphate and triphenylphosphate. In particular, the
interaction between antimony trioxide and chlorinated material to
produce the intermediate, antimony oxychloride, at elevated
temperatures, has proven an efficacious flame retardant. In the
preferred embodiment, all three classes of compounds were
compounded into the elastomeric material so as to impart to it
maximum fire-retardant properties.
Layer 17 is generally 0.045 to 0.140-inches thick and, preferably,
0.060 to 0.140-inches thick. Since the combination of the glass
fiber, asbestos tape and elastomeric material forms a layer which
is highly impermeable to gases, this type of insulation presents a
problem when the insulated wire or cable is exposed to fires. Thus,
gases that are formed in the lower layers of insulation are trapped
by the outer layers of glass fiber, asbestos tape and elastomeric
material so that, as a result of increasing pressure of the trapped
gases, the insulation comes apart. This problem is resolved in the
present invention by having the lay of the layer of glass fiber and
the asbestos tape layer in the same direction, to form a loose
combination of layers. With respect to cables, the lay of the
wires, the glass fiber layer and the asbestos tape layer are all in
the same direction. The loose layers, that is, the layers of
insulation having their lay in the same direction, expand under the
influence of the pressure of the trapped gases allowing the trapped
gases to pass, or travel, beneath the insulation and parallel to
the axis of the wire or cable. This prevents the insulation from
being torn apart.
Although such a loose covering of insulation is not usually
desirable, it is used in a case where it is preferred to use an
insulation system having the two external layers of asbestos tape
and elastomeric material instead of asbestos it should be noted
that the insulation system of this embodiment has fireproofing
properties which are even better than those of the insulation
system wherein intumescent material is used.
The basic insulation system of the present invention comprises a
layer of primary insulation which is then covered by a layer of
glass fiber. The glass fiber layer has at least one layer of
silicone rubber on one of its sides. Preferably, both sides of the
glass fiber layer are covered with silicone rubber. The layer of
silicone rubber is, at this point, covered by a layer of asbestos.
This is the basic insulation system of the present invention, which
has the exceptional fireproofing properties described above. In
addition, if an external layer of asbestos braid or asbestos felt,
or any other loosely-packed asbestos fiber material is used, then
the asbestos material can be impregnated with intumescent material
to impart to the insulation additional fireproofing properties. On
the other hand, if an external layer of asbestos tape is used, then
an outer layer of elastomeric material, having suitable
fire-retardant properties, is used to cover the asbestos tape. In
this embodiment, the lay of the glass fiber layer and the asbestos
tape layer is in the same direction so as to form a loose covering
over the conductor.
In FIG. 4, there is illustrated a cable insulated according to the
present invention. A plurality of individual metal conductors 18,
wherein each conductor may be composed of stranded or solid metals
such as copper or copper coated with tin, as the case may be, are
shown covered by a layer of primary insulation 19, generally 5--500
mils in thickness and, preferably, in a thickness of from 10--250
mils. The primary insulation 19 is selected from the same class of
material as primary insulation 11. Covering the primary insulation
19 on the insulated wires 18, there may or may not be a layer of
intermediate covering material 20. The intermediate covering
material 20 may be neoprene, polyvinylchloride or any other
thermosetting or thermoplastic insulation, which may be applied by
extrusion or any other suitable manner. Generally, the layer of
intermediate covering material is of 30--100 mils thickness and,
preferably, is of 40--80 mils thickness. To the intermediate
covering material 20 there is applied a covering of a layer of
silicone rubber 21, generally in a thickness of 1--10 mils and,
preferably, in a thickness of 2--5 mils.
Immediately adjacent and above the silicone rubber layer, there is
applied a glass fiber layer 22, generally of 3--10 mils thickness
and, preferably, 4--7 -mils thick. The glass fiber layer 22 is
subsequently covered by a layer of silicone rubber 23, again in a
thickness of generally 1 to 10 mils and, preferably of 2--5 mils.
As explained with reference to FIGS. 1--3, the second silicone
rubber layer 23 may or may not be present. However, a second
silicone rubber layer imparts additional fireproofing properties.
Then, as in the embodiment illustrated in FIGS. 1 and 2, the second
layer of silicone rubber 23 is covered by a layer of asbestos braid
24, impregnated with intumescent material of a thickness of
generally 20--90 mils and, preferably, 25 to 50 mils. The primary
insulation layer 19, the silicone rubber layers 21, 23, the glass
fiber layer 22, and, the impregnated asbestos layer 24, are as
described with reference to FIGS. 1 and 2.
With respect to cables, as in the case of a single electrical
conductor, when the asbestos used to cover the second layer of
silicone rubber comprises asbestos braid or any other asbestos
material composed of loose fibers, the asbestos is impregnated with
intumescent material. However, when the external asbestos layer is
composed of asbestos tape or any other closely woven asbestos
material, the asbestos is covered with a layer of elastomeric
material having good fireproofing properties, as has been explained
previously in connection with FIG. 3. As in the case of single
electrical conductors, the elastomeric layer covering the asbestos
tape is generally 0.025 to 0.200 -inches thick and, preferably,
0.050 to 0.100 -inches thick. Further, in the case where asbestos
tape or other closely woven asbestos is used, the lay of the cable,
the glass fiber layer and the asbestos layer is preferably in the
same direction.
It will thus be seen that the objects set forth above, among those
made apparent from the preceding description, are efficiently
attained and since certain changes may be made in the above article
without departing from the spirit and scope of the invention, it is
intended that all matter contained in the above description and
shown in the accompanying drawing shall be interpreted as
illustrative and not in a limiting sense.
It is also to be understood that the following claims are intended
to cover all the generic and specific features of the invention
herein described and all statements of the scope of the invention
which, as a matter of language, might be said to fall
therebetween.
* * * * *