Note: Descriptions are shown in the official language in which they were submitted.
WO 9610292Q PCT/GB95/01638
-1-
FIRE-RESISTANT WIRES
This invention relates to fire-resistant wires capable of maintaining
electrical
circuit integrity when the wires, or cables containing them, are exposed to
fire.
Known examples of such wires are wrapped with inorganic fire-resistant
material in order to continue to provide electrical insulation surrounding the
electrical conductor where the usual layers of organic polymer insulation have
been
melted or burnt away. Of various known inorganic fire-resistant wrappings,
mica
"paper" is generally preferred, and for convenience the following description
will
therefore refer to mica wrapping while not restricting the present invention
to that
material.
Mica paper reinforced by a backing material of woven glass or of
polyethylene film is produced as tape that can be spirally wrapped onto
electrical
conductors to give a degree of fire resistance. The mica wrapped conductor is
then
normally coated with a conventional polymer to impart the required
electrical/mechanicat properties. In a fire, the polymer is destroyed but the
electrical integrity of the cable is maintained by the mica layer and the
insulating
char from the polymer. It is generally understood that the more mica the
better
from a fire performance point of view and tapes are available with different
weights
of mica (e.g. 80, 120, 160 g./sq m).
Conventionally mica tapes are applied by spiral wrapping with an overlap of
typically 30% to 50% (of tape width) to maintain protection when the wrapped
conductor is flexed, since the overlaps tend to open on flexing. High overlaps
are
preferred to achieve the required degree of fire resistance. It is a problem
that the
thicker, heavier tapes are harder to wrap successfully, especially on small
diameter
conductors, leading to tape damage, wrinkling and a poor wrapped surface
making
subsequent processing more difficult and final appearance andlor performance
of
' the wire less satisfactory. Therefore, if a higher level of fire protection
is needed,
the options are normally 2o increase the weight of the mica tape and suffer a
' deterioration in the wrap quality, or to apply 2 layers of tape giving a
greater
diameter. It is also a problem that the overlapped tape tends to suffer from
breakage of some of the glass weave reinforcement, causing glass fibre
protrusions
from the wrap. This problem gets worse the smaller the conductor diameter and
11'0 94!02920 YG fI(:B951t1i63~
2-
the thicker the tape, and is partly due to the low elongation to break
(typically 3'~)
of these tapes. Geometrical calculations suggest that considerably higher
elongation
is needed to cope with the overlaps, but this may be compensated by some
compression of the tapes in practice. This problem has resulted in the tape
suppliers developing the polymer-t3lm-backed alternatives, although the mina
paper
itself remains as inextensible as ever, and there is doubt as to the
reliability of the
tape not to delaminate during wrapping and the reduced fire performance that
must
be incurred by replacing glass and silicone binder tapes with polyethylene-
backed
tapes. These problems are exacerbated by the fact that the mica tape is
supplied in
discrete lengths requiring splicing every few hundred metres to allow
continuous
longer lengths of wrapped conductor to be produced. Splicing is normally done
using a suitable adhesive tape applied to one side of the mica tape, which
results in
increased wrapping thickness where the splices occur.
These problems may be better understood from accompanying schematic
sketches A to G, wherein
Sketch A indicates an "ideal" 50~ overlap wrap on a wire;
Sketch B indicates a more realistic degree of overlap achieved in practice;
Sketch C indicates a wire having a single wrap with a typical 33 % overlap;
Sketch D indicates a double layer of wraps similar to that shown in Sketch
B;
Sketch E indicates a stranded wire having a double layer of wraps similar to
that shown in Sketch C;
Sketch F indicates (exaggerated) imperfections in lrnown wrappings; and
Sketch G indicates increases in wrap thickness caused by the aforementioned
mica splicing rages.
Ideally, as shown in Sketch A, the successive turns of mica wrap M of
perceived width W would have an overlap of exactly 50'x, giving a double
thickness of mica cape over the entire surface of the wire conductor C frnrn a
single
wrap. In practice, this ideal overlap is difficult to achieve, and Sketch B
indicates a
W0 96f02920 PCTIGB95101638
-3-
more realistic result where gaps G occur, taking into account the finite
thickness of
the mica tape (M) which necessitates a lower degree of overlap to achieve a
smooth
wrap. Sketch C indicates a single wrap of mica tape M of perceived width W
with
an overlap (shaded) of about 33%. The perceived width W is slightly greater
than
the true width of the tape M owing to the spiral wrapping angle of the tape
about
the conductor axis. Because of the gaps G lowering the effectiveness of the
wrapping, two mica tapes M1 and M2 are preferably used as indicated in
Sketches
D and E to ensure that at least two layers of the mica wrap are present at all
points.
Sketch F shows, with some exaggeration for clarity, the unevenness which
occurs
in these known wrappings of mica tape M on conductor C, and indicates the
undesirable breakage of the brittle reinforcing fibres F (usually glass] which
tends
to occur due to stresses in the overlapped tapes.
These imperfections in the mica wraps, and even the aforementioned
increases in thickness due to splices in the mica tape, can be tolerated when
a thick
coating (1 millimetre or more) of paly~meric insulation is to be applied over
the
mica wraps, as shown in Sketch G. The mica tape M of width W is wrapped on
the conductor C as before, but the splicing tape S is present on two or more
adjacent turns, since the splicing tape S usually covers a length of the mica
tape
which is greater than that required to complete one turn around the conductor
C,
especially on conductors of small diameters. The resulting increase in
wrapping
thickness by one or two times the thickness of the splicing tape S (up to four
times
the thickness of S if splices of two tapes (Sketches D and E) overlap, so this
is
avoided in practice) can clearly be seen. This, and the roughness and fibre
breakages indicated in Sketch F, tend to be swallowed up when thick coatings
of
insulating plastics P, of thickness 1 millimetre or more, are applied by known
methods such as extrusion over the mica wraps, as indicated (not to scale) on
the
left-hand side of Sketch G. Such crude aver-coating is undesirable, however,
for
wiring in situations where the weight and volume of the wire must be
minimised.
In high-quality wires having thinner coatings of insulating plastics P', as
indicated on the right-hand side of Sketch G, all the aforementioned problems
rnay
have an increasingly significant effect on the appearance andlor performance
and/or
manufacturing efficiency of the mica-wrapped fire-resistant wire. The lumpy
splice
areas may even have to be cut out in some places , thus limiting the
continuous
lengths of wrapped wire which can be produced.
CA 02194993 1997-O1-13
The present invention alleviates the aforementioned
problems in surprisingly simple way.
The invention provides an electrical wire having a
conductor carrying at least one electrically-insulating under-
layer of fire-resistant material over which is spirally
wrapped an electrically-insulating covering gape of inorganic
fire-resistant material with adjacent turns of the tape
overlapping one another by less than 5% of the tape width.
Preferably, t:he under-layer also comprises a tape of
inorganic fire-resistant material spirally wrapped around
(preferably in contact with) the conductor. The overlapping
regions of the covering tape will preferably be positioned
between the overlapping regions of the under=Layer tape, so as
to maintain protection on flexing of the wire which will tend
to open the small overlaps of the covering tape.
It has been found, according to the present
invention, that reducing the covering tape overlap as above,
thus approaching or possibly even achieving a true "butt edge"
wrapping with zero overlap, produces a very rnuch smoother
outer surface of the spiral wrap. A single layer of this
spiral wrap with a continuous under-7..ayc~r may be useful for
some purposes, depending on what further materials are to be
applied, but it is preferred to use an under--layer comprising
a second tape of inorganic fire-resistant material spirally
wrapped as aforesaid. Preferably, adjacent turns of the
underlayer tape overlap one another by less than 20%, (more
preferably less than 15%), of the second tape width.
For avoidance of doubt, it is too be noted that the
-
27065-338
CA 02194993 1997-O1-13
term "spiral" is used herein in the sense of winding
continually and advancing as if along a cylinder, the cylinder
being in fact represented by the conductor in. this invention,
which is preferably of substantially circular cross-section
although other cross-sections are not necessarily excluded.
The term "tape" is used herein to refer to any
elongate body of material capable of being spirally wrapped
around a wire conductor:, without limitation to any particular
format, construction or materials except as specifically
stated herein.
This invention is especially useful for wires having
a coating of organic polymer material of: thickness less than
0.8 millimetre, preferably less than 0.6
- 4a -
27065-338
219499~i
W096I02920 PCTIGB95101638
S -
mm, more preferably less than 5 mm, and especially less than l7.4 mm,
overlying
and preferably in contact with the outermost surface of the said wrapped
tope(s).
Preferably, the adjacent turns of the underlayer tape overlap one another by
not less than 2 % , preferably not less than 4 % , more preferably not less
than 8 i~ , of
the respective tape width.
Although two or more tapes of different materials and/or widths andlor
weights per unit area may be useful for some purposes, it is generally
preferred to
use two (or possibly more) tapes which are of similar width and thickness,
preferably substantially identical. Preferably, the outer covering tape will
have
turns overlapping less than the turns of the underlayer tope(s).
The reasons for the clearly superior smoother wrapging of the present
invention are not fully understood, but it appears (without limiting the
invention to
this theory) that the preferred fibrous support of the covering tapes may be
able to
enhance the smoothness by providing a degree of "play" or looseness which
enables
the reduced overlaps to partially amalgamate or settle closely against (or
partially
"into") each other, thus relieving some of the overlap tension and unevenness.
The
conventional larger overlaps extending nearer to the central portions of the
tape are
apparently unable to do this. The preferred fibrous backing appears to produce
a
greater smoothing effect than is obtained by similarly reduced overlapping of
polymer-backed mica tapes. The aforementioned breakage of brittle fibres is
also
reduced, presumably due to reduction in tension in the overlaps according to
the
presentinvention.
In one preferred kind of tape for wrapping according to the present
invention, the fibrous support of each tape is an open-weave cloth, preferably
a
square weave having the weft fibres or filaments substantially normal to the
length
of the tape. In such open weaves for the purposes of the present invention
adjacent
fibres in the weave are preferably spaced apart by a distance at least equal
to the
average fibre thickness, more preferably at least twice the average fibre
thickness,
and preferably not more than 20 times (mare preferably not more than 10 times)
the
average fibre thickness. The preferred fibrous materials are woven glass
cloths.
It is especially preferred that polymer-backed tapes of mica paper or other
inorganic fire-resistant material be used as the covering tape, since they
tend to
produce a surprisingly smooth profile and may be superior to the cloth-backed
tapes
21~4g93 ~
W09610292t) , ' PCT/GB95101G3~
-6-
in avoiding defects caused by loose or broken fibres. The covering tape
hacking
preferably comprises a substantially continuous film of organic polymeric
material,
for example polyethylene: or polyester film. Tapes having polymer coatings or
films applied aver the aforementioned fibrous (cloth) backings may be
advantageous ~
in combining the preferable qualities of the fibre or cloth and the polymeric
(film)
backings.
..
Itt the two-(or more)-layered wrappings according to this invention, it is
preferred that the overlapping portions of the covering (outer) tape are
positioned
substantially mid-way between the overlapping portions of the underlayer tape.
The benefits of the present invention may be more clearly appreciated on
referring to Figures 1 and 2 of the accompanying drawings, wherein
Figure I shows schematically in partial section a two-layered reduced-
overlap construction according to this invention; and
Figure 2 indicates the smoother appearance of the wrapping in comparison
with the known result indicated in Sketch F.
In Figure 1, the wire conductor 10 is shown carrying a first spiral wrap of
glass-cloth-backed mica tape 20, the preferred open square weave of the glass
cloth
support of the tape being suggested by one illustrated strip 22. The tape is
wrapped
so that successive turns overlap by about 10% of the tape width (not to scale)
and,
as shown schematically, these overlaps 24 tend may to flatten slightly as
indicated
to provide a somewhat smoother wrapping. Larger overlaps may also be
acceptable
in this under-layer. A second covering wrap of a similar mica tape 30 is shown
having its overlaps of about 2'~ of tape width positioned in the preferred
arrangement mid-way between those of the underlying tape 20. These minimal
overlaps tend to produce noticeably smoother wrapping, as aforesaid.
It is a great and unexpected advantage of the smoother wrapping achieved
by the present invention that heavier tapes can be used to achieve a given
degree of
fire resistance With effectively only two layers of the tape material, where
four
layers (two tapes at maximum possible overlap) were required before. For
example, two glass-cloth-backed mica tapes of 80 glmi weight wrapped as shown
in Sketch D provides a coverage tending towards the theoretical 4-layer
maximum
of 320 glmz, This maximum level of coverage is actually achieved at paints
where
W09Gl02920 PCT/GB95/O1G3$
-7_
four layers of the tape material are superimposed, but the average coverage
over the
whole wire will be somewhat less due to the gaps which occur as hereinbefore
described.
It has been found that two glass-cloth-backed mica tapes of 120 glmt weight
wrapped as shown in Figure I can advantageously achieve substantially the same
level of fire protection with the coverage tending towards the theoretical
minimum
of only 240 g/m2 (slightly more on average over the whole wire, due to the
small
overlaps). Furthermore, when a splicing tape 40 occurs on the mica tapes, it
appears to benefit to some extent from the "amalgamation" effect of the
reduced
overlaps according to the present invention, as indicated schematically at
splice
overlaps 42 and 44, thus reducing substantially the problematic splice-tape-
thickness-doubling effect illustrated in Sketch G.
As a result, a thin (0.35 mm) coating of polymeric insulation 50 can be
extruded by methods known her se over the wrapped tapes 20 and 30 with only a
relatively small and acceptable thickening 55 occurring at the splice. A
thinner,
smoother wire is thus produced according to this invention, with improved
production efficiency, to meet the same fire-resistance standards which
previously
required much higher percentage overlaps of lighter tapes and/or thicker
polymer
coatings.
Fire resistance may be measured on ftre test samples consisting of 2 of
mica-wrapped wires with no additional polymer insulation twisted together with
an
overall copper braid applied. The ftre tests are performed according to IEC331
(published by International Electrotechnicai Commission, 1 rue de Varembe,
Geneva) with the flame enhanced to 950°C ~ 50°C. Voltage is
applied to the
conductors with the braid connected to earth. Tests are terminated if the
cable
maintains circuit integrity for more than 60 minutes.
The present invention may be practised using any of the known materials
and production methods suitable for the production of insulated electrical
wires,
especially fire-resistant electrical wires. A preferred example will now be
described by way of further illustration of the invention.
Example
Description of Production of 1.5 mmz fire resistant wire
PCTdGB95101638~
wo vsdoavao c~ ~.~.~ (~. f~
_ g _
I) Conductor = 7 strands of 0.52 mm tin plated copper conductors assembled
into a round circular conductor to a nominal diameter of 1.50 tntn.
2') Conductor wrapped with 2 layers of glass cloth backed mica tape with a
mica content of 120 glmz. Total tape thickness 0.12 mm. Both tapes 5 mm
overall ,
width and applied in the same direction with the same nominal pitch (advance
per
revolution}.
3) Wrapping perforated on an industrial cable wrapping machine equipped with
2 concentrically mounted tape application heads.
ø) Wrapping machine set up to give the same speed and direction of rotation to
each application head of nominal 9.9 trim pitch and a left hand wrap
direction.
5) Adjustment to be made between the two heads to ensure top rage is formed
with the correct registration to the lower tape.
6) Overall diameter of the wrapped assembly is 1.93 mm.
7) Tapes applied to give an approximate butt edge to the top tape with a
typical
0 overlap to the bottom tape.
g) Splices made in the tape to be performed using a suitable thin adhesive
splicing tape applied to the glass side of the tape with care that the
splicing tape
does not protrude at the sides ofthe mica tape.
9} Splices in each tape to be staggered so as not to coincide.
10) Wrapped conductor so produced can then be insulated using standard wire
insulating lines.
11) A tube extrusion or similar technique (known per se) is used to enable the
slight diameter increase caused by splices in the wrapped product to be
accepted.
12) Using above technique it is possible to insulate the wrapped conductor
with
a thermoplastic halogen free insulating compound (known per se).
WO 9G102920 PCTtGB95l01638
i3) Extrusion to a wall thickness of 0.35 mm is quite practical of such a
material over such a wrapped conductor.
14) Diameter increases due to tape splices will produce an increase in
insulation
diameter of the same order as of the wrap itself. These splices need not be
removed as fire resistant properties and insulation electrical and mechanical
properties are not impaired.
15) Insulated wires so produced may be constructed by known methods into
multicare electrical cables consisting of further extrusions and possibly
steel armour
layer as required.
16) Well designed cables so produced will be capable of withstanding fire
resistance (circuit integrity) as defined by IEC331 with flame enhanced to
950°C.
