Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
8~55
This invention rela~es to electric cables and more
particularly to cables with polymeric insul.ation, sheathing or
other components that present a low hazard under fire
conditions. Cables that present no significant primary fire
hazard, in the sense that the insulation cannot be ignited by
the effect of an electrical fault in a properly protected
circui-t, are now readily available. There is however a risk
that polymeric materials will burn if pre-heated -to a high
temperature by an external source, such as an external fire,
10 with the disadv~ntages that: (i) in certain circumstances fire
may be transmitted along ca~le runs to other parts of a
building; (ii) dense smoke may be generated; and (iii~ since
P~C and/or other halogen-containing materials are commonly
present toxic and corrosive fumes ~such as hydrogen chloride
gas and/or hydrochloric acid droplets) may be produced.
In accordance with the present invention, an electric
cable comprises at least one conductor with insulation of
polymeric material, a bedding layer surrounding the insulated
conductor and made from a composition comprising a curable
20 alkene homo-polymer or copolymer, at least 55% of inert mineral
filler, a low smoke plasticiser, a curing agent for the
polymer, and an antioxidant, and an enclosing sheath of low
flammability material. Preferably all the components are free
of halogen-containing materials or at Least substantially so.
I~ there is more than one conductor one (or
exceptionally more than one but not all) of them may be bare.
If the electrical and physical performance
requirements of the cable allow it the insulation ls preferably
also of curable, mineral-filled, plasticised alkene homopolymer
30 or copolymer composition, but the proportion of filler that is
appropriate will be lower than for the bedding. Preferably the
weight of filler is not in any instance more than 55% of the
-- 2 --
whole composition of the insulation.
The beclding cornposition will commonly include up to
80~ ot the filler ancl may include even more. A preferred
filler is hydrated alumina o suitable particle size, used
alone or mixed with calcium carbonate and/or china clay; a
proportion of pigment can be included especially in the
insulation where it can be used for colour coding. Preferred
pol~ners fox the bedding are the curable ethylene-propylene
copolymer rubbers (EPR) and ethylene-propylene-diene
10 terpolymers (EPDM). Preferred plasticisers are polyisobutylene
and paraffinic resins, oils or waxes whi~h may advantageously
be used together. A preferred range of compositions comprises,
in parts by weight:
Polyrner: curable alkene polymer 15-35
Filler: alumina trihydrate with or
without calcium carbonata and/or
china clay 55-80
Plasticiser(s): polyisobutylene and/or paraffin
wax 7-20
Processing aid: stearic acid and/or inorganic
stearate up to 5
20 Curing agents and antioxidants:up to 5
The sheath is preferably of suitable polymeric
material, and may be of the same composition as the bedding,
but materials of higher oxygen index are preerred: ethylene-
vinyl acetate copolyrners ~EVA) compounded with at leas-t 5S% of
an inert mineral Eillqr and other conventional ingredients,
with or without curing agents, are very suitable.
`'~;~'"' ' :
Similar compositions based on mi~tures of EVA with EPR or EPDM,
and silicone rubber and flame-retardant grades of cross-linked
polyethylene can also be used.
A ma~or improvement in properties can be obtained by
interposing a heat-barrier layer of suitable low-~lammability
material between the bedding layer and the sheath. Suitable
materials include:
(1) flexible metal tape or foil;
(2) heat-resistant plastics tape, such as polyimides
(e.g. those sold under the trademark KAPTON); and
(3) fibre-reinforced bonded mica tape.
The heat-barrier also offers some resistance to emission of
volatile materials or smoke from the interior of the cable.
For some types of cable additional components of a
conventional kind, for example wire or tape armour and/or a
metal sheath, ma~ be included, for example between the parts of
a subdivided bedding layer and/or between the parts of a sub-
divided sheath.
EXAMPLES
-
1. Tinned copper wires each of 0,85 mm diameter
are insulated with 0 6 mm radial thickness of a conventional
insulating compound (hereinafter called compound EP~ 1)
compri.sing:
~thylene~propylene/diene monomer rubber (~PDM) 30-35%
China clay 50%
Paraffinic Plastlcisers 10-15%
Curing agents and antioxidants 5%
(all percentages are by weight of the compound)
355
, .
~ Pairs Or these insulated wires are twisted together with a
¦ right-hand lay o~ about 40 mm~ and seven such t;wlsted pa:crs
¦ laid up together with a le~t-hand lay of about 100 mm, An
extruded bedding layer with a nominal radial thickness o~ ] mm
encloses the laid up cores, this bedding layer is of the
following compound (compound EPR ~J:
EPDM 22%
Hydrated alumina (nominal particle si,~e 6
1 micrometre) 2%
Plasticisers: Polyisobutylene 7.7~J ll
Par~ffin wax ~.3%) ~
Processing aid:Stearic ac-Ld 1%
` Convent;onal curing agents and antioxidants l!~o
Over this bedding ~s applied a copper tape screen~made of two
apes each (nominally) 18 mm wide and 0~1 mm thick, breaking
joint with the edges of each tape nominally abutted. A second
bedding layer ~.5 mm thick and made of compound EPR 2 is
extruded over the screen.
:
A heat barrier is next formed by lapping on tw~
silicone-resin bonded woven-glass reinforced mica tape,s. Eaoh
tape is 0.].5 mm thick and ~5 mm wide and applLed with its edges
~,l nominally abutting, khe two tapes breaking joint by half the width
of ~he tape~
An outer sheath of a standard nylon sheathing compound
~Grilon ~ype 6J with a radial thickness o~ ~.4 mm completèS t~e
cable~ ~hich has an overall diameter o~ 16.5 mm,
2, This is the same as example l except that the n~rlon
sheath is replaced by a she~h 1,0 mm thick (increasing the `
overall diameter to about 17.5 mm) of the following compound
* ~ r~
. .:,
_ 5 w
. : ~
.' ~
ss
j (Compouncl EVA lJ:
¦ 60~o Ethylenc - I~O~o Vln~l a&etate copolymer2470
Processing aid: Stearic acld 2%
Hydrated alumina ~nominal particle size 0%
. 1 micrometre) 7
Antioxidants (and curing agents if desired) 1~%
3. This cable is the same as examples 1 and 2 llp to
and including the first bedding layer Over this is applied
a single glass/mica tape of the same kind and size as in the
preceding e~amples but applie~ with 20~ overlap. Two copper
kapes (also of the same dimensions as in the preceding examplesJ
are applied directly over the glas~s/mica tape~ without a second . .
bedding layer. This cable is completed by an outer sheakh of~a
conventional commercially-available flame-retardant silicone rubber
j ~ compound ("Silastic(~ 603"J with a radial thickness o~ 1.8 mm
¦ (overall diameter abou~ 16 mm).
4, The conductors, insulation and strandin8 of this
: cable are as in example 1. ~ bedding layer of compound EPR 2
is extruded over the laid-up cores to a radial thickness of 5 mm.
Two "Kapton" (krade mark) polyimide tapes~ o.o8 mm thick and
nominally 30 mm wide, are lapped on to the bedding la~er,
breaking joint. A sheath, 1.8 mm thick, of khe same sil-icone
¦ rubber compound as llsed in example 3, oompletes a cable with
an overall diameker of about 23;7 mm.
5. The aores of khis cable are screened concentri.c pairs,
. wlth a tinned copper inner conductor 1.53 mm in diamete:r, inner
dielecbric of compound BPR 1 with a radial thickness of 1 mm,
~¦ outer oonductors made of thirt~-five 0.3 mm diameter tinned
I - 6 - :1 ,, ~ . ~ `
,
copper wires lapped on wi~h a lay o~ around ~8 mm, outer
dielectric also of compound EPR l and also ] mm thick~ and a
braided screen made up o~ tinned copper wires each o.]5 mm in
diameter, applied 16 spindles 4 ends with a lay o~ 11 mm, two-
over-~wo-under-two.
Seven such cores are laid up with one axial core
and khe remaining six surrounding it and having a right hand
lay of about 230 mm A bedding o~ compound EPR 2 is extruded
over ~he laid-up cores and has a radi21 thickness o~ 2 5 mm.
~wo layers o~ the same polyimide tapes as in example 4 are
lapped in the same malmer on the bedding, and an outer sheath
of the same silicone rubber compound as in examples 3 ~nd 4,~
with a radial thickness of 2 mm, completes a cable with an
overall diameter of around 30 mm.
6. m is is exactly the same as Example 5 except that
the polyimide tapes are omitted, slightly reducing the overall
diameter.
7. m is is exactly the same as Example 5 except that
the silicone rubber sheath is replaced by a sheath of compound
jh
EVA 1 with a radial thickness of 1~5 mm (overall diameter 29 mm).
8-11. These are exactly the same as Examples 1 - 4
respectlvely except thak the solid wire conductors are replaced
by tinned copper strands~ made up o~ seven wires each 0.3 mm
in diameterJ the radial thickness o~ t,he insulation is 0.8 mm,
and the overall dimensions are correspondingly increased.
12-13. These are the same as Examples 8 and 9 respectively
except that the sheath is o~ standard nylon sheathing compound~
0.4 mm thick (as in Example 1).
. . . ..
1 , , ''
~ . ..... ,,.. ~
~- : .
.~ '
14. Th:is is -the same as Example 2 except that the
heat barrier is formed by -two "~apton" (trade mark) polyimide
tapes, 0.08 mm thick and nominally 30 mm wide, lapped onto the
bedding layer, breaking joint.
The accompanying drawing illustrates the invention
and is a diagrammatic cross-section through a cable having a
core 1 comprising at least one conductor 2 with insulation 21
of polymeric material, bedding layer 3 of the composition
defined, and a sheath 4 of low flammability material.
.
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