Note: Descriptions are shown in the official language in which they were submitted.
: ~587~5
This i1~vention is concerned ~rith electric
- cables having good fire resis-tant properties.
A known class of fire resistant cables is
the so-called mineral insulated cable ~hich contains no
organic material at all. This type of cable has been
; de~eloped to withstand heating to 800C for 4 hours
without its func*ioning being adversely affected~ It
is ~ell known, howe~er, that ~ineral insulated cables
; need speci~l equipmen-t for their installation and the- 10 formation of terminations wlth such cables is very time-
consuming~
l~e ha~e now developed a multi-core electric
cable that is easier to install than mineral insulated
cable and which meets the major operating requirements of
the latter as a wiring cable. In particular, during laboratory
tests, the cable we have de~eloped has continued to function
after being heated for 4 hours at oO0 C, though it is
damaged by such -treatmen*.
According to the present invention, there is
provided a multi-core electric cable, which comprises two
or more electric conductors, each of which is insulated
with a coating of silicone rubber insulation1 a bare earth
wire, and a sheath consisting of a laminate of a thermo-
, .
~ - plastic polymeric composition~aluminium or copper foil and
,i
an extruded sheathing of a fire retardant polymeric com-
~i position around the laminate, the sheath enclosing the
insulated conduc~tors and the earth wire with the metal layer
1 of the laminate on the inside and in contact with the earth
- ~ wire throughout the length of the cable.
The metal layer constitutes an efficient electro
',
2 -
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s
static screen l~hich is particularly advantageous where
these cables are installed near electronic equipment,
such as computers.
The cable construction according to the in-
~ention is suitable for cables in which the cross-
sectional area o~ each conductor is up to 4 square milli-
metres and which comprise from 2 to~19 separate conductors.
By way of example only, this construction may be used for
2 and 5 pair 0~5 mm telephone and signal cablcs for use
in a high fire risk area, such as a power station.
Silicone rubber compositions su1table for use
AS electrical insulation and adapted for coating on con-
ductors by extrusion are well know~ and commercially
available. An~ o~ known compositions of this kind may be
used in the cable according to the in~ention.
The earth wire is preferably formed of tinned
copper. It may be laid up helically with the insulated
conductors or laid straight, in each case in contact with
- the metal la~er of the laminate.
Suitable thermoplastic polymers for the polymer/
aluminiu~ or copper laminate are, for example, polyvinyl
chloride (pvc)~poly~in~lidene chloride, polyethylene and
polypropylene, of which polyvinyl chloride is preferred.
Such laminates are either known in the art and generally
available or can be produced in the same ~ay as the l~nown
laminates; preferred laminates are those having a metal
layer having a thickness of 100 to 250 microns. To form the
sheath around the core consisting of the insulated con-
, ductors and the earth wire, the polymer/metal laminate,
in the form of tape having a width somewhat greater than
- ~ - 3 -
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l~SI~ 5
the circumferencc of the core, is fed longitudinally on
to the core and is formed (by suitable guide surfaces)
around the core so as to encircle the latter completely,
with onc edge of the laminate overlapping the other and
forming a longitudinal seam along the length of the
cable. The ,outer sheathing of ~ire retardant polymer
composition is then extruded on to the outer surface o~
the laminate.
- The outer coating may be formed of any fire
retardant pol~ner composition~ The polymer present in the
composition may, for example be PVC, polyvinylidene
chloride, polyethylene, polypropylene or a thermoplastic
; rubber, and the polymer may be cross-linked by any suitable
means. The composition contains one or more fire retardants,
the choice of fire retardant being mainly dependent on the
particular polymer present. Suitable fire retardan-ts are,
, ~or example, chlorina-ted paraf~in, antimony trio~ide,
~ . ~
'' magnesium carbona*e, zinc borate and aluminium hydrate.
~ Suitable compositions of this kind are kno~n in the art
-' 20 and generally available.
We have found that the combination of silicone
rubber insulation for the conductors and a sheath of a fire
retardant thermoplastic polymer, such as PVC, ~hich has an
~ internal liningof me~al~the metal lining being in contact
3 25 with a bare earth wire which forms a part of the core,
;~ provides a cable having a favourable com~ination of
~ properties. Thus the cable is mechanically robust so that
`I,it is suitable for surface mounting and _ __
~S~37~5
i~ easy to in~tall. It also continues to ~unction9
even though damaged by very severe overheating.
Cables in accorda~ce with the invention ha~ing
from 2 ~o 19 cores and with ~he individual conductors
having cross-sectional areas of from 0~5 to 4.0 ~m 9
were manu*actured from the following l~aterials:
Conducts~r~- Plain annealed copper
complying with BS 6360
Non-insulatea earth con- - Tinned annealed copper
tinuity conductor~compl~ing with BS 6360
~ 10
Core insulation- Silicone rubbsr complying
with BS 689g 1969
:~ PVC-coated alu~inium foil - A~nealed aluminium strip
: with a film of PVC securely
bonded to one surf`ace. The
metal strip was annealed,
commercially pure al~minium
having a thickne~s of 0.150
mm ~ 000125 ~m. The
weight of the PVC layer was
2 gm per s~uare me~re with
a tolerance of - 0.5 gm +
.1 gm per square metre.
:;
PVC outer sheath- Type 2 PYC c~mplyi~g with
. - BS 6746 1969.
. ' ' ' . '
`' 20 The make up, dimensions and weights of the
cables that were manufactured are shown in Table 1
below.
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~51~5
____ _ ________~ _
. ~ ~ U~ ~ o U~O o o o o U~ U~O U~ o o
o ~ ~ ~a~ o ~ o ~--~ o ~oo ~ ~ co 1-- ~ ;r
¢
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Cl ~ ~ .~ ., .,
o
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~ ~o o o o o o o o o o o o ~ o o o o
m ~
o
h
~ ~ ~ a. . . .
~I O oq C~
_
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O . ~4 ~
Z; ~ ~ ~. ~, ~ . .... . O ... ~ .... . .
.~d ~ ~ OOOO OOOO OOOO OOO O O
. H .
C~ H
_
, ~1 ~ . ~ ~0
CO ~ ~CO U~ ~00 U~ ~CO o~ ~
.~1 .~ ~ ~ .~
,Q ~ ~ ~ ... ... , . . ~ 1 . ...
S~ ~ X ..
.,
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~ ~ ~~0 co u~ ~ 0 oo ~ ~ 0 co ~ ~ co co c~ co
o ~ ~ e1 ~ 1~ N I t`~ [' C'3 ~ ~ r` ~ 1 ~\ r~ 1"
r1 S~l ~
O
Gq t~ O
O ~ X.i .
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~ O U~ 0 0 It~ 0 0 i~ 0 0
r~
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Lengths of cer*ain of the above-dePZcribed cables
were subjected to the following tests.
Fire Resistance
.
A gas flame at a tempe.rature of 80oc was
applied to the cable with working voltage applied to
the cable~ After six hours, there wa~ no electrical
breakdown.
The PVC sheath ~180 meets the requir~ment~ of
BS 4066 for flame re*ardant cables~
AnQther flre resistance test i~ defined in
C~E~G~Bo Specification 09990~ which requires the cable
to be su~jected to a gas flame a* a temperature of
1000C and to continue to operate for a minimum of twenty
minute~ZZ. Application of this test to the cable of the
invention showed that after one hour at this temperature~
the cable wa~ still functioning satisfactoriiy~
i
i ~ ~ Mechanical
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' ~ 20 The ca~le wa~ bent through 180 on a mandrel
; ten time~ the diameter of the cable, straightened and
then bent through i80 in the opposite direction. This
procedure was repeated five times; no kinking or creaq-
in f tha outer ~heath de~eloped.
, 25
' Volta~e
Z ~ The breakdown voltage o~ samples of cahle~
according to the~lnvention (having 2 cores ~ith a nDmlnal
cross-sectional area of 2.5 mm2 and having 3 cores ~ith a
nominal cross-~Zect1onal area o~ 1~5 mm2) wa~ determined
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I ~ - 7
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follot~ing the bending test described aboveO The result~
are given in Table 2 beloW.
In the case of the 3 x 0.15 mm2 cable, failure
took the form of flashover at the endsO The sheath of
a further s~mple was then ~tripped back 150 mm to elim-
inate end flashover and the breakdown voltage again
deter~ined. The results are gi~e~ in Table 2 below.
: Table 2
io _ _ ~
Si~e mm Core colour Volt~e at Form of
. kV failure
_ _ _ ~_
,After bending
: 2 x 2.5 Red 11 Breakdown
A 15 Bla~k 11 Breakdown
3 x 1~5 Red 11 Fl~hover
. Yellow 12 Fla~ho~er
~` Blue 13 Breakdown
With 150 mm tails :
3 x 1.5 Red 16 Breakdo-~n
. Yellow 18 Breakdown
. . . . Blue . _ _ Bre akdown
These ~oltage~reakdown strengths give a very
good margin of ~afety over voltage surges from contactor~
and fluorescent tubes which may well be of the order of
5kVo
, .
, : ~oisture Resistance
, .
3~ The breakdown v~ltage of lengths of cores of
,,
5871~ii
certain of the above_described cables were determined,
both before and after the bending test described above.
Further samples of the same core~ were immersed in water
for 24 hours and the breakdown voltage then determined.
The results of these tests are gi~en in Table 3 below.
Table 3
_. _ , ~ ~
Description 3 x 1.5 mm2 2 x 2.5 mm 2 x ~ mm . :
_ , ,, _ _
Bre~kdown voltage
lOkV lOkV lOkV
of cable/earth
Breakdown ~oltage . lOk~ ; 10kV lOkV
after bending test .
Breakdown voltage .
on core immersed lOk~ lOk~ lOk~
in water at 20C
for 24 hours ~ . - _ _
Lengths o~ the cable were stored in containers
; so as to maintain a 100% relative humidity condition in
20~ the cable. After ei$ht weeks, the cables were opened
up and no corrosion of the Rluminium foil or the bare
earth conductor was found.
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