Note: Descriptions are shown in the official language in which they were submitted.
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This Invention relates to sheathed cables of the fire-
retardant type.
~ ire-retardant cables are cables designed to meet
stringent requirements concerning flame propagation as well as
smoke and acid gas evolution. Up to now, the requirements have
been adequately met by covering sheathed cables, either smooth
or corrugated, with a flame retardant poiyvinylchloride (PVC)
jacket. The above coverings adequately meet the requirements as
far as flame propagation is concerned but emit a significant
amount of smoke and acid gas when burning.
It is therefore the object of the present invention to
provide a fire-retardant cable having not only limited flame
propagation characteristics but also low smoke and acid gas evo-
lution during burning.
The fire retardant cable, in accordance with the in-
vention, comprises at least one insulated conductor, a metal
shield surrounding the conductor or conductors, and a protective
nylon sheath having a thickness between 10 and 25 mils covering
the metal shield.
The metal shield is corrugated or smooth and made of
copper or aluminum.
The nylon covering is normally applied by vacuum ex-
trusion using any conventional cable extruding equipment.
The invention will now be disclosed, by way of example,
with reference to the following examples:
Example 1
Two types of nylon were extruded over a l-inch diameter
corrugated aluminum sheathed cable. The first nylon material was
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a 6 12 nylon CDupont's Zytel 153HS) which was vacuum drawn over
the convolutions of the cable to produce a tight fitting jacket.
The second nylon material was a plasticized nylon (Dupont's Zytel
69) which was vacuum extruded over the ca~le~ The thickness of
both nylon sheaths ~as about 15 mils.
To evaluate the cable for flammability and smoke evolu-
tion, the two cables were subjected to the Ontario Hydro Vertical
Tray Fire Test according to Sp~cification L-891SM-77. This test
is done to ensure that cables will contribute little to the pro-
pagation of a fire. In this test, six cable lengths are placed
in a cable tray twelve inch wide, the cable tray vertically moun-
ted, and a 70,000 BTU/hr strip burner flame is applied to the ca-
ble for 20 minutes. The extend of charring must be less than 150
cm after the burner has been removed. Both cable constructions
burned to approximately 75-~0 cm above the face or leading edge
of the burner. The flexible nylon, Zytel 69, showed a slight
tendency towards greater burn and slightly denser smoke but both
cables could not generate enough heat of combustion to sustain
the burning of the cables to the top of the tray. Tables I and
II detail the results of the fire test for both constructions.
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TABLE I
Jacket Zytel 153HS
Vertical Test F~re Tray
TIME OF BURN OBSERVATIONS.
(min.~
1 No propagation
2 No propa~ation - flame height 30 cm
3 Cables are starting to burn ~ flame height 75 cm
4 Flame height still 75 cm
Still no further propagation - low smoke emission
6 No propagation past flame source
7 Ditto
8 Ditto
9 Ditto
Ditto
11 Aluminum armour burst on one cable
12 Slight re-propagation ~ flame height 75-80 cm
13 Two cables on left side of burner face burning on
their own - no increase in smoke emission
14
Propagation subsided
16 No change
17 Cable on ext~eme right of ~urner burst
18 Same as before
1~ Same
No flame propagation past flame source.
Am~ient Temp C 13 C
Flame Spread ~cm~ 73 (on 2 outer cables~, 66 on
other 4 cables
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Char (cml No char
After Burn Cmin.~ None
Smoke Prod, Minimal~slight greyish smoke
Relative Humidity 46
T~BLE II
Jacket Zytel 69
_rtical Test Fire Tray
TIME OF BURN OBSERVATIONS
(min.)
1 Cables are just commencing to burn - Flame height
30 cm
2 Flame spread to 60 cm - cables are burning on their
own above point of flame source
3 - Propagation 75 cm
4 No change
Propagation of flame subsided ~ smoke production
minimal
6 No further propagation of fire
7 No change
8 No change
9 No change
No change
11 No change
12 2nd cable on tray from LHS burst - slight increase
in burning
13 Burning of exploded cable subsided
14 No furth.er increase in flame spread
~,/ 15 3rd cable on tray from LHS of burner burst (alumi-
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num armour is beginning to melt~
16 3rd cable on tray from RHS of burner burst
17 Slight increase ;n flame height due to burning ga-
ses from interior of cable
18 Burning has subsided again
19 No change
No change - test terminated. -
Ambient Temp C 14~0
Flame Spread (cm~ 80
Char (cm) No char
After Burn (min.~ None
Smoke Prod. Minimal but slightly more dense
than DuPont 153 HS (6-12 Nylon).
Relative Humidity 46%
The UL-83 Steiner Tunnel Test ~ULC-S102-1978) was also
performed on both cables and the results are tabulated in the
following Table III:
TABLE III
UL83 Steiner Tunnel Test
Properties Zytel 153HS j Zytel 69 Spec.
Flame Spread 10.3 15 25 max~
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Smoke Developed 20 25 50 max.
Remarks/Observation 1. Drip at 45 sec 1. Some bur-
2. Some burning ning on the
on the floor floor
3. Metal Covering 2~ 2 out of
burst at 8 min. metal cove-
rings burst
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The above results indicate that ~ot~ constructions
pass, with the Zytel 153HS jacketed construction sho~ing supe-
rior fire resistance and less smoke development. Criteria for
passing flame spread and smoke development specifications (25 and
50 respectively in Table I~I~ are outlined in the Canadian Buil-
ding Code, 1975.
A 30 mil nylon jacket was also extruded over a 1" dia-
meter corrugated aluminum sheathed cable. The nylon material was
Nylon 6-12 (Dupont's Zytel 153HS~. The cable was evaluated for
flammability and smoke evolution following the above Vertical
Tray Fire Test. The nylon restricted the fire for approximately
5 minutes; however, within 12 minutes, the entire tray of cables
was consumed indicating that the nylon is not self-extinguishing
under these conditions. It has been found that a thinner wall
jacket of less than 25 mils contributes less fuel to the fire al-
lowing the cable to extinguish itself due to the heat sink effect
of the shield upon which the nylon jacket is extruded. Therefo-
re, the thickness of the nylon jacket must be kept below 25 mils.
The physical properties of the above two nylon jacketed
cables with a wall jacket of about 15 mils were compared to a
conventional PVC jacketed cable having a jacket thickness of
about 45 milsl and the results of the test are given in the follo-
wing Table IV:
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TABLE IV
Physical Properties of Shielded Cables
~.
_ Zytel-153HS . 2y~el 69 PVC Spec~ C22.2
(CSA)
Tensile Strength psi (MPa~ 8800 C60,7) 4700 (32.4) 2100 (14.5) 1800 (12,4)
min.
Elongation % 340 330 350 250 min.
Cold Bend at -40C Pass Pass Pass No cracks
Cold Impact at -40 C Pass Pass Pass No cracks
Cold Impact at -50C No cracks 2 out of 5
cablescrack _
Fast Flex or Bend at -40C No cracks No cracks No cracks
Abrasion CSA
1. Cycles to Wear Through 200 370 540 _
2. Wear through per mil 12 15 12
Janco Abrasion*
inches per mil 1.7 6,3 2,2
Cut Through Resistan~e
Speed 0.2 cm/min
Blade 43 cutting edge
: Off Cable**-Peak (Ne~tons~ 2574 1188 146
Valley CNewtons~ 1134 1462 382
Peak - Newtons/mll 151 50 3
Valley - Newtons/mil 67 61 4
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*Abrasive tape moves over the tested surface under the pressure
created by weight of 4.25 pounds.
: "Inch~s per mil" refers to the length of the abrasive tape which
needs to pass to abrade one mil depth of sample.
**Of~Cable - Peak, Valley" means sample cut from cable at peak or
valley of the corrugated shield.
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The reduced wall thickness C15 mils ~or nylon as com-
pared to 45 m;ls for PVCI vasl~ improves the handling of the ca-
ble, stripping and its performance at low temperature. While
both cable constructions meet CS~ cold bend and impact tests at
-40C, they also perform well in the non-specified fast flexing
or bending test at -40C which is a measure of their performance
that could be encountered in the field. To differentiate bet-
ween the two nylons, impact tests were performed at -50C with
results indicating that the 6-12 nylon (Zytel 153HS) is supe-
rior with no cracks shown. Stripping of the jacket, an impor-
tant criteria for handling, was evaluated in the field by elec-
tricians using PVC jacketed cables r Their reaction was that there
was no problem in stripping either nylon jacket in comparison to
the normal PVC jacketed cable.
Abrasion tests were performed on the nylon and PVC
compounds to determine at what wall thickness the nylon is compa-
rable to the PVC compound. CSA abrasion tests yield similar re-
sults for related compounds when expressed as wear-through cy-
cles/unit thickness of jacket. However, the idiosyncracies of
each test and material must be more fully explained. The PVC
clogs the sandpaper in the abrasion tester, unllke the hard 6-12
nylon, thus reducing the abrasiveness of the paper causing a
falsely good réading. This argument also applies to the soft
nylon (Zytel 69) where in this case the nylon lubricates the
sandpaper producing better results than anticipated. For con-
trast, the hard nylon appears to wear through quickly because it
does not remain in the interstices of the sandpaper surface.
In the case of the Janco Abrasion test, cables jacketed
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with Zy-tel 153HS again appeared to be quite infexior to both PVC
and the flexible nylon~ When expressed as inches of tape to wear
through a given thickness of jacket, the nylon does show similar
performance to PVC. The results are most unexpected, and to ex-
plain this phenomenon, the lubrication of each compound must be
considered. However, according to the test, equal jacket thick-
nesses are required to produce equal performance upon abrasion.
Both nylons are far ~uperior to PVC in resistance to
cut through. Zytel 153HS has superior resistance to cutting over
the plasticized nylon.
In conclusion, the above tests have shown that the
physical performance which includes flexibility, handling, strip-
ping plus bending and impacting at low application temperatures
has been improved with the use of a nylon jacket having a thick-
ness in the order of 10 to 25 mils as compared to a PVC jacket
having a thickness of 4S mils. When compared to the current
construction of regular shielded cable with a PVC jacket at wall
thickness of 45 mils, a cable jacketed with nylon Zytel 153HS at
wall thickness of 10-2S mils performs favourably in regards to
physical properties with the exception of abrasion resistance.
In tests performed using CSA and Janco apparatus, the Zytel 153HS
appeared inferior when comparing cable performance. On an equal
thLckness basis, the PVC compound and nylon are comparable in
abrasion resistance. As explained above, there appears to be
subtle reasons why these unexpected results occurred.
Both nylon jacketed cables meet the requirements of
the 1975 Canadian Building Code for the UL Steiner Tunnel Tests,
fire spread rating and smoke rating. Zytel 69, a plasticized
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nylon, gave slightl~ inferior results than Z~tel 153HS, a 6-12
copolymer, ~ut this may be attributed to a slightly higher wall
thickness. In previous tests, shielded cable with the PVC
jacket could not meet the smoke requirements of this stringent
test.
Another important feature of the present invention is
that nylon does not evolve any acid gas when burning.
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