Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
MJP/8519358
TERMINATIONS FOR MINERAL INSULATED ELECTRIC CABLES
This invention relates to terminations for
electric cablss of the kind having a metal sheath and
insulation of compacted mineral powder which fills the
sheath (mineral insulated cables). Such cables are
inherently resistant to heat and to fire exposure and
will continue to function unless the metal sheath is
melted or destroyed by oxidation; for example, a
mineral insulated cable with copper conductors and
sheath will function for several hours at 1000C in air
of normal atmospheric composition.
However, the mineral insulation of such cables is
sensitive to moisture, and the terminations currently
used to provide a moisture-proof seal are much less
resistant to heat and fire than the cables are, and may
cause system failure when the cables themselves are
still in a serviceable condition.
The present invention provides terminations with
fire-performance characteristics comparable with those
of the cables.
In accordance with the invention, a mineral-
in~ulated cable termination including a sealing pot
secured to the cut-back end of the cable sheath and
enclosing the whole of the mineral in~ulation that is
exposed at the cut-back end of the sheath together with
an adjacent section of the, or each, exposed cable
conductor and filled with a permanently-pasty and
permanently-adherent sealing medium comprising a mineral
filler and a fluid organic binder is characterised by
the fact that the sealing medium i9 convertible to a
ceramic body on heating sufficient to pyrolyse and/or
volatilise the organic binder.
Preferably the pot i9 made of a material or
materials at least as heat-resistant as the cable, but
this is not essential as the ceramic body formed under
fire conditions may (in favourable cases) be adequate to
maintain a minimum level of insulation resistance even
though the pot has been melted or otherwise destroyed:
thus a conventional brass pot with an organic closure
disc may be acceptable. Otherwise a stainless steel or
copper pot with a ceramic closure disc is recommended.
The sealing medium must have a high adhesive
affinity for the metal(s) of the cable sheath and
conductor(s) as well as for the mineral insulation; and
it must be convertible to ceramic form at a temperature
above the maximum service temperature but not
substantially higher than the highest temperature which
the pot will withstand. For use with standard
mineral-insulated cables in which the metal components
are of copper, we prefer that conver~ion to ceramic form
is rapid in at least part of the temperature range from
500 to 800G.
Suitable mineral fillers include bentonite, talc,
kaolin and other clays, magnesium silicate, mica,
magnesium oxide, zinc oxide, glass, and mixtures of
these. Silica can be used in admixture with one or more
of these.
Since carbon-base organic binders are liable to
carbonisation in at least some high-temperature
conditions, we prefer to use a non-carbonisable silicone
binder and more especially those silicone polymers which
decompose in the solid state to leave a residue of
silica which contributes positively to the
ceramification process; these requirements are met by
the 'copolymeric siloxanes' in which there is a
structural framework consisting essentially of silicon
and oxygen atoms only with carbon-based side-chains all
attached v~a side-chain silicon atoms, and the
Applicants at present believe that this structure is
necessary among silicone binders. Suitable polymers can
be made by hydrolysis and co-condensation of a
tetrafunctional silane and a trialkyl monofunctional
silane, e.g. tetra ethoxy silane and trimethyl ethoxy
silane (or their chloro analogues) as more fully
described in UK Patent 2046283B (and see also US Patent
2676182). So far as the applicants are aware, such
polymers are not at present offered on the open market
as such, but they are believed to be made and used in
the manufacture of silicone adhesives and coatings.
Silicone binders suitable for use in the invention and
thought to be of this kind can be extracted from the
adhesives sold by Dow Corning Limited under reference
l t,
numbers 280A and 282 and the coating sold under the
7~r~ G~e ~1 a r ,4
de3ignation 'Toray Silastic TS1417'.
A proportion of a silicone, such as a silicone
fluid, which decomposes at least partly in the vapour
phase to give a powdery silica deposit may be present,
and is helpful in securing the required ambient-
temperature properties. Ordinary polydimethyl siloxane
fluids are suitable for this purpose.
The ceramifiable silicone adhesives described in
U.S. Patent 4255316 may be suitable for use as the
sealing medium of the present invention, or the
proportion of the ingredients may be varied to secure
better ambient physical and adhesive properties.
Example l:
Toray Silastic TS1417 appears to be a dispersion
of mica in a solution in xylene o~ a first silicone
polymer (polymer A) of the kind described in U.S. Patent
2676182 and a qecond silicone polymer (polymer B)
designed to flexibilise the coating (curing agents would
be added when the material is conventionally used).
The coating material (as bought and without any
curing agent) was centrifuged to separate the mica and
the resulting clear solution was mixed with a silicone
fluid sold by Dow Corning Limited under the designation
Silicone Fluid DC200/300,000 cs and with a filler-grade
talc (-200 mesh, less than 70~m) in the ratio of three
parts silicone ~luid and 60 parts talc to each ten parts
of total polymers A and B. The xylene was removed ~rom
the mixture by distillation at 160C to give a putty
similar in consistency and adhesiveness to conventional
mineral-insulated-cable termination sealants.
A 2.5m length of a 440/600V Z-core 3mm2
copper-conductor copper-sheathed mineral insulated cable
was terminated at one end with a conventional seal. An
experimental termination was made by stripping back 1.2m
from the other end, screwing on a conventional brass pot
and filling it with the putty just described. A closure
disc of hard silicone rubber was applied.
The cable was loaded at 500V d.c. and 250mA using
a load resistor and a 3-phase transformer, and voltage
withstand tests made at intervals by applying 2kV A.C.
~or 1 minute.
No significant change in electrical properties
were detectecl on heating the experimental termination
(without any gland or other protection) in a ~urnace at
920C ~or 30 minutes. On inspection a~ter cooling it
was found that the brass pot had melted but the putty
had been converted to a ceramic body with sufficient
cohesion to prevent the melted brass from shorting
across the conductors. (no impact was applied during
this test).
Example 2
In a fully-synthesised example, 15.6g of
tetraethoxy silane and 13.3g o~ trimethyl ethoxy silane
were dissolved in a mixture of 40 ml ethanol (industrial
methylated spirit, 'denatured' by addition of small
i.L~
-- 6
amounts of methanol), 10 ml water and 0.4 ml of lN
hydrochloric acid. The solution was refluxed ~or 8
hours and the solvents removed by evaporating under
atmospheric pressure at 100C. A polymeric silicone
condensate was obtained as a viscous liquid.
This was mixed with china clay powder (Grade E,
from ECC International Limited, predried at 200C for 3
hours) in the proportion 5 parts of the product, 8 parts
of china clay to produce a putty substantially
equivalent to the one prepared and used in Example 1.
Example~ 3-6
In each of these examples, 100 parts of Dow
Corning silicone adhesive 280A (55 parts solids) was
mixed with 27.5 parts Dow Corning slicone fluid
200/60,000 cS and with fillers as follows (all predried
at 200C for 3 hours):
Example 3: 132 parts china clay powder, grade E;
Example 4: 82.5 parts china clay powder, grade E
and 165 parts magnesium oxide (grade HMD5 from Steetley
Refractories Limited);
Example 5: 74.5 parts of calcined clay (sold
under the Trade Mark Polestar 501 by ECC International
Limited) and 149 parts of magnesium oxide, grade HMD5
Example 6: 66 parts of calcined clay (Polestar
51) and 132 parts of silica flour (grade 35/200S from
Richard Baker Harrison Limited).
Solvent was evaporated at 150C to give in each
case soft, sticky mastic putties. They were packed into
201~3-81~
standard mineral-insulated cable terminations, and each
passed the water ingress test of British Standard
BS 6081~
The terminations made with the putty of Example 4
passed the fire test according to IEC Specification 331
(3 hours at 750C in a gas flame, carrying rated voltage
while fused at 3A) and also withstood 15 minutes at
850OC in a tube furnace. In all of these examples, the
mastic putty was converted, on application of a flame,
to a ceramic body sufficiently coherent to avoid
conductor-to-conductor short circuits (but not to
maintain water resistance).
