Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF SUPPRESSING SUPERSATURATION IN UNDERGROUND ELECTRICAL
CABLES
Field of the Invention
The present invention relates to methods of enhancing
the dielectric strength of electrical distribution cables, and
more particularly, preventing supersaturation in large diameter
cables that are being treated with fluids for restoration of
dielectric strength.
Background of the Invention
It is a well-known phenomenon that underground
electrical distribution cables typically include an electrical
conductor surrounded by a semi-conducting polymeric shield,
which is then jacketed with a polymeric insulation jacket. The
polymeric insulation jacket may then be further layered with a
semi-conducting insulation shield, and finally, an outer
polymeric protective jacket is typically applied over the
insulation shield. The conductor may be stranded from multiple
wires, or less commonly a solid conductor core may be utilized.
It is a well-known phenomenon that after such electrical
distribution cables are buried in the ground for extended
periods of time, the polymeric insulation jacket of the cable
may undergo deterioration that reduces its dielectric
properties and can lead to failure. This situation, which is
particularly prevalent with polyolefin insulations, is referred
to as electrochemical tree formation, and is caused by the
diffusion of moisture into the polymeric insulation. This
process can greatly reduce the useful life of electrical
cables.
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As a result, techniques have been developed for treating such installed cables
with an anti-treeing agent that retards the entry of moisture into the
insulation layer.
A tree retardant or anti-treeing agent is typically a low-viscosity liquid
that can be
introduced into the interstitial voids assisting between the strands of a
stranded
conductor cable, which then diffuses out through the shielding and into the
polymeric
insulation jacket. Alternately, when a solid conductor is utilized, anti-
treeing agents
can be injected underneath the outer protective jacket and diffuses inwardly
through
the insulation jacket. Known techniques for treating cables in this manner are
disclosed, for example, in U.S. Patent Nos. 4,372,998 to Bahder and 5,372,840
to
Kleyer et al., disclosures of which are hereby expressly incorporated by
reference.
For large diameter cables (>500 kcm or >240 mm2) with stranded wound or
loose conductors, the amount of fluid that can fit in the interstices of the
strands may
exceed the amount of fluid required to optimally treat polymeric cables.
Because
these cables all have varying electrical loads in use, they exhibit
corresponding
resistive energy-induced temperature swings. As the temperature of the
polymeric
insulation varies, so too does the solubility of fluids (such as anti-treeing
treatment
fluids residing within the cable core and absorbed into any insulation
jacket), and
hence a condition of "supersaturation" can occur as the temperature cycles
down.
The fluid is forced from the polymeric phase of the insulation jacket and into
tiny
microvoids, which are created by the mechanical pressures resulting from the
thermodynamic equilibrium associated with the change of phase from the anti-
treeing
fluid as it passes from being dissolved in the polymeric solid into a free
liquid.
During the next increase in temperature still more fluid is drawn into the
polymeric
phase, and the cycle repeats until the swell of the polymer reaches a point
where the
mechanical strain bursts the cable and it fails catastrophically.
The failure mechanism described above has been observed by the inventors in
two classes of cases. In the first class, 750 kcm feeder cables were treated
with an
anti-treeing agent sold commercially by Utilx Corporation, Kent, Washington,
under
the trademark CableCURE 2-2614 (as disclosed in U.S. Patent No. 4,766,011,
issued
to Vincent et al., the disclosure of which is hereby expressly incorporated by
reference) fluid for a period from 1990 to 1991 at Arizona Public Service
(APS).
Reservoirs of fluid were left attached for 60 days as this was the standard
practice for
all cables treated prior to this time frame. The application to large diameter
cables
was new. A large number of these cables failed in-service due to the
supersaturation
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mechanism described above. The procedure of leaving a pressurized soak bottle
attached to cables larger than 3/0 in size was discontinued.
A second class of observations involved an experiment at Cable Technology
Laboratories (CTL) undertaken on behalf of Orange & Rockland utilities. A 4/0
(relatively small) diameter cable was thermally cycled with a pressurized
reservoir of
Cab1eCURE fluid attached. The cable failed as described above. In actual field
application, no reservoir is attached to such a cable, so that there has not
been a
chance for such a failure mechanism if proper procedures are followed. The
problem
was thought to have been solved by eliminating the external pressurized
reservoir.
Until the current unexpected problem, which is the inspiration for the present
invention this procedural change solved the problem. While eliminating the
pressurized reservoir was and is sufficient for many cables, certain large
diameter
conductors, especially those with thinner conductor shields and/or thinner
insulation
have experienced failure due to supersaturation. FIGURE 1 is a Cable Field
Report
(CFI) for such a cable. The 1000 kcm cable was treated on February 2, 1998 and
failed on July 30, 1998.
FIGURE 2 is a micro-infrared spectrographic analysis of the cable described
in FIGURE 1, labeled Texas Utilities (TU.) 00023210. Four radial scans
quantifying
the anti-treeing agent sold commercially by Utilx Corporation under the
trademark
Cab1eCURE/XL fluid (as disclosed in U.S. Patent No. 5,372,841, issued to
Kleyer et
al., the disclosure of which is hereby expressly incorporated by reference)
were taken
(90 apart from each other and labeled 1 St Quarter through 4th Quarter) from
the
conductor shield out to the insulation shield and are plotted. An insert of a
well-
treated cable labeled OG&E Cable Phase B is provided for comparison. The
integrated quantity of fluid in the dielectric of the TU cable is
approximately twice
that of the OG&E cable.
The dilution of dielectric enhancement fluids (i.e., anti-treeing agents) has
been proposed for other purposes. Bertini teaches in U.S. Patent 5,200,234,
the
disclosure of which is hereby expressly incorporated by reference, that
diluents can
be used to treat cables from the outside in. This prior art teaches that since
there is
such a gross oversupply of fluid in the annulus of the conduit contemplated in
that
disclosed method, that dilution is an economic requirement for outside-in
treatment
to be feasible. The prior art did not consider supersaturation an issue. The
TU
failure is an unexpected result of an inside-out injection.
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Summary of the Invention
The present invention involves the dilution of the
active ingredient, i.e., the "dielectric enhancement fluid"
or "anti-treeing agent", used in treating cables having a
high ratio of conductor interstitial volume (vl) to conductor
shield solubility plus insulation solubility plus insulation
shield solubility (v2). A diluent material is selected so as
to be substantially insoluble in the polymeric insulation,
sufficiently low in initial viscosity to enable introduction
into the cable interior, and to miscible with the dielectric
enhancement fluid.
According to one aspect of the present invention,
there is provided a method for enhancing the dielectric
properties of an electrical cable having a central stranded
conductor encased in a polymeric insulation, the cable
defining an interstitial void space (vl) between the strands
of the conductor, comprising: (a) determining a volume (v2)
of a dielectric enhancement fluid to be absorbed by the
cable to reach a predetermined level of dielectric
enhancement; (b) computing the ratio of (vl/v2) for the
cable; (c) if (vl/v2) is greater than a predetermined maximum
ratio determined to avoid supersaturation of the polymeric
dielectric enhancement fluid after treatment and during
long-term use, then diluting a quantity of the dielectric
enhancement fluid with a sufficient quantity of a diluent to
produce a mixture of diluent and dielectric enhancement
fluid, such that when the volume (vl) of the mixture is
introduced into the cable, the cable will have been supplied
with a volume (v3) of the dielectric enhancement fluid within
the mixture such that the ratio (v3/v2) is less than a
predetermined maximum ratio of 2.0; and (d) introducing the
mixture into the cable.
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According to another aspect of the present
invention, there is provided a method for enhancing the
dielectric properties of an electrical cable having a
central stranded conductor encased in a polymeric
insulation, the cable defining an interstitial void space
(vi) between the strands of the conductor, comprising: (a)
determining a volume (v2) of a dielectric enhancement fluid
to be absorbed by the cable to reach a predetermined level
of dielectric enhancement; (b) computing the ratio of (vl/v2)
for the cable; (c) if (vl/vz) is greater than a predetermined
maximum ratio determined to avoid supersaturation of the
polymeric dielectric enhancement fluid after treatment and
during long-term use, then diluting a quantity of the
dielectric enhancement fluid with a sufficient quantity of a
diluent to produce a mixture of diluent and dielectric
enhancement fluid, such that when the volume (vl) of the
mixture is introduced into the cable, the cable will have
been supplied with a volume (v3) of the dielectric
enhancement fluid within the mixture such that the ratio
(v3/v2) is less than the predetermined maximum ratio; and (d)
introducing the mixture into the cable.
Brief Description of the Drawings
The foregoing aspects and many of the attendant
advantages of this invention will become more readily
appreciated as the same become better understood by
reference to the following detailed description, when taken
in conjunction with the accompanying drawings, wherein:
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FIGURE 1 is a cable field report for an electrical cable that experienced
failure due to the supersaturation that preferred embodiments of the present
invention
address;
FIGURE 2 provides a large chart showing the results of micro-infrared
spectrographic analysis of the supersaturated electrical cable documented in
FIGURE
1, with a smaller inset chart of a treated electrical cable that has not
experienced
supersaturation for comparison; and
FIGURE 3 is a cable geometry data sheet providing interstitial volume
parameters for a representative cable suitable for treatment in accordance
with the
present invention.
Detailed Description of the Preferred Embodiment
The present invention involves the dilution of the dielectric enhancement
fluid in treating cables, having a high ratio of stranded conductor
interstitial volume
to conductor shield plus insulation solubility, with a diluent material.
The present inventions provides a method of treating electrical distribution
cables for dielectric enhancement. In a preferred embodiment, underground
electrical distribution cables are treated after insulation. However, the
present
invention may also be adapted for use in the treatment of new cables prior to
installation. Other types of electrical cables, such as submarine cables, may
also be
advantageously treated in accordance with the present invention. While the
present
invention is primarily directed to treatment of stranded conductor electrical
cables,
including a plurality of strands defining an interstitial volume vj, the
present
invention may also be adapted for treatment of cables having a solid conductor
core
which defines instead a volume vl between the polymeric insulation jacket and
the
conductor.
As used herein, the term "large diameter cable" refers to a cable having an
area that is computed to be greater than 250 kcm, i.e., greater than 120 mm2.
While
useful for cables greater than 250 kcm (a unit of area denoting one thousand
circular
mils), the present invention has particular utility for treatment of cables
greater than
500 kcm (120mm2) in area.
The term "dielectric enhancement fluid" is intended to mean any of a variety
of known anti-treeing agents or other anti-treeing agents that may be
specifically
developed for dielectric enhancement of electrical cable insulation. Suitable
anti-
treeing agents for use in practicing the present invention include, without
limitation,
the aromatic radical containing silanes disclosed in U.S. Patent No. 4,766,011
to
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Vincent et al., including phenyltrimethoxysilane and
phenylmethyidimethoxysilane;
and the organosilanes disclosed in U.S. Patent No. 5,372,841 to Kleyer et al.,
including phenyltrimethoxysilane, diphenyldimethoxysilane,
phenylmethyldiethoxysilane and phenylmethyldimethoxysilane.
The term "diluent" is intended to refer to a material that is at least
initially
fluid and that has a sufficiently low viscosity to facilitate injection into a
cable; that
is substantially insoluble in the polymeric insulation materials that are
utilized in
electrical cables; that are compatible with electrical cable materials and
accessories,
without causing degradation thereof; and that is miscible with the dielectric
enhancement fluids selected.
Preferably diluents have an initial viscosity that is less than 500 cps, and
more
preferably that is less than 10 cps. While low initial viscosity is necessary,
and
diluents that stay liquid are suitable, preferred diluents are selected such
that the
diluent increases in viscosity or even gels during a predetermined period of
time after
injection into the cable, such as with in 24 hours after injection. This
reduces the
likelihood and impact of spills from accidental cutting of a treated cable.
A preferred diluent is substantially insoluble in the insoluble polymeric
insulation materials used in electrical cables. For conventional insulation
materials,
i.e., polyethylene and variants such as ultrahigh molecular weight
polyethylene, the
diluent preferably has a solubility of less than 1.0 weight per cent, and more
preferably less than 0.1 weight percent. For other polymeric insulation
materials
including EPR and EPDM elastomers, a solubility of less than 1.0 weight
percent is
suitable.
It is also preferred that diluents used in the present invention be
environmentally benign, have a flash point that is greater than or equal to
the flash
point of the dielectric enhancement fluid with which the diluent is to be
mixed, and
which is toxicologically benign.
The silicone water block fluid sold commercially by Utilx of Kent,
Washington CableCURE/CBTM (as disclosed in U.S. Patent Nos. 4,845,309, issued
to
Vincent et al., and 4,961,961, issued to Vincent et al., the disclosures of
which are
hereby expressly incorporated by reference) meets all of the criteria above
and is
most preferred for use in the present invention. Other suitable diluents for
use in the
practice of the present invention include the silicone water block fluids
disclosed in
U.S. Patent No. 4,845,309 to Vincent et al., the disclosure of which is hereby
incorporated by reference. Still other suitable diluents are disclosed in U.S.
Patent
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No. 5,200,234 to Bertini, the disclosure of which is hereby incorporated by
reference.
These diluents include polydimethylsiloxane oil, fluorosilicone oil, mineral
oil, and
certain high molecular weight vegetable oils. Diluents similar to these
materials are
also encompassed within the scope of the present invention providing they meet
the
requirements set forth above with respect to low initial viscosity, low
polymeric
insulation solubility, compatibility with cable materials and accessories, and
miscibility with the dielectric enhancement fluid.
The present invention is adapted for use with large diameter cables. In order
to determine whether the present invention is advantageous for use in treating
a
particular cable, the interstitial void space (v i) between the strands of the
conductor
core is computed. Based on experience and empirical data, the volume (v2) of
dielectric enhancement fluid required to optimally treat the cable to achieve
a
predetermined level of dielectric enhancement is then determined. Volume v2 is
thus
the amount of dielectric enhancement fluid that will be absorbed by the
polymeric
insulation and shield materials. The ratio of interstitial volume to required
dielectric
enhancement fluid treatment volume (vl/v2) is then computed. If this volume
vl/v2
is above a predetermined maximum threshold, then the dielectric enhancement
fluid
is diluted with a diluent in accordance with the present invention prior to
application
to the cable. This predetermined maximum ratio of v1/v2 is preferably no
greater
than 2.0, more preferably no greater than 1.6, still more preferably no
greater than 1.4
and most preferably is between 1.3 and 1.4. Larger ratios above 1.4 may be
preferred
in certain instances, however, such as when a low temperature fluctuation
during use
is anticipated, or when a high degree of materials intended to diffuse through
the
cable insulation ("fugitive" materials) are included in the dielectric
enhancement
fluid.
When the ratio of v1 /v2 is greater than the predetermined maximum threshold,
then a sufficient quantity of diluent is added to the dielectric enhancement
fluid,
either prior to or during introduction into the cable interior, to produce a
mixture of
diluent and dielectric enhancement fluid. Sufficient diluent is added such
that when
the volume v, of the mixture is supplied to the cable interior (substantially
filling the
interstitial void space), the cable will have been supplied with a net volume
(v3) of
the dielectric enhancement fluid (not including the diluent), wherein (v3/v2)
is less
than the predetermined ratio. Preferably, the mixing of this solution is
carried out
and is followed by introduction of the mixture to the cable interior. If the
preferred
diluent disclosed above is utilized, after introduction, the diluent gels
within the cable
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interior while the dielectric enhancement fluid diffuses into the polymeric
insulation
and shield materials of the cable.
As a representative example, FIGURE 3 is a Cable Geometry Data Sheet for
the TU cable. The section of Attachment 3 labeled "Mass Absorption, Silicone"
indicates that 14.283 pounds of dielectric enhancement fluid will be absorbed
by the
cable's conductor shield, 14.837 pounds of fluid will be absorbed by the
polymeric
insulation jacket, and 0.946 pounds of fluid will be absorbed by the
insulation shield
for proper treatment, or a (v2) total of 30 pounds. The interstitial volume
(vl) is
about 70 pounds. Hence the ratio of interstitial volume to that required for
treatment
(vl/v2) is 70/30 or 2.33.
Because this ratio is in excess of the predetermined measurement ratio, in
this
case 1.4, dilution back to a ratio of dielectric enhancement fluid contained
in the
diluted mixture to interstitial void space of 1.4 is required to eliminate the
possibility
of supersaturation.
Excess dilution is to be avoided. For example, a ratio of 1.0 is not desirable
since some fluid diffuses all of the way out of the cable and there must be
some
residual fluid in the strands within the diluent in order to provide
sufficient free
energy (an entropy driving force) to allow diffusion into the conductor
shield.
Preferably, there should be at least the same concentration of the remaining
fluid in
the diluent as can be absorbed/adsorbed in the strand shield, which is
typically 16%.
Hence the optimum ratio (v3/v2) is between 1.3 and 1.4.
For any cables with a treatment ratio greater than 1.4 (or other predetermined
ratio as determined herein), dilution back to 1.3 to 1.4 is desired for
reliable post-
treatment dielectric performance. Table 1 provides some examples of cable
sizes and
their ratio of interstitial volume to fluid requirements sorted by this ratio.
The
headings in this table are abbreviated as follows: AWG - American Wire Gage;
mils
- thickness of insulation in mils; Cd - cable code; kV - electrical rating in
kV; str. -
strand count; Inter. - interstitial volume; and required volume of fluid
absorbed for
treatment. A horizontal line is drawn between those cables with ratios less
than 1.4
and those with ratios greater than 1.4. All cables, that have a treatment
ratio in excess
of 1.4 would benefit from treatment in accordance with the preferred
embodiment of
the present invention.
The preferred embodiment of the present invention provides that dilution in
cables with vI /v2 ratios in excess of 1.4 will improve the reliability of
treated cables.
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TABLE 1
Ratio of Interstitial Volume to Volume Required for Treatment
VI VI
AWG. mils Cd W. Str. (Inter.) (Re uired Ratio
NO.2 420 00 35 7 1.0 19.912 0.050
NO.2 345 00 35 7 1.0 15.392 0.064
NO.2 320 00 25 7 1.0 14.020 0.070
NO.2 295 00 25 7 1.0 12.716 0.078
NO.2 260 00 25 7 1.0 11.004 0.090
NO.4 220 00 15 7 0.8 8.846 0.095
M016 197 00 20 7 0.5 5.335 0.096
NO.2 220 00 15 7 1.0 9.210 0.107
1/0 280 01 25 7 1.6 14.039 0.114
NO.4 175 00 15 7 0.8 7.256 0.116
NO.2 175 01 15 7 1.0 7.661 0.125
M035 175 00 10 7 0.9 6.792 0.131
NO.2 175 00 15 7 1.0 7.662 0.137
NO.2 175 02 15 7 1.0 7.477 0.139
1/0 220 02 15 7 1.5 9.840 0.151
1/0 175 01 15 7 1.6 9.022 0.176
4/0 580 00 46 19 9.7 53.121 0.182
NO.1 420 00 35 19 4.2 20.862 0.200
1/0 420 00 35 19 5.2 22.225 0.236
NO.1 345 00 35 19 4.2 16.188 0.257
500 900 00 138 37 29.9 114.603 0.261
2/0 420 00 35 19 6.6 23.670 0.280
NO.1 320 00 25 19 4.2 14.765 0.282
1/0 345 00 35 19 5.2 17.380 0.302
3/0 420 00 35 19 8.3 25.346 0.329
1/0 320 00 25 19 5.2 15.900 0.330
2/0 345 00 35 19 6.6 18.650 0.355
NO.1 260 00 25 19 4.2 11.625 0.358
1/0 295 00 25 19 5.2 14.488 0.362
4/0 420 00 35 19 10.5 27.297 0.385
2/0 320 00 25 19 6.6 17.112 0.387
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Ratio of Interstitial Volume to Volume Required for Treatment
VI V]
AWG. mils Cd kV. Str. (Inter. Re uired Ratio
250 525 00 69 37 15.8 39.256 0.401
500 620 00 49 37 29.3 72.278 0.406
3/0 345 00 35 19 8.3 20.127 0.415
1/0 260 00 25 19 5.2 12.624 0.415
NO.1 220 00 15 19 4.2 9.749 0.427
3/0 320 00 25 19 8.3 18.523 0.451
750 800 00 115 61 44.8 96.698 0.463
4/0 345 00 35 19 10.5 21.851 0.481
2/0 260 00 25 19 6.6 13.697 0.483
1/0 220 00 15 19 5.4 10.848 0.498
1000 880 00 99 61 72.2 142.467 0.507
4/0 320 00 25 19 10.5 20.171 0.521
NO.1 175 00 15 19 4.2 7.845 0.531
M240 510 00 60 61 29.1 54.752 0.532
250 420 00 35 37 15.8 28.507 0.553
3/0 260 00 25 19 8.3 14.949 0.558
2/0 220 00 15 19 6.6 11.637 0.569
1/0 175 00 15 19 5.2 8.651 0.606
M095 200 00 20 19 8.2 13.376 0.616
4/0 260 00 25 19 10.5 16.415 0.641
3/0 220 00 15 19 8.3 12.783 0.653
M070 216 00 20 19 7.0 10.241 0.679
350 420 00 35 37 22.1 32.112 0.688
250 345 00 35 37 15.8 22.875 0.689
2/0 175 00 15 19 6.6 9.526 0.695
4/0 220 00 15 19 10.5 14.128 0.744
250 320 00 25 37 15.8 21.133 0.746
M120 227 00 20 37 12.6 16.653 0.758
1973 880 01 99 61 117.4 149.861 0.784
3/0 175 00 15 19 8.3 10.552 0.791
350 345 00 35 37 22.1 26.061 0.847
500 420 00 35 37 31.5 36.775 0.856
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Ratio of Interstitial Volume to Volume Required for Treatment
VI VI
AWG. mils Cd kV. Str. (Inter.) (Required) Ratio
4/0 175 00 15 19 10.6 12.302 0.861
1750 880 01 99 127 131.6 148.875 0.884
350 320 00 25 37 22.1 24.180 0.913
250 260 00 25 37 15.8 17.227 0.915
M065 135 00 11 19 5.3 5.747 0.918
M380 440 00 88 61 44.4 46.236 0.961
M240 195 01 15 19 23.4 22.777 1.029
500 345 00 35 37 31.5 30.205 1.043
250 220 00 15 37 15.8 14.840 1.062
300 160 00 10 37 12.7 11.653 1.089
350 260 00 25 37 22.1 19.940 1.107
500 320 00 25 37 31.5 28.151 1.119
750 420 00 35 61 52.6 42.910 1.225
350 220 00 15 37 22.1 17.330 1.274
250 175 00 15 37 15.8 12.362 1.275
M240 235 00 20 61 28.9 22.616 1.279
500 260 00 25 37 31.5 23.495 1.340
600 260 00 35 37 31.5 23.471 1.342
1500 420 00 46 91 116.1 82.494 1.408
M325 266 01 23 61 44.0 30.748 1.432
M325 266 02 23 61 44.0 29.991 1.468
750 345 00 35 61 52.6 35.614 1.476
M120 197 01 20 7 21.9 14.577 1.503
M325 260 99 23 61 43.9 29.138 1.508
350 175 00 15 37 22.1 14.601 1.512
500 220 00 15 37 31.5 20.608 1.528
750 260 00 25 61 42.5 27.621 1.537
750 220 01 15 61 44.1 28.220 1.562
750 320 00 25 61 52.6 33.318 1.577
750 260 01 25 61 49.8 30.849 1.616
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Ratio of Interstitial Volume to Volume Required for Treatment
vl VI
AWG. mils Cd kV. Str. (Inter.) Re uired) Ratio
1000 345 00 35 61 70.0 40.769 1.717
600 220 00 15 37 31.9 18.236 1.751
500 175 00 15 37 31.5 17.776 1.771
1000 320 00 25 61 70.0 38.271 1.829
750 220 00 15 61 50.8 26.153 1.941
750 175 00 15 61 51.1 25.001 2.046
M400 145 00 10 61 45.2 21.140 2.138
1000 260 00 25 61 70.0 32.551 2.151
1500 420 01 46 91 149.5 64.403 2.321
1000 175 00 15 61 70.0 30.066 2.328
1000 175 01 15 61 67.4 28.651 2.351
1000 220 00 15 61 70.0 28.954 2.418
750 380 00 46 61 75.0 30.307 2.474
M800 175 00 20 91 113.5 39.243 2.892
1500 220 00 46 127 102.9 29.740 3.461
560 100 01 5 37 51.4 5.182 9.923
While the preferred embodiment of the invention has been illustrated and
described, it will be appreciated that various changes can be made therein
without
departing from the spirit and scope of the invention.
