US 2351056 A
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June 13, 1944. J. LEPETIT ELECTRIC CONDUCTOR Filed Feb. 25, 1939 m m u m ma MiG/V5670 J 4,, -/f. WM/0w; 1756010189 Patented June 13, 1944 ELECTRIC CONDUCTOR Jean Lepetlt, Clichy, France; vested in the Alien Property Custodian Application February 25, 1939, Serial No. 258,571
In France March 2, 1938 4 Claims.
The present invention relates to the manufacture of electric conductors composed of one or a plurality of singleor divided conducting or resistant cores which are insulated from a metal sheath by a powdered mineral insulating material.
It has already been proposed, for manufacturing conductors of this category, in particular when the finished product is obtained by elongation of a suitably dimensioned blank, to use a mineral insulating material in a highly compacted and totally dehydrated state.
The insulating material usually employed hitherto in such cables is ordinary magnesium oxide.
The applicant has observed that, when the bare ends of such a conductor are left exposed to the atmosphere, a decrease occurs in certain of the dielectric qualities.
Thus if a section of conductor. the ends of which are not provided with fluid-tight devices and wherein the free edge of the dielectric is in direct contact with the atmosphere, is exposed in an atmosphere of average hygrometric degree,
temperature and pressure, it is observed that its insulating resistance per kilometre which was, for example, 10,000 megohms at the end of its manufacture, gradually decreases.
It is an easy matter to follow the law ofvariation of this resistance proportionally to the time of exposure and to plot the curve of same. Experience shows that, after a more or less long time, said curve reaches a value below which it practically 'does not fall.
In the case of a conductor which is insulated with ordinary magnesium oxide, said value is about 100,000 ohms when the radial thickness of the insulating material is 1.5 mm. and in the above circumstances of exposure.
Now, if the conductor is cut at a certain length, for example at a few centimetres, from-each of the ends of the section. substantially the initial value of insulating resistance is found again. The variation of insulating resistance observed is an end phenomenon and, independently of the insulating resistance per kilometre of the conductor itself or of the lineformed by same, it is possible to define a "limiting insulating resistance per free en In the above mentioned case. the mean value of the lim ting insulating resistance per free end is about'200.000 ohms.
It would appear that these phenomena originate from a certa n reabsorotion of the atmosrheric moisture by the insulating material. in the part adiacent its edge which is exposed to contact with the air.
The decrease of the insulating resistance may be considerable, in particular when the insulating material is ordinary magnesium oxide.
It is necessary to have recourse to end devices in order to obtain a satisfactory fluid-tightness, in the absence of which devices the absolute value of the insulating resistance of the line quickly falls and reaches substantially one half of the value of the limiting insulating resistance of each end. The drawing illustrates a cross-section of a conductor made in accordance with this invention. According to the present invention, for the manufacture of electric conductors of the category in question, insulating materials are employed such that the conductors manufactured have a higher limiting insulating resist ance per free end than that obtained with ordinary magnesium oxide, i. 'e. higher than 0.2 megohm for a radial thickness of insulating material of 1.5 mm.
Moreover, the insulating material may be so chosen that the absolute value of the limiting insulating resistance of the line itself is higher than a predetermined value taking into account the number of free ends.
If it is desired, for example, to guarantee the line an absolute insulating resistance value of 500,000 ohms under the above mentioned average conditions, for a conductor having a radial thickness of insulating material of 1.5 mm., if there are only two free ends an insulating material will be chosen of which the limiting insulating resistance per free end is not lower than 1 megohm.
Among the insulating materials which have this property, oxides or compounds of calcium or of other similar substances will be chosen in particular.
The applicant has found, in fact, that although very hygroscopic, calcium oxide placed under the conditions to which it is subjected between the core of the conductor and the metal sheath, gives the means of obtaininis a very much higher limiting insulating resistance per end than what had hitherto been obtained and in particular very much higher than that of conductors insulated with ordinary magnesium oxide.
In a practical example in which calcium oxide was employed, the limiting insulating resistance per end was found to be higher than 20 megohms, for a radial thickness of insulating material of 1.5 mm.
Powdered insulating materials which are capable of swelling are generally suitable for this purpose, that is to say those, such as calcium oxides or compounds, which cannot absorb moisture or in a general manner combine with air or its constituents without increasing in volume.
The insulating material is, in fact, bound by the owing to a natural action such as that of the carbonic acid of the air, are also suitable.
With conductors thus constructed, it is furthermore possible either to eliminate any end device, or to make use of less fluid-tight devices which are consequently less, costly to manufacture.
For th manufacture,'the powdered insulating material used is totally dehydrated before the usual mechanical treatments of shaping, drawing, etc., theinsulating material being introduced into the rough shape, either in the powdered state, or in the shape of agglomerated blocks in a highly compact state, or in any other shape, or by means of any other methods. It is possible, in particular, to effect the total dehydration by heating the insulating material under appropriate conditions until it no longer loses weight; in practice, for ordinary calcium exide, such dehydration is complete when it has been subjected to a temperature of 900 to 1000 C. for several hours.
The invention also covers the use of insulating materials having the properties claimed, and in particular calcium oxide, either in the state of homogeneous mixtures, or by radial or longitudinal juxtaposition of different insulating mate rials in contact with each other. Theinvention covers in particular th wires, cables or conductors of the category in question which have the properties described and in which the dielectric is binary and is formed by calcium oxide and magnesium oxide, whatever be the proportion of the two substances. The applicant ha proved. that in this. case, the highest limiting insulation resistance is obtained when the proportion of.
calcium oxide relatively tothe whole is between '25 and 40%. I
Parts CaO 30 MgO 70 B203 1 Good results have also been obtained by using a homogeneous mixture comprising:
Parts MgO 91 These proportions are onlygiven by way of example.
usual treatments such as agglomeration in the shape ofinsulating blocks, dehydration, introduction into the rough shape, etc.
Moreover, without exceeding the scope of the invention, the improvedinsulating material such as calcium oxide, boron oxide or the like may 7 be introduced into the homogeneous ,powdered insulating material only in th zone of each end -of--t he conductor. f
The invention also covers above.
The conductors may be manufactured as above set forth and using for example at least for those portions adjacent the ends a mixture containing from 25 to 40% calcium oxide and 75 to 60% of magnesia. Thereafter each of the ends of the conductor may be dipped in' a boric acid solution.
I claim: 1. An electric conductor comprising a core; a
' metallic sheath and a powdered insulating material located between said core and sheath, said insulating material comprising a mixture of calcium oxide and magnesia, said calcium oxide comprising between 25 and 40% of said mixture and saidmagnesia the remainder of said mixture. I
2. An electric conductor comprising a ,core, a metallic sheath and a powdered insulating material located'b'etween. said core andsheath, said insulating material comprising a mixture of calcium oxide, magnesia and boric oxide, said calcium oxide comprising between 25 and 40% and said boric oxide comprising about 1% of. said mixture and said magnesia the remainder of 'said mixture.
3. An electric conductor comprising a a core, a metallic sheath and a powdered insulating material located between said core and sheath, said insulating material located at the ends'of' said conductor comprising a mixture of about 25 to 40% of calcium oxide and to 60% of magnesia.
4. An electric conductor comprisinga. core, a metallic sheath and a powdered insulating material located between said core and sheath, said insulating material comprising a mixture :of about 25 to 40% 'of calcium oxide, 74" to 59% of magnesia and 1% of boric oxide.
electric conductors 1 in which the dielectric is composed as stated