US2075927A - Shielding system - Google Patents

Description

April 6, 1937. I A. E. BOWEN 2,075,927

SHIELDING SYSTEM 2 Filed April 25, 1936 FIG! - INVENTOR if v v T A.E.80WEN f ATTORNEY other for a material distance.

Patented Apr. 6', 1937 PATENT OFFICE SHIELDING SYSTEM Arnold Everett Bowen, Red Bank, N. J., assignor to American Telephone and Telegraph Company, a corporation of New York Application April 25, 1936, Serial No. 76,490

12 Claims.

This invention relates to shielding systems for intelligence transmitting circuits and more particularly to systems for shielding telephone and telegraph lines from a power line in proximity thereto.

As is known in the art, it frequently occurs that a telephone or telegraph line is positioned in the vicinity of a transmission line, the lines, for example, being substantially parallel to one an- As a result of such physical relation of the lines and the consequent electromagnetic association thereof, highly undesirable voltage and current disturbances appear in the telephone or teiegraph line due to periodic variations in the power line and also due to unbalances and short circuits in the power system. Consequently, the transmitting characteristics of the telephone or telegraph line are materially affected and altered so that unfaithful transmission of speech and music or signals obtains.

One general object of this invention is to effectively shield one transmitting system from another transmitting system in proximity thereto.

0" More specifically, one object of this invention is y; attainment of a low resistance shielding circuit is enabled.

In accordance with one feature of this invention, shielding system is composed of a closed metallic loop having a wire portion adjacent and substantially parallel to the disturbing power line and another wire portion adjacent and substantially parallel to the disturbed telephone or telegraph line. The wire portions may be coextensive with the proximate portions of the power and telephone or telegraph lines or may occupy but point in the exposure between the two lines.

In a modification, the closed loop may comprise two conductors each having a portion adjacent the disturbing line and another or transposed portion adjacent the disturbed line. The two portions may be cross-connected at a suitable point in the exposure between the two lines.

Suitable'impedances, which may be resistive or reactive or a combination thereof, may be introduced at suitable points in the shielding systern to provide the requisite impedance and phase relationships to insure substantially perfect shielding in any particular instance.

The invention and the various features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Figure 1 is a diagrammatic view showing a shielding system illustrative of one embodiment of this invention and the relation thereof to a disturbing and a disturbed line;

Fig. 2 illustrates a modification of the shielding system shown in Fig. l in which conductors forming a part of the system are adjacent only portions of proximate sections of the disturbing and disturbed lines;

Fig. 3 is another diagrammatic view showing another shielding system in accordance with this invention, comprising transposed conductors;

Fig. 4 illustrates a modification of the shielding system shown in Fig. 3 wherein suitable impedances are provided in the shielding circuit; and

. Fig. 5 illustrates another modification of the shielding systems shown in Fig. 3.

Referring now to the drawing, there is illustrated in each of the figures a disturbing system lli, which may be, for example, a power line, and a disturbed system I I, which may be, for example, a telephone or telegraph line. The two lines may extend in proximity to each other and may be 0 substantially parallel to each other to form an exposure of length Z. The telephone or telegraph line may be grounded at one end as shown. In order to shield the disturbed line against longitudinal induction, a shielding system is provided in the enclosure formed by proximate conductors in the disturbed and disturbing systems.

In one illustrative embodiment shown in Fig. 1, the shielding system comprises a conductor 52 positioned adjacent and substantially parallel to the disturbing or power line H), and another conductor 93 positioned adjacent and substantially parallel to the disturbed or telephone or telegraph line I I. The conductors l2 and I3 may be connected at one end of the exposure by a cross conductor l4 and at the other end by a crossconnection including an impedance [5. This impedance may be a resistance, an inductance or a capacitance or a combination of two or all of these. i

As shown in Fig. 2, the shielding system need not be coextensive with the exposure between the disturbing and disturbed lines It! and H, respectively. That is, the conductors l2 and I3 in the disturbed circuit may extend only from one end of the exposure to a distance fl from this end.

It will be understood, of course, that the shielding effect of the shielding circuit is dependent upon a variety of factors among which are the size of the conductors l2 and I3, the length of these conductors, the frequency of the currents in the disturbing system and the space relationship of the disturbing and disturbed lines. The requisite parameters for the shielding system in order to produce substantially perfect shielding may be ascertained in the manner set forth hereinafter.

If the current in the disturbing line in is designated as I1 and the current in the shielding system is designated as I2, and the length of the shielding conductors l2 and I3 is large in comparison with the distance between the lines It] and H, as it is in actuality, the electromotive forces extant around the shielding and disturbed circuits in Figs. 1 and 2 may be expressed, respectively, by the equations IlZl0-llZ+I2 zl2llzl3ll)fl vd (2) where V4 is the terminal voltage on the disturbed circuit ii.

Eliminating I2 from Equations 1 and 2,

The voltage which would have been induced il in the absence of shielding circuit ill-i3 is given by V4'=I1Z1011, so that the shield factor which represents the effect of adding the shielding conductor, is obtained by dividing Equation (3) by ilzlo ii, and me be expressed as which is equal to '7 r Z l- 2( l0 l. Z if) (Z13 ll 1.. 11) ie-ii Equation 4 may then be written as where Inasmuch as the several mutual impedances above-mentioned are complex quantities, p will be complex and may be Written as where both P1 and P2 are real.

To ascertain the requisite parameters of the shielding circuit to give substantially perfect shielding, the shield factor 1 is made 0 and, ex-

pressing p, ZlZ-IZ and Zia-1s in rectangular form,

Equation (6) may be written thus:

Equations 9 and 10, it will be seen, involve three variables, namely, 2;, Rig-12 and Xiz-iz. Hence, if any one of these three is fixed the equations may be solved for the remaining two.

As an illustration there may be considered a situation wherein the spacing between lines H3 and H, [B and l3, l2 and H, and i2 and i3 is 400 feet, and wherein the shielding conductors I2 and I3 are spaced three feet from the lines 59 and II, respectively. The conductors l2 and I 3 may be No. 4/0 AWG copper wire, the frequency of the current in the disturbing or power line H) may be 60 cycles, and the earth resistivity may be approximately 10 meter-ohms. Under these conditions, P1:-.56, P2:.86, R1z 12=.G66, R12-13=.0l4 and Xl2l3:-G20, each being expressed in ohms per kilo feet. Under these conditions the values of o and Xi2i2 requisite for perfect shielding may be obtained from Equations 9 and 10, to wit:

Inasmuch as Xl2-12 as determined above is negative, it follows that the series impedance i5 in the shielding system of Figs. 1 and 2 must be a condenser. If the reactance of the wire parts of the shielding circuit, with ground return, be designated as Xl2-l2vr ohms per kilo feet, the requisite capacitance, C, for perfect shielding may be ascertained from the relation 1 12-12t- 12-i2) (11) Where w: 1r times the frequency. Under the conditions assumed, X12 12w:o.l97 chm per kilo foot so that 1 m5bro6 faracl kllv feet Hence, if the exposure between the power line H] and the telephone or telegraph lines were kilo feet in length, the series impedance i5 would need to be a condenser having a capacitance of 680 microfarads.

The various specific parameters given heretofore are, of course, merely illustrative of one particular application or" this invention and it Will be understood that the invention may be practiced in instances where the distance between the disturbing and disturbed lines is greater or less than 400 feet, and also that various size wires may be employed for the shielding conductors l2 and 53. It may be pointed out, for example, that with a 4/0 AWG copper shielding conductor for spacings of the disturbing and disturbed lines of 209 feet or less it is desirable that the shielding conductors be coextensive with the exposure, that is that i=1, although even this length of shielding conductor will not suifice to reduce the voltages to zero. However, with this wire size and for separations greater than 200 feet, perfect shielding can be had, and it "'has been found that f decreases rapidly with increasing separation and that, for example, for a separation of 1000 feet the shielding system need extend through only about one-tenth of the exposure to obtain perfect shielding. As illustrative of specific instances, it may be noted that for a separation of about 50 feet it has been ascertained that the product cl should be about 7,000 microfarad kilo feet and that for separations of 200 feet and 1,000 feet this product should be about 4,000 and 60,000 microfarad kilo feet, respectively.

It may be pointed out also that, in general, if the distance between the disturbing and disturbed lines is decreased, the size of the shielding wires and of the series capacitance must be increased in order to secure perfect shielding. For example, it has been ascertained that with the distance between lines [0 and I2, and H and I3 of the order of 3 feet and for a shielding system extended through the whole exposure (i=1) for relatively small separations of the order of feet or less, the shielding conductors should be of the order of 1,000,000 circular mils in crosssection for satisfactory shielding, and that this cross-section decreases rapidly with increasing separations until at a separation of 1,000 feet, a No. 8 AWG shielding conductor will enable perfect shielding. It has been ascertained also, for example, that for separations of the order of 50 feet the requisite value of cl is approximately 6,500 microfarad kilo feet and for separations of about 1,000 feet the necessary value is about 1,600 microfarad kilo feet.

In the foregoing specific illustration, the calculations have been based upon a power lineshield conductor spacing of three feet. Although such spacing is satisfactory for relatively low potential lines, it will be appreciated that for higher potential lines greater spacings will be necessary. The requisite parameters for the shielding system to assure the desired high degree of shielding may be ascertained in the manner set forth heretofore. It may be noted, however, that it has been ascertained that as the separation between the shielding conductor and the power line is increased, the resistance of the shielding wire required to assure perfect shielding decreases and the series capacitance becomes larger. Neither, however, varies rapidly with separation.

In another embodiment of this invention shown in Fig. 3, the shielding system may comprise cross-connected transposed shielding wires. In this figure, the shield wire l2 includes a portion Ha adjacent the disturbing or power line In and a portion 12b adjacent the disturbed or telephone or telegraph line, the two portions being transposed at a distance from one end of the exposure between the disturbing and disturbed lines, the point of transposition being displaced materially from the longitudinal midpoint of the enclosure. Similarly, the shielding wire l3 includes transposed portions I3a and 131), the former portion being adjacent the telephone or telegraph line H and the latter portion being adjacent the power line I0. The shielding wires l2 and 13 are connected at one end by a cross wire [4 and at the other end by a series impedance [5, which preferably is adjustable in both magnitude and phase angle.

The particular parameters required to secure desired shielding may be ascertained in a manner similar to that set forth heretofore with reference to the shielding systems illustrated in Figs. 1 and 2. For convenience in analysis, the

I1Z1o-11lI2(Z13 i1Z12-11)26l=V4 (13) From Equations 12 and 13 it follows that the Equation 14, it will be seen, is identical with Equation 4 if Consequently Equation 14 may be rewritten as Equation 6 above if in the latter 11:5 Hence the requisite parameters for the shielding system illustrated in Fig. 3 may be ascertained in the same manner as heretofore given in the discussion of Figs. 1 and 2.

As a specific illustration, the case may be considered wherein the several factors, that is, size of shielding conductor, frequency, earth resistivity and separations are the same as in the specific illustration described heretofore with reference to Figs. 1 and 2. In this case, for perfeet shielding u should be .093 and X1242 should be .060 as ascertained heretofore. 5 may then be ascertained from the relation and the necessary capacitance for the series impedance [5 in the shielding circuit shown in Fig. 3 may be calculated from the relation so that if the length of the exposure were 20 kilo feet, in Fig. 3, the capacitance required for perfect shielding would be 252 microfarads. This value, it will be seen, is less than half that required in a shielding system of the types shown in Figs. 1 and 2.

The requisite value of this capacitance and also the magnitude of the current I2 in the shielding system may be reduced, if desired, by the insertion in the shielding system of lumped impedances l8, :1, l8 and i9 connected in series with the shielding conductors l2 and I3 and another lumped impedance 20 cross-connected between the shielding conductors as shown in Fig. 4. These impedances may be resistive, inductive, or capacitive or a combination of two or more of these.

It will be understood, of course, that the specific parameters above-determined for the shielding system of Fig. 3 are merely illustrative of one particular application of this invention. It may be pointed out also that, in general, variations in such factors as the wire size, separations of the disturbing and disturbed lines, and spacings between these lines and the shielding conductors 75 result in variations in the parameters of the shielding system shown in Fig. 3 commensurate with the variations mentioned hereinabove in the discussion of the systems shown in Figs. 1 and 2.

In another embodiment of this invention illustrated in Fig. 5, the closed metallic shielding circuit comprises, in addition to the shielding conductors l2 and E3 located adjacent the disturbing line if! and the disturbed line H, respectively, two conductors I20 and [3c disposed closely adjacent and substantially parallel to each other and positioned between the conductors i2 and it. A series impedance i5 is connected as shown and may be a resistor, inductor, or capacitor or any combination of the three. The parameters of the shielding system necessary for perfect shielding may be ascertained in the same manner as described in detail heretofore in connection with Figs. 1 to 4, inclusive.

Although several embodiments of this invention have been shown and described, it will be understood, of course, that the shielding systems thus shown and described are but illustrative of the invention and that various modifications may i be made therein without departing from the scope and spirit of this invention as defined in the appended claims.

What is claimed is:

I. In combination, a pair of transmitting lines disposed in the vicinity of each other and a shielding circuit comprising a closed metallic loop having a portion adjacent one of said transmitting lines and another portion adjacent the other of said lines.

2. In combination, a power line, a telephone line subject to inductive interference from said power line, and a shielding circuit comprising a closed metallic loop having a wire portion adjacent and substantially parallel to said power line and another wire portion adjacent and substantially parallel to said telephone line.

3. In combination, a power system including a line, a telephone system including a line in the vicinity of said first line and forming an inductive exposure therewith, and a shielding system comprising a closed metallic loop having a wire adjacent said power line and another wire adjacent said telephone line, said wires being of such length that the shield factor is substantially zero.

l. In combination, a transmission line, a second transmission line subject to inductive disturbance by said first line and forming an exposure therewith, and a closed metallic loop shielding system including a pair of wires each adjacent one of said lines, said wires being of less length than said exposure.

5. In combination, a transmitting line, a second transmitting line in the vicinity of and subject to inductive disturbance by said first line, and a closed metallic shielding system including a pair of conductors each having a portion adjacent said first line and another portion adjacent said second line.

6. In combination, a power line, a telephone line in the vicinity of said power line and form-' ing an inductive exposure therewith, and a closed metallic shielding circuit including a pair of conductors each transposed intermediate the ends of said exposure, each of said conductors having a portion adjacent said power line and a portion adjacent said telephone line.

'7. In combination, a power line, a telephone line in the vicinity of said power line and forming an inductive exposure therewith, and a shielding system comprising a closed metallic loop including a pair of wires each having a portion adjacent said power line and a transposed portion adjacent said telephone line, said wires being transposed at a common point intermediate one end of said exposure and the longitudinal midpoint thereof.

8. In combination, a pair of transmitting lines disposed in the vicinity of each other and forming an inductive exposure, and a shielding system adjacent said exposure including a closed metallic circuit having a portion adjacent each of said lines and means including a pair of adjacent conductors cross-connecting opposite ends of said portions.

9. In combination, a power line, a telephone line in the vicinity of said power line and forming an inductive exposure therewith, and a closed metallic shielding circuit comprising a wire adjacent and substantially parallel to said power line, a wire adjacent and substantially parallel to said telephone line, and conductors including closely adjacent substantially parallel portions crossconnecting opposite ends of said wires.

19. In combination, a pair of transmitting lines I one subject to disturbance by the other and a closed metallic shielding circuit comprising a pair of wires each disposed adjacent one of said lines, and impedance in series with said wires of such magnitude that the shield factor is substantially zero.

11. In combination, a power line, a telephone line in the vicinity of said power line and forming an inductive exposure therewith, and a closed metallic shielding circuit comprising a pair of wires each transposed intermediate the ends of said exposure and each having a portion adjacent said power line and a portion adjacent said telephone line, and an impedance of predetermined phase angle and magnitude connecting said wires at one end.

12. In combination, a power line, a telephone line in the vicinity of said power line and forming an inductive exposure therewith, and a closed shielding system comprising a wire adjacent said power line, a wire adjacent said telephone line, a conductor connecting one pair of opposite ends of said wires, and a connection between the other opposite ends of said wires including a conductor adjacent said first conductor and a series impedance of predetermined magnitude and phase angle.

ARNOLD EVERETT BOWEN.

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