US3633194A

US3633194A - Tamperproof barrier

Info

Publication number: US3633194A
Application number: US226286A
Authority: US
Inventors: Erich W Kothe
Original assignee: ANOCONDA WIRE AND CABLE CO
Current assignee: Ericsson Inc
Priority date: 1962-09-26
Filing date: 1962-09-26
Publication date: 1972-01-04
Anticipated expiration: 1989-01-04

Abstract

A tamperproof barrier is disclosed which can be used for wall structures or cable structures. The barrier comprises two electrically conducting surfaces separated by a dielectric spacer. A high potential is impressed between the two conducting surfaces such that any penetration or break in the dielectric spacer will result in a corona discharge. The discharge can be detected by an ionization detector which in turn can activate an alarm.

Description

United States Patent [72] Inventor Erich W. Kothe White Plains, N.Y. [21] Appl. No. 226,286 [22] Filed Sept. 26, 1962 [4S] Patented Jan. 4, 1972 [73] Assignee Anaconda Wire and Cable Company [54] TAMPERPROOF BARRIER 16 Claims, 4 Drawing Figs.

[52] U.S. Cl 340/248, 340/276, 174/105, 174/107 [51] Int. Cl G08b 21/00 [50] Field of Search 179/78; 324/54; 174/105, 107,102.2, 102, 127; 340/248, 253, 256, 276

[56] References Cited UNITED STATES PATENTS 2,081,517 5/1937 VanHoffen l74/102.2 UX

2,270,274 ll 1942 Davis 340/276 X 2,686,909 8/1954 Poulson 340/276 2,937,336 S/1960 Gooding... 324/54 3,015,774 l/l962 Eigen 324/54 3,088,995 5/1963 Baldwin.'...... 174/127 3,098,893 7/1963 Pringle et al.... l74/l02.2 3,146,300 8/1964 Beckius et al... 174/127 3,160,871 12/1964 Rubenstein II. 340/276X Primary Examiner-Richard A. Farley Assistant ExaminerBrian L. Ribando An0meyVictor F. Volk ABSTRACT: A tamperproof barrier is disclosed which can be used for wall structures or cable structures. The barrier comprises two electrically conducting surfaces separated by a dielectric spacer. A high potential is impressed between the two conducting surfaces such that any penetration or break in the dielectric spacer will result in a corona discharge. The discharge can be detected by an ionization detector which in turn can activate an alarm.

PATENTED m 4 i872 SHEET 1 0F 2 TELEPHONE CABLE CORE 46 BAND PAS FILTER 447 I 3839 P48 F/g. 2 \\IONI.ZATION DETECTOR AND :7 l6 I2 ALARM I0 I! ISIVETJTOR.

ERICH W. KOTHE BY V M M TAMPERPROOF BARRIER My invention relates to barrier walls of novel structure and alarm means associated with such walls responsive to any penetration thereof. Particularly my invention relates to tapproof communication cables comprising said walls.

Many efforts have been made to construct barrier walls that could not be penetrated without generating a warning signal, but heretofore no scheme of manufacturing such a wall has been proof against the ingenuity of persons skilled in methods of burglary and wiretapping. Certain of such prior-art barrier walls comprised two conducting surfaces with a thin layer of insulation between them. The conducting surfaces were connected across the terminals of a battery and any carelessly applied metal tool that pierced the wall would short the circuit between the two conducting surfaces and ring an electric alarm. Walls of'this typewere easily penetrated without raising an alarm by being careful not to short circuit the two conducting surfaces while cutting through them, one at a time, and by cutting or melting away the dielectric layer with a torch or hot air gun. Sometimes it was possible to dissolve the dielectric with solvent, or eat away the metal with acid. The alarm could also be rendered inoperative by cutting the circuit leads. Where the walls described above were in the form of concentric conducting shields used to protect telephone cables from tapping, they could be penetrated with electrically insulated probes that were prevented from short-circuiting the shields by the presence of the coating of insulation on the probes. In the case of cables it has been suggested to prevent inductive tapping from outside of the cable jacket by superimposing a masking noise in the cable shield. This noise is not effective, however, against tapping with probes that penetrate into the cable core.

I have overcome the shortcomings of prior-art barrier wall structures by the novel means of creating an environment where ionization phenomena will raise an alarm upon any unauthorized tampering or penetrating to areas protected by barrier walls. These phenomena may be described as follows. When a high electrical potential is applied between two surfaces corona will occur inany voids or air spaces between the surfaces as a result of ionization of gases in the voids. Across a -mil void, at one atmosphere, corona has been reported to occur at a stress of approximately 160 volts/mil, converting some of the power supply energy of the potential source to the radio frequency spectrum with frequencies as high as 100 megacycles. With 25-mil voids, 90 volts/mil may be sufficient to induce corona. The corona is accompanied by discharge'of electrical energy over a wide frequency range, and it can be detected by known types of ionization detectors. These detectors can be associated with alarms such as alarm bells or sirens or warning lights in a known manner.

I have invented a tamperproof barrier comprising two electrically conducting walls with electrical insulation between them. The insulation is free from corona-producing voids and makes void-free contact with both of the walls. My barrier also comprises means for applying a high potential between the walls sufficient to induce corona discharges in voids that might develop between them, and alarm means responsive to corona discharges between the walls. I have also invented a tamperproof cable utilizing my barrier, wherein the conducting walls take the form of elongated electrical conductors. My potential applying means may be a transformer and my barrier may then also comprise a filter to exclude the transformer output from the alarm. Alternatively the filter might be omitted and the barrier comprise an electrical bridge with the high-voltage source connected across a first diagonal of the bridge and the alarm connected across a second diagonal. The walls are then connected across one of the legs of the bridge.

An embodiment of my invention also comprises a tamperproof barrier connected to alarm means through a tamperproof cable, both barrier and cable being of the type described herein. My invention has utilization in a cable comprising a core comprising at least one signal-carrying conductor. The core is advantageously surrounded by a grounded shield and at least one of the surrounding conductors may beferromagnetic.

I have invented a tamperproof'barrier comprising'two electrically conducting walls with insulation between them, means impressing a potential of at least 3 kv. between the walls and alarm means responsive to corona discharges between the walls. I have also invented a tapproof waveguide protectedby the barriers of my invention, and the methodof energizing a tamperproof barrier comprising the steps of applying-an electric potential between the conducting walls, increasing the potential until the initiation of corona is detected on an ionization gage, immediately reducing the potential to the extinction I point of the corona and then setting and holding the potential at a value between 50 percent and 90 percent of the extinction potential but preferably over 3,000 v.

A more thorough understanding of my invention can be gained from a study of the appended drawing.

In the drawing:

FIG. 1 is a perspective view, partly sectionalized, of a safe and connecting cable utilizing barrier walls of my invention.

FIG. '2 is a sectional view of a cable made to my invention with a diagram of associated wiring and alarm apparatus.

FIG. 3 is a sectional view of a waveguide made to my invention.

FIG. 4 is a diagram of a bridge-circuit used inan embodiment of my invention.

Referring to FIG. 1 a safe 10 is fabricated ofbarriers I] made to my invention. These barriers 11 have an outer metal wall 12 which is preferably grounded and an inner metal wall 13 which is maintained at a high electrical potential as hereinafter to be explained. Between the walls 12 and 13 there is an insulating layer 14 having a thickness sufficient to prevent appreciable current flow between the two walls 12. and 13 at the voltage impressed between them. The insulation 14 is free from any voids or pockets which might contain ionizable gases and it is important thatthere should be no areas of ionization between the insulation 14 and the metal walls 12, 13. This may be accomplished by vulcanizingthe insulation to the walls when the fonner is a vulcanizable material. I have found, however, that the means whichbest combines.

ease of fabrication with reliability of operation is-to coat the insulation 14 with electrically conducting layers of organic material on both sides before attaching it .to the metal walls. These conducting organic layers are designated by numerals 16, 17 in FIG. 1. Under these circumstances any voids which develop between the metal walls 12, 13 andthe organic sandwich consisting of the layers 17, I4, 16 will not result in ionization or corona because the metal and the surfaceof the organic material will have the same electric potential. However,

if anyone tries to penetrate through the barrier 11 by cutting a section out of the metal wall 12 he will initiate a corona discharge in the insulation 14 assoon as he cuts beyond the conducting layer 16. This discharge will be detected by meanshereinafter to be described and will set off an alarm. It is virtually impossible to cut into the insulation 14 without generating corona due to the high voltage impressed between the con ducting walls l2, l3. Thiscan be explainedasfollows. The insulating wall 14 is comprised of a material selected'for its freedom from corona-producing tendencies. Suchmaterials are butyl rubber, polyethylene and epoxy resins when properly compounded and fabricated. When such an insulation. is pierced by a cutting tool or probe the tool not only is likely to carry small air bubbles on its surfaceinto the insulation but the sharp point or edge of the tool presents a node for-electrical stress concentration which, as the point or edge ap'- proaches the high-voltage layer 17, finally causes the insulation to break down with a resulting corona discharge. In addition to warning of any deliberate penetration of the layer 14 the system shown in FIG. l.will also sound an alarm in case of" cessfully tapped by melting away the dielectric layer with flame torches or hot air guns. These methods, or the use of solvents or acids to penetrate the dielectric, cannot be employed without creating voids that will be instantly detected in the barrier here described. I have shown my barrier 11 fabricated into the safe having a door 18 similarly constructed. To preserve the electrical separation of the metal walls 12, 13 it is necessary that the edges of the barrier such as an outer frame 19 of the door 18 and an inner frame 20 of the opening of the safe 10 should be of insulating material. A suitable material in this case is nucleated glass which has great physical strength along with high electrical resistivity. Although I have shown a safe 10 it will be understood that my barrier will have many uses in the form of a single panel. Such a panel might form a partition between a masonry wall and a metal vault and protect the vault from any attempts to gain entry through the wall.

The detection and alarm means protecting the safe 10 is remote therefrom and connection between the safe 10 and the alarm (not shown) is made through a cable 21 of which an enlarged section is shown in FIG. 1. The cable has an energized central metal conductor 22 which is connected within the safe 10 to the wall 13 of the barrier 11 and to an inner wall 23 of the door 18. The cable 21 has an outer grounded conductor 24 which may be a lead sheath, but is by no means limited thereto and this sheath 24 is electrically connected to the metal wall 12 and to an outer wall 26 of the door 18. In making the aforesaid connections between the cable 21 and the safe 10, known, corona-free, high-voltage cable-jointing techniques such as the use of potheads should be employed. Between the conductors 22 and 24 is a layer of void-free insulation 27. Layers 28, 29 of electrically conducting organic material are interposed between the layer 27 and the respective conductors 22 and 24. The system comprising the safe 10 and cable 21 along with remote energizing and alarm means not shown is tamperproof, for the cable is protected against tampering in the same manner as the barrier 11. Any penetration of the insulation 27 will cause corona that would set off the alarm. Indeed, if the cable 21 were broken or severed, a corona discharge would immediately occur between the conducting areas 28 and 29 at the severed edge and excite the alarm at the remote station. Although the alarm may, as hereinbefore stated, be a bell or siren, I am using the word alarm" to include any corona detecting or indicating equipment used to detect the initiation of corona discharges in my barrier for, indeed, any such equipment will serve to alarm a person viewing the same to the presence of corona in the barrier.

FIG. 2 shows a tapproof communication cable 30 utilizing the barrier of my invention to prevent the insertion of probes in proximity to the signal-carrying conductors of the cable. These conductors which may be any of a range of known types such as twisted pairs, triads, quads, concentric pairs, all either shielded as by braids, or unshielded, are included in a core 25 of known type which may be inclosed in a jacket (not shown). Surrounding the core 25 is an electrically conducting wall or shield which may be but is not limited to an extruded metal sheath or a wrapping of metal tape. The shield 35 is grounded and shields the conductors in the core 25 from any high-voltage potentials that may be applied to the outer layers either for the purpose of generating corona or for masking or jamming. Surrounding the shield 35 is a layer of electrically conductirig organic material 31. This may be a layer of rubber tape rendered conducting by the inclusion of carbon black or a conducting rubber extrusion or it may be a conducting paste. The important property of the material 31 is its compatibility with a layer of insulation 32 surrounding it so that the material 31 is in close contact with the insulation 32 over all of their common area. The insulation 32 is of a material and thickness sufficient to withstand the ionizing voltage that will be applied across it and it is surrounded by a layer 33 of conducting organic material which may be the same as the material 31. Over the layer 33 another metal wall or shield 34 is applied which may be similar to the shield 35. Over this there is another conducting organic layer 36, a thickness of insulation 37 which may be of the same material and thickness as the insulation 32, another conducting organic layer 38, a final metal wall 39 and an overall protective jacket 41 of an abrasion resistant material such as, but not limited to, polyethylene and neoprene. In order to shield the cable 30 magnetically the wall 39 and also the walls 34 and 35 may advantageously be constructed of steel or other permeable material. At a point remote from the section of the cable 30 which may be exposed to tampering, potential is applied to the shield wall 34 by means of a transformer secondary 42, a grounded side 43 of which is connected to the

walls

35 and 39. A series resistance 44 prevents the flow of high currents in the event of a short circuit of the wall 34 to ground. The secondary 42 is a part of a corona-free transformer 46 of which known types are available and which serves to raise the voltage across the

walls

34, and 39, 35 to a value high enough to generate corona in any voids that might appear in the insulation. I prefer that this voltage should be of the order of 20,000 v. and it is convenient to use a 60-cycle source for this current. However, a detectable corona can be generated with insulations of practical thickness by impressed potentials as low as 3 kv. Although it is convenient to employ a 60-cycle voltage source this may be converted to DC to avoid the high charging currents for a long length of cable. Thus the coil 42 of FIG. 2 may be replaced with other suitable AC or DC sources of high voltage within the scope of my invention. Where an AC voltage is impressed across the walls a band pass filter 47 is connected to the transformer in series with an ionization detecting instrument 48 which preferably includes a siren or alarm bell. Corona discharges have a wide frequency band and will excite the alarm 48 while the filter 47 prevents excitation by the low frequency of the impressed potential. Alternatively the need for the filter 47 can be eliminated by the arrangement of FIG. 4 where a source of high potential 50 is connected across a diagonal of a corona-free bridge 49 and the walls of the barrier are connected across one leg 51 of the bridge with the bridge impedances 52, 53, 54, 55 selected to balance out the transformer potential. The alarm 48 is connected to the other diagonal of the bridge 49 and will be excited by any corona discharges across the leg 51. I

In FIG. 3 I show a wave guide 66 having a conducting tube 67 covered by a thickness of insulation 68 which is surrounded in turn by a conducting layer 69. A second wall of insulation 71 surrounds the layer 69 and an outer conducting layer 72 surrounds the wall 71. The

conductors

67 and 72 are grounded while a high potential is applied between conductor 69 and ground. When it is necessary, each of the insulating

layers

68, 71 may be interposed between layers of conducting organic material to insure that there are no ionizable voids between the insulation and the conducting walls.

To supply a DC potential to the wall 69 an autotransformer 73 provides a potential that is increased by a step-up transformer 74 and rectified by a full-wave rectifier 76. Any 60- cycle residue is removed by a power supply filter 77 and a band suppression filter 78 provides a high impedance in the RF between the wall 69 and ground. Thus any RF detected by the ionization detector 48 can only be due to the ionization of voids in the

insulations

68, 71.

Although in FIGS. 2 and 3 l have shown the alarm means wired across both the inner and outer layers of insulation my barrier will be effective if it is only wired across one layer, for example the outer layer. In FIG. 2 an alternative embodiment of my invention might have the conductor 35 grounded as at present and the coil 42 remaining ungrounded and applied between the walls 34 and 39. In this case the jacket 41 would be made of insulating material sufficient to insulate the cable at the applied voltage. Corona discharges produce energy over a wide range of the frequency spectrum and are detectable by radio equipment up to I00 megacycles. When such equipment is used it will pick up any discharges between the conductors 34 and 35 even though it is wired only to the conductors 34 and 39. For this reason it would then also be necessary to have the insulation 32 void-free and possibly also include the conducting organic layers 31,33 which serve to minimize the chance of ionizable voids forming between conductor and insulation.-

The voltage that is applied across the dielectric of my barrier must be free of any significant radio frequency component, especially in the RF band which is selected for detection. Alternating current may beused since 50- and 60-cycle power sources are universally available. Normal power line voltages 100 v. and 220 v. would be stepped up to a voltage level, normally above 3,000 v., that will ionize air across a small gap. Freedom from RF noise is required at the barrier electrodes and is available at the output terminals of high-voltage corona-free" transformers of the type employed in cable corona detection equipment. Less expensive (non-coronafree) transformers may be used, with the provision for proper RF filtering between the transformer and barrier electrodes, especially in the RF band selected for detection. The detection equipment, consisting of a shielded single-band radio receiver, is coupled directly to the barrier electrodes. The output of the receiver is used to trip a relay which in turn trips any desired alarm.

In an alternating current system, sufficient power supply capacity is necessary to supply the charging current due to the capacitance susceptance of the barrier. Since the dielectric is charged only during the initial switching surge in a DC system, a DC power supply with a capacity limited to the leakage current requirements is sufficient. A DC power supply consisting of a HV transformer, rectifier and filter can be used. Since RF energy is shunted across the capacitors of the filter, a coronafree transformer is not required. A band suppression filter in the RF band used for detection is required between the power supply and barrier electrodes, the detector being coupled to the electrodes. It should be understood that the connections shown in'FlGS. 2 and 3 between the conducting barrier walls and the electrical circuitry are only diagrammatic and that in the actual structure of my barriers ground connections to the interior conductors such asthe

walls

35 and 67 and the connections between energy sources and the conductors 69. and 34 will be made at terminal'points, using known, corona-free, high-voltage cable terminating or jointing apparatus such as oil-filled potheads.

The voltage level at which each installation is operated will be dependent upon such factors as electrode configuration, electrode dimensions, and in cables and waveguides, their length. I have found that a preferable method of operation is to raise the voltage to a level at which corona is produced, then drop back to the corona extinction voltage. The operating level is then set percent below this extinction voltage to eliminate any spurious triggering due to normal voltage surges. This procedure is carried out simply by means of an auto transformer such as the transformer 73 at the primary winding of the power transformer.

l have invented a new'and useful article and method for which I desire an award of Letters Patent.

1 claim:

1. A tamperproof barrier comprising:

A. a first electrically conducting wall,

B. a second electrically conducting wall,

C. electrical insulation between said walls,

a. said insulation being free from corona-producing voids,

and

b. said insulation being in corona-free contact with said first and said second walls,

D. a transformer having a second coil applying an electric potential across said first and said second walls,

a. said potential jbeing sufficient to induce corona discharges in voids that might develop between said walls,

E. an electric bridge,

a. said coil being connected across a first diagonal of said bridge and b. said walls being connected across one leg of said bridge, and

F. an alarm responsive to corona dischargefrequencies a. connected across a second diagonalof said bridge. 2. A tamperproof cable comprising: A. at least one pair of communicationsconductors, B. a grounded shield over said conductors, C. an electrically conducting organic'layer a. surroundingsaid shield b. in electrical contact therewith over substantially the entire outer surface thereof,

'D. a first layer of electrical insulation surroundingsaid-organic layer,

a. said insulation beinginintimate=void freecontact with said organic layer,

E. a first electrical conductor a. surrounding said insulation and b. in corona-free contact therewith,

F. a second layer of electrical insulation a. surrounding said conductor and b. in corona-free contact therewith,

G. a second electrical conductor a. surrounding said second layer b. in corona-free contact therewith,

c. said second conductor being grounded,

H. means applying an electric potential to saidfirstconductor, and

l. alarm means responsive to corona discharges within said layers of insulation.

3. A tapproof communications cable comprising:

A. a cable core comprising at least one communication means selected from the group consisting of insulated wire pairs, shielded pairs, and coaxial pairs,

B. a first electrical conductor a. grounded,

b. surrounding said core,

C. a first electrically conducting organic layer a. surrounding said conductor and b. in electrical contact therewith,

D. a first layer of electrical insulation a. surrounding said organic layer,

b. said insulation being free from corona-producing voids,

and

c. in void-free contact with said organic layer,

E. a second electrically conducting organic layer a. surrounding said insulation and b. in void-free contact therewith,

F a second electrical conductor a. surrounding said second organic layer-and b. in electrical contact therewith,

G. a third electrically conducting organic layer.

a. surrounding said second conductor and b. in electrical contact therewith,

H. a second layer of electrical insulation a. surrounding said third organic layer,

b. said second layer of insulation being'free from coronaproducing voids and c. in void-free contact with said third'organiclayer,

l. a fourth electrically conducting organic layer a. surrounding said second layer ofinsulation and b. in void-free contact therewith,

J. a third electrical conductor a. surrounding said fourth organic layer b. in electrical contact therewith,

c. said third conductor being grounded,

K. a protective jacket surrounding said third conductor,

L. means applying an electric potential a. across said second conductor and at least one of-said grounded conductors,

b. said potential being sufficient to induce corona discharges in voids that might develop in'said layers of insulation, and

M. alarm means responsive to corona discharges in said layers of insulation.

4. A tamperproof waveguide comprising:

A. a tubular conductor,

Ba first layer of electrical insulation surrounding said conductor,

C. a first electrical conducting layer surrounding said insulation,

D. a second layer of insulation surrounding said first conducting layer,

a. insulation of said layers being void-free and b. in void-free contact with said conducting layer,

B. a second conducting layer surrounding said second layer of insulation a. in void-free contact therewith,

F. means applying an electrical potential from one to the other of said conducting layers said potential being sufficiently high to produce corona discharges in any voids subsequently occurring between said conducting layers, and

G. alarm means responsive to corona discharges in said second layer of insulation.

5. A tamperproof barrier comprising:

A. a first electrically conducting wall,

B. a second electrically conducting wall,

C. a void-free layer of insulation between said walls,

D. a source of adjustable alternating current applying a potential across said walls,

E. means rectifying said potential,

F. a power supply filter connected between said rectifying means and said walls,

G. a band suppression filter having a high impedance at radio frequencies connected between said power supply filter and one of said walls, and

H. alarm means responsive to electrical energy at radio frequencies connected between said walls.

6. The method of energizing a tamperproof barrier comprising electrically conducting walls separated by a layer of voidfree electrical insulation comprising the steps of:

A. applying an electric potential between said walls,

B. increasing said potential until the initiation of corona is detected on an ionization gage, and

C. immediately reducing said potential to the extinction point of said corona,

D. setting said potential at a level greater than 3,000 v. and less than the potential at which corona was extinguished, and

E. holding said potential at said level.

7. The method of energizing a tamperproof barrier comprising electrically conducting layers separated by a wall of voidfree electrical insulation comprising the steps of:

A. applying an electric potential between said walls,

C. immediately reducing said potential to the extinction point of said corona,

D. setting said potential at a value between 50 percent and 90 percent of the corona extinction potential, and

E. holding said potential at said value.

8. A safekeeping system comprising:

A. an interior region to be protected against unauthorized intrusion from an unprotected exterior region,

B. a monitor barrier forming the boundary between said protected and said unprotected regions comprising:

a. a first electrically conducting wall,

b. a second electrically conducting wall,

c. a wall of electrical insulation free from corona-producing voids between said conducting walls and in coronafree contact therewith,

C-. means applying a voltage across said first wall and said second wall,

' a: said voltage being sufficient to produce corona discharges in any voids subsequently occurring between said walls, and

D. alarm means connected to at least one of said conducting 7 walls responsive to corona discharges between said walls. 9. The system of claim 8 wherein said voltage is at, least 3 kilovolts.

10. The system of claim 8 wherein said interior region contains at least one signal-carrying conductor.

11. The system of claim 10 wherein said voltage is at least 3 kilovolts.

12. A safekeeping system comprising:

a. a first electrically conducting wall,

b. a second electrically-conducting wall,

c. a wall of electrical insulation free from corona-producing voids between said walls,

d. an electrically conducting organic layer in void-free contact with said wall of electrical insulation between said wall of electrical insulation and said first wall, and

e. an electrically conducting organic layer in void-free contact with said wall of electrical insulation between said wall of electrical insulation and said second wall,

C. means'applying a voltage across said first wall and said second wall,

a. said voltage being sufficient to produce high-frequency corona discharges in voids between said walls, and

b. said voltage having a frequency substantially lower than said high-frequency corona discharges,

D. alarm means connected to at least one of said conducting walls responsive to corona discharges between said walls, and

E. means in series with said alarm means filtering out said voltage.

13. The system of claim 12 wherein said voltage is at least 3 kilovolts.

14. The system of claim 13 wherein said interior region contains at least one signal-carrying conductor.

15. The system of claim 14 wherein said voltage is at least 3 kilovolts.

16. A safekeeping system comprising:

a. a first electrically conducting wall,

b. a second electrically conducting wall,

C. means applying a voltage across said first wall and said second wall,

a. said voltage being sufiicient to produce corona discharges in voids between said walls,

D. alarm means responsive to corona discharges,

E. a tamperproof cable connecting said barrier to said alarm means comprising:

a. a first elongated electrical conductor,

b. electrical insulation surrounding said conductor,

1. said insulation being free from corona-producing voids, and

2. said insulation being in corona-free contact with said conductor, and

c. a second elongated electrical conductor,

1. surrounding said insulation 2. in corona-free contact therewith,

(1. said first conductor being electrically connected to one of said conducting walls and said second conductor being electrically connected to the other of said conducting walls whereby said alarm means will respond to the penetration of said barrier and to the penetration of said cable.

Claims

1. A tamperproof barrier comprising: A. a first electrically conducting wall, B. a second electrically conducting wall, C. electrical insulation between said walls, a. said insulation being free from corona-producing voids, and b. said insulation being in corona-free contact with said first and said second walls, D. a transformer having a secondary coil applying an electric potential across said first and said second walls, a. said potential being sufficient to induce corona discharges in voids that might develop between said walls, E. an electric bridge, a. said coil being connected across a first diagonal of said bridge and b. said walls being connected across one leg of said bridge, and F. an alarm responsive to corona discharge frequencies a. connected across a second diagonal of said bridge.

2. A tamperproof cable comprising: A. at least one pair of communications conductors, B. a grounded shield over said conductors, C. an electrically conducting organic layer a. surrounding said shield b. in electrical contact therewith over substantially the entire outer surface thereof, D. a first layer of electrical insulation surrounding said organic layer, a. said insulation being in intimate void-free contact with said organic layer, E. a first electrical conductor a. surrounding said insulation and b. in corona-free contact therewith, F. a second layer of electrical insulation a. surrounding said conductor and b. in corona-free contact therewith, G. a second electrical conductor a. surrounding said second layer b. in corona-free contact therewith, c. said second conductor being grounded, H. means applying an electric potential to said first conductor, and I. alarm means responsive to corona discharges within said layers of insulation.

2. said insulation being in corona-free contact with said conductor, and c. a second elongated electrical conductor,

2. in corona-free contact therewith, d. said first conductor being electrically connected to one of said conducting walls and said second conductor being electrically connected to the other of said conducting walls whereby said alarm means will respond to the penetration of said barrier and to the penetration of said cable.

3. A tapproof communications cable comprising: A. a cable core comprising at least one communication means selected from the group consisting of insulated wire pairs, shielded Pairs, and coaxial pairs, B. a first electrical conductor a. grounded, b. surrounding said core, C. a first electrically conducting organic layer a. surrounding said conductor and b. in electrical contact therewith, D. a first layer of electrical insulation a. surrounding said organic layer, b. said insulation being free from corona-producing voids, and c. in void-free contact with said organic layer, E. a second electrically conducting organic layer a. surrounding said insulation and b. in void-free contact therewith, F. a second electrical conductor a. surrounding said second organic layer and b. in electrical contact therewith, G. a third electrically conducting organic layer a. surrounding said second conductor and b. in electrical contact therewith, H. a second layer of electrical insulation a. surrounding said third organic layer, b. said second layer of insulation being free from corona-producing voids and c. in void-free contact with said third organic layer, I. a fourth electrically conducting organic layer a. surrounding said second layer of insulation and b. in void-free contact therewith, J. a third electrical conductor a. surrounding said fourth organic layer b. in electrical contact therewith, c. said third conductor being grounded, K. a protective jacket surrounding said third conductor, L. means applying an electric potential a. across said second conductor and at least one of said grounded conductors, b. said potential being sufficient to induce corona discharges in voids that might develop in said layers of insulation, and M. alarm means responsive to corona discharges in said layers of insulation.

4. A tamperproof waveguide comprising: A. a tubular conductor, B. a first layer of electrical insulation surrounding said conductor, C. a first electrical conducting layer surrounding said insulation, D. a second layer of insulation surrounding said first conducting layer, a. insulation of said layers being void-free and b. in void-free contact with said conducting layer, E. a second conducting layer surrounding said second layer of insulation a. in void-free contact therewith, F. means applying an electrical potential from one to the other of said conducting layers said potential being sufficiently high to produce corona discharges in any voids subsequently occurring between said conducting layers, and G. alarm means responsive to corona discharges in said second layer of insulation.

5. A tamperproof barrier comprising: A. a first electrically conducting wall, B. a second electrically conducting wall, C. a void-free layer of insulation between said walls, D. a source of adjustable alternating current applying a potential across said walls, E. means rectifying said potential, F. a power supply filter connected between said rectifying means and said walls, G. a band suppression filter having a high impedance at radio frequencies connected between said power supply filter and one of said walls, and H. alarm means responsive to electrical energy at radio frequencies connected between said walls.

6. The method of energizing a tamperproof barrier comprising electrically conducting walls separated by a layer of void-free electrical insulation comprising the steps of: A. applying an electric potential between said walls, B. increasing said potential until the initiation of corona is detected on an ionization gage, and C. immediately reducing said potential to the extinction point of said corona, D. setting said potential at a level greater than 3,000 v. and less than the potential at which corona was extinguished, and E. holding said potential at said level.

7. The method of energizing a tamperproof barriEr comprising electrically conducting layers separated by a wall of void-free electrical insulation comprising the steps of: A. applying an electric potential between said walls, B. increasing said potential until the initiation of corona is detected on an ionization gage, and C. immediately reducing said potential to the extinction point of said corona, D. setting said potential at a value between 50 and 90 percent of the corona extinction potential, and E. holding said potential at said value.

8. A safekeeping system comprising: A. an interior region to be protected against unauthorized intrusion from an unprotected exterior region, B. a monitor barrier forming the boundary between said protected and said unprotected regions comprising: a. a first electrically conducting wall, b. a second electrically conducting wall, c. a wall of electrical insulation free from corona-producing voids between said conducting walls and in corona-free contact therewith, C. means applying a voltage across said first wall and said second wall, a. said voltage being sufficient to produce corona discharges in any voids subsequently occurring between said walls, and D. alarm means connected to at least one of said conducting walls responsive to corona discharges between said walls.

9. The system of claim 8 wherein said voltage is at least 3 kilovolts.

11. The system of claim 10 wherein said voltage is at least 3 kilovolts.

12. A safekeeping system comprising: A. an interior region to be protected against unauthorized intrusion from an unprotected exterior region, B. a monitor barrier forming the boundary between said protected and said unprotected regions comprising: a. a first electrically conducting wall, b. a second electrically conducting wall, c. a wall of electrical insulation free from corona-producing voids between said walls, d. an electrically conducting organic layer in void-free contact with said wall of electrical insulation between said wall of electrical insulation and said first wall, and e. an electrically conducting organic layer in void-free contact with said wall of electrical insulation between said wall of electrical insulation and said second wall, C. means applying a voltage across said first wall and said second wall, a. said voltage being sufficient to produce high-frequency corona discharges in voids between said walls, and b. said voltage having a frequency substantially lower than said high-frequency corona discharges, D. alarm means connected to at least one of said conducting walls responsive to corona discharges between said walls, and E. means in series with said alarm means filtering out said voltage.

13. The system of claim 12 wherein said voltage is at least 3 kilovolts.

15. The system of claim 14 wherein said voltage is at least 3 kilovolts.

16. A safekeeping system comprising: A. an interior region to be protected against unauthorized intrusion from an unprotected exterior region, B. a monitor barrier forming the boundary between said protected and said unprotected regions comprising: a. a first electrically conducting wall, b. a second electrically conducting wall, c. a wall of electrical insulation free from corona-producing voids between said conducting walls and in corona-free contact therewith, C. means applying a voltage across said first wall and said second wall, a. said voltage being sufficient to produce corona discharges in voids between said walls, D. alarm means responsive to corona discharges, E. a tamperproof cable connecting said barrier to said alarm means comprising: a. a first elongated electrical conductor, b. electrical insulation surrounding said conductor,