U.S. patent number RE28,692 [Application Number 05/540,147] was granted by the patent office on 1976-01-20 for monitoring apparatus and method for electrical protection systems.
This patent grant is currently assigned to American District Telegraph Company. Invention is credited to Raymond J. Highstone, deceased, Manfred W. Muehter.
United States Patent |
RE28,692 |
Highstone, deceased , et
al. |
January 20, 1976 |
Monitoring apparatus and method for electrical protection
systems
Abstract
.Iadd. 1. Apparatus for increasing the security of an electrical
protection system in which a protected location is conductively
coupled to a central station which comprises at least two
conductive means for coupling said protected location to said
central station, at said central station, a first supervisory
circuit and a second supervisory circuit, each supervisory circuit
including a source of supervisory current coupled to a
corresponding one of said conductive means and current-sensitive
means coupled to said corresponding conductive means to detect
predetermined changes in supervisory current therein, and at said
protected location, a first protection circuit and a second
protection circuit respectively corresponding to said first and
second supervisory circuits at said central station, wherein each
protection circuit is provided with an input terminal, a shunt
terminal and an output terminal, and wherein at least one of said
protection circuits is provided with at least one detection device
connected between said input terminal and said output terminal so
that operation of said detection device alters the flow of
supervisory current in said protection circuit, Means for
connecting the input terminal of each of said protection circuits
to a corresponding one of said conductive means to couple each
protection circuit to a corresponding supervisory circuit at said
central station, so that supervisory current is directed to each
protection circuit, a ground point, switching means interposed
between said ground point and said output terminals of said first
and second protection circuits, said switching means including:
first relay means and second relay means respectively corresponding
to said first and second protection circuits, wherein each of said
relay means is provided with a winding having first and second
terminals, an armature, and a contact, means for connecting the
second terminal of each relay means to said ground point, means for
connecting the armature of each relay to the first terminal of the
winding of the other relay, two first conductor means for
connecting the output terminals of said first and second protection
circuits to said respective first terminals of said windings of
said correspondingly first and second relay means, and two second
conductor means for connecting the shunt terminals of each of said
first and second protection circuits to said contacts of said
second and first relay means, respectively, wherein each of said
shunt terminals is located in said respective first and second
protection circuits at a point selected to shunt a predetermined
portion of the respective protection circuit in response to the
energizing of the relay means corresponding to the other protection
circuit.
Inventors: |
Highstone, deceased; Raymond J.
(late of Scottsdale, AZ), Muehter; Manfred W. (Lanoka
Harbor, NJ) |
Assignee: |
American District Telegraph
Company (New York, NY)
|
Family
ID: |
27058985 |
Appl.
No.: |
05/540,147 |
Filed: |
January 10, 1975 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
516896 |
Dec 28, 1965 |
03401384 |
Sep 10, 1968 |
|
|
Current U.S.
Class: |
340/508; 340/550;
340/511; 340/647 |
Current CPC
Class: |
G08B
29/06 (20130101); G08B 29/08 (20130101) |
Current International
Class: |
G08B
29/00 (20060101); G08B 29/06 (20060101); G08B
29/08 (20060101); G08B 013/22 () |
Field of
Search: |
;340/276,409 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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447,265 |
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Mar 1948 |
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CA |
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616,363 |
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Mar 1961 |
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CA |
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758,263 |
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May 1967 |
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CA |
|
Primary Examiner: Trafton; David L.
Attorney, Agent or Firm: Smith; Charles B.
Claims
What is claimed is:
1. Apparatus for increasing the security of an electrical
protection system in which a protected location is conductively
coupled to a central station which comprises at least two
conductive means for coupling said protected location to said
central station,
at said central station, a first supervisory circuit and a second
supervisory circuit, each supervisory circuit including a source of
supervisory current coupled to a corresponding one of said
conductive means and current-sensitive means coupled to said
corresponding conductive means to detect predetermined changes in
supervisory current therein, and
at said protected location, a first protection circuit and a second
protection circuit respectively corresponding to said first and
second supervisory circuits at said central station, wherein each
protection circuit is provided with an input terminal, a shunt
terminal and an output terminal, and wherein at least one of said
protection circuits is provided with at least one detection device
connected between said input terminal and said output terminal so
that operation of said detection device alters the flow of
supervisory current in said protection circuit,
means for connecting the input terminal of each of said protection
circuits to a corresponding one of said conductive means to couple
each protection circuit to a corresponding supervisory circuit at
said central station, so that supervisory current is directed to
each protection circuit,
a ground point,
switching means interposed between said ground point and said
output terminals of said first and second protection circuits, said
switching means including:
first relay means and second relay means respectively corresponding
to said first and second protection circuits, wherein each of said
relay means is provided with a winding having first and second
terminals, an armature, and a contact,
means for connecting the second terminal of each relay means to
said ground point,
means for connecting the armature of each relay to the first
terminal of the winding of the other relay,
two first conductor means for connecting the output terminals of
said first and second protection circuits to said respective first
terminals of said windings of said corresponding first and second
relay means, and
two second conductor means for connecting the shunt terminals of
each of said first and second protection circuits to said contacts
of said second and first relay means, respectively, wherein each of
said shunt terminals is located in said respective first and second
protection circuits at a point selected to shunt a predetermined
portion of the respective protection circuit in response to the
energizing of the relay means corresponding to the other protection
circuit.
2. The method of detecting attempts to compromise an electrical
protection system including a central station connected to a
protected location by at least two conductors, at least two
protection circuits at said protected location, and at least two
supervisory circuits at said central station individually connected
to supply separate supervisory current to each of two corresponding
protection circuits by means of two of said conductors, which
comprises the step of:
shunting a portion of either of said protection circuits in
response to a predetermined change in supervisory current in the
other one of said protection circuits so that a predetermined
change in supervisory current in either one of said protection
circuits is accompanied by a selected change in supervisory current
in the other one of said protection circuits.
3. An electrical protection system in which a protected location is
conductively coupled to a central station which comprises:
first conductive means and second conductive means for coupling
said protected location to said central station,
at said central station, a first supervisory circuit and a second
supervisory circuit corresponding to said first and second
conductive means, wherein each supervisory circuit includes a
source of supervisory current coupled to said corresponding
conductive means and current-sensitive means coupled to said
corresponding conductive means to detect predetermined changes in
said supervisory current therein, and
at said protected location, a first protection circuit and a second
protection circuit respectively coupled to said first and second
conductive means to carry supervisory current supplied by said
corresponding supervisory circuit to a ground point, and
switching means interposed between said ground point and said first
and second protection circuits for shunting a portion of either of
said protection circuits in response to a predetermined change in
supervisory current in the other one of said protection circuits so
that a predetermined change in supervisory current in either one of
said protection circuits is accompanied by a change in supervisory
current in the other one of said protection circuits.
4. Apparatus as defined in claim 3 wherein said first protection
circuit comprises at least two current-adjusting resistors so that
said switching means shunts one of said current-adusting resistors
in response to said predetermined change in supervisory current in
the other one of said protection circuits.
5. Apparatus as defined in claim 3 wherein said second protection
circuit comprises at least one detection device so that operation
of said detection device alters said supervisory current carried by
said first protection circuit.
6. Apparatus as defined in claim 3 wherein each of said first and
second protection circuits includes at least one detection device
so that operation of said detection device in either one of said
protection circuits alters said supervisory current carried by the
protection circuit including said operated detection device.
7. Apparatus as defined in claim 3 wherein said switching means
comprises:
a first relay means including an armature, a contact, and a winding
with an input terminal and a ground terminal,
a second relay means including an armature, a contact, and a
winding with an input terminal and a ground terminal,
means for connecting said armature of said first relay means to the
input terminal of said winding of said second relay means,
means for connecting said armature of said second relay to said
input terminal of said winding of said first relay means, and
means for connecting said ground terminals of said windings of said
first and second relay means to said ground point. .Iadd. 8.
Apparatus as defined in claim 1 wherein at least one of said first
and second protection circuits includes at least one additional
shunt terminal beyond that connected by the second conductor means
to the contact of the relay means, and wherein said apparatus
further includes
switch means corresponding in number to the number of said
additional shunt terminals, each of said switch means having a
first and second terminal,
third conductor means for connecting the first terminal of each of
said switch means to one of said additional shunt terminals,
and
fourth conductor means for connecting the second terminal of each
of said switch means to the second conductor means connected to the
shunt terminal of the protection circuit which contains the
additional shunt terminal to which the first terminal of the switch
is connected by said third conductor means,
whereby the elements of either of the protection circuits included
in the shunted portion of the protection circuits can be
varied..Iaddend..Iadd. 9. Apparatus as defined in claim 8 wherein
the first and second protection circuits each include at least one
additional shunt terminal..Iaddend..Iadd. 10. Apparatus as defined
in claim 3 further including means for varying the portion of
either of the protection circuits which is shunted..Iaddend..Iadd.
11. Method as defined in claim 2 further including varying the
portion of either of the protection circuits which is
shunted..Iaddend.
Description
The present invention relates to electrical protection systems, and
more particularly to electrical protection systems having apparatus
for improving the security of such systems by detecting attempts to
compromise the systems.
In direct wire central station burglar alarm systems a number of
protected places such as factories, banks and mercantile
establishments, are individually connected to a central station by
means of separate lines or conductive paths, usually ordinary
telephone lines leased for the purpose. Each of the protected
premises is typically provided with a protection circuit including
one or more devices which are arranged to detect the entry of an
intruder into the protected premises by producing an electrical
variation in the corresponding line which results in an alarm
signal at the central station.
Direct wire central station burglar alarm systems are customarily
designed to provide an alarm signal upon a departure of a
predetermined magnitude from the normal value of a supervisory line
current supplied from the central station through the corresponding
conductive path to the protection circuit at the protected
premises. Detection devices of various types are located at the
protected premises and are so arranged as to break and/or ground
the protection circuit upon being actuated by some activity of an
intruder. The resulting decrease and/or increase in the normal
supervisory current is detected by a so-called "drop" or
supervisory circuit located at the central station which also
serves to annunciate the alarm signal to the attendants at the
central station.
In brief, the customary drop consists of a break detecting relay
and a ground detecting relay connected in series with the line, the
break relay responding to a predetermined decrease in line current
and the ground relay responding to a predetermined increase in line
current. Each relay is provided with suitable contacts and
auxiliary circuitry to actuate visible and audible annunciators
upon line current changes of sufficient magnitude to actuate the
corresponding relay. Of course, suitable electronic circuitry may
be used in place of the current-sensitive relays, an example of
which is the solid state circuit described and claimed in the
copending United States patent application of Manfred W. Muehter,
Ser. No. 309,761, filed Sept. 18, 1963.
While difficult, it is at least theoretically possible for a
skilled and knowledgeable intruder to compromise such burglar alarm
systems by tampering with .[.thhe.]. .Iadd.the .Iaddend.protection
circuit. For example, if the transmission lines could be bridged
external to the protected premises through a device having
approximately the characteristics of the electrical load
represented by the detection apparatus comprising the internal
protection circuit, an entry could be then effected to the premises
without producing an alarm signal at the central station. Under
such circumstances, the device acts as a dummy load which, if it
can be substituted undetected for the internal protection circuit,
will permit disconnection of the actual detection apparatus from
the circuit. The dummy load must obviously electrically resemble
the internal circuit closely enough so that the line current will
not be changed sufficiently to actuate either the break or ground
relays in the central station drop.
Alternatively, it is also theoretically possible under certain
conditions to ground the transmission line between the central
station and the protected premises thereby negativing the effect
produced upon the protection circuit by the operation of the
detection devices at the protected premises. Usually this would be
of significance under daytime conditions, and would be of concern
particularly in connection with holdup alarms.
In the present invention, attempts to compromise an electrical
protection system are immediately detected at the central station
by providing at the protected premises at least two protection
circuits which are interconnected so that a compromise attempt on
one protection circuit will cause the other protection circuit to
produce an alarm signal at the central station. The two protection
circuits are interconnected at the protected premises by means of a
pair of switching devices respectively interposed between each of
the two circuits and the local ground connection, and each of the
two protection circuits is connected to a separate supervisory
circuit at the central station via a separate supervisory signal
path. The two switching devices are arranged so that an attempt to
compromise either one of the protection circuits which results in
the operation of the corresponding switching device will shunt a
portion of the other protection circuit, thereby increasing the
normal supervisory signal current in the second circuit by an
amount sufficient to produce to an alarm signal in the
corresponding supervisory circuit at the central station.
Apart from the detection of attempted tampering with protection
circuits, the present invention also provides useful advance
warning of impending electrical troubles in the two protection
circuits provided at the protected premises. For instance, slowly
deteriorating insulation tends to cause increasing line leakage,
and if the leakage in one of the protection circuits reaches a
point sufficient to cause operation of the corresponding switching
device, then a portion of the other protection circuit is shunted
thereby operating the ground relay in the drop at the central
station. Similarly, increasing circuit resistance, indicative of
dirty or corroded terminals and contacts, in one of the protection
circuits may also be detected at the central station by the
supervisory circuit of the other protection circuit. The line may
then be repaired while still serviceable instead of passing
unnoticed to the alarm level at which time the line would be out of
service until repairs could be effectuated.
Those skilled in the art will immediately recognize from the
following description and appended drawings that the principles of
the present invention may be applied with advantage to various
types of electrical protection systems and to other electrical
systems designed for the transmission of data over extended lines.
However, as it is anticipated that the invention will find most
immediate utility in central station direct wire burglar alarm
systems, the invention will be described in terms of its
application thereto. The description is intended for illustrative
purposes only and in no manner to limit the invention.
The invention will now be described in greater detail which
reference to the appended drawings, in which:
FIG. 1 is a schematic wiring diagram of a direct wire burglar alarm
system embodying the basic principles of the invention and
illustrating an installation where detection devices are divided
between two protection circuits; and
FIG. 2 is a schematic wiring diagram of a direct wire burglar alarm
system embodying the principles of the invention and especially
adapted for locations where it is not desired to divide the
detection devices between two protection circuits.
FIGURE 1
Referring now to FIG. 1, two supervisory circuits 13-1 and 13-2 at
a central station 10 are shown connected to a protect premises 11
by two separate signal paths 12-1 and 12-2, each of which may be a
telephone line or similar direct wire connection. The return may be
by separate wire connection or, as shown, by ground return.
Typically, the lines 12-1 and 12-2 may be as much as twenty miles
in length and may pass through several telephone exchanges (not
shown). At the protected premises 11, the lines 12-1 and 12-2 are
connected to the input points of corresponding local protection
circuits 14-1 and 14-2, each circuit including conventional
detection devices which produce break and/or ground signals, and
thence via the output points of circuits 14-1 and 14-2 through
conductors 17-1, 17-2 and switching unit 15 to ground point 18. For
convenience of description, local protection circuits 14-1 and 14-2
have been shown as identical in construction, although it is to be
understood that in general the two circuits will include different
arrangements of detecting devices. Thus local protection circuit
14- 1 is illustrated in highly simplified form as comprising the
series connection of a current adjusting resistor 30-1, a foil
circuit 31-1, armature 34-1 and back contact 33-1 of a protection
control switch 32-1, another foil circuit 36-1, armature 38-1 and
back contact 40-1 of a door protection switch 37-1, a current
adjusting resistor 41-1, foil circuits 42-1 and 44-1, and thence to
ground at 18 via switching unit 15. The door protection switch 37-1
has a front contact 39-1 connected to foil circuit 42-1 at point
45-1, and the protection control switch 32-1 has a front contact
35-1, which, when made with armature 34-1, provides a shunt through
conductor 43-1 to a point between foil circuits 42-1 and 44-1 for
all circuit components 36-1, 37-1, 41-1, and 42-1.
Within circuit 14-1, the foil circuits 31-1, 36-1, 42-1, 44-1 may
consist of conventional lead foil strip adhered to the glass panels
of windows and doors and carry the normal supervisory current
supplied from the corresponding supervisory circuit 13-1 at the
central station 10. By way of example, if an intruder breaks a
glass panel in an effort to enter the premises, the current
carrying foil will be severed and thereby result in the production
of a "break" alarm signal by the corresponding supervisory circuit
13-1 at the central station 10. Typically, such a break signal
might be represented by a decrease in supervisory current in line
12-1 from a normal value of 15 milliamperes to 9 milliamperes or
less. A similar form of protection for walls, floors and ceilings
is sometimes provided by means of foil or fine wire applied to the
surfaces thereof in such manner that it will be ruptured if the
surface is breached. Items 31-1, 36-1, 42-1, 44-1 of the drawings
are intended to represent this form of protection regardless of
whether installed on glass panels or other building structure
elements.
Also within circuit 14-1, if a door protected by the switch 37-1 is
opened, the armature 38-1 will transfer from the back contact 40-1
to the front contact 39-1 and result in the production of a brief
"break" alarm signal (while the armature 38-1 is in motion)
followed by a "ground" alarm signal by circuit 13-1 at the central
station 10. Typically, a ground signal might be represented by an
increase in supervisory current in line 12-1 from the normal value
to 21 milliamperes or more.
It should be understood that the actual protection circuits at the
protected premises will usually be substantially more complex and
sophisticated than the simple circuits shown and will include a far
greater number of protective devices. It should also be understood
that severing of one of the foil strips 31-2, 36-2, 42-2, 44-2 in
protection circuit 14-2 will result in the production of a "break"
alarm signal by corresponding supervisory circuit 13-2 at central
station 10, and that opening of a door protected by switch 37-2 in
protection circuit 14-2 will result in the production of a brief
"break" alarm signal followed by a "ground" alarm signal by
supervisory circuit 13-2 at central station.
The other end of line 12-1 is coupled within circuit 13-1 at the
central station 10 to the positive terminal of a source of
potential 22-1 (which typically might be a fifty-two volt battery)
through a series circuit comprising test equipment 21-1, the
winding of the ground relay G-1 and the winding of the break relay
B-1. The negative terminal of the normal operation battery 22-1 is
grounded at point 23-1. Test equipment 21-1 may comprise any one of
a number of well known arrangements, for example, equipment 21-1
may be designed in accordance with the principles disclosed in the
copending United States patent application of Lawrance H. Mitchell,
Ser. No. 331,771, filed Dec. 19, 1963.
As previously indicated, each of the circuits at the protected
premises will generally comprise a number of devices for the
protection of doors, walls and windows. In the case of mercantile
establishments particularly, it has been found convenient to shunt
certain of the protective device.Iadd.s .Iaddend.out of one or both
of the circuits during normal business hours and thus avoid the
production of alarm signals as the front door or various windows
are opened in the ordinary course of business. The shunt feature is
shown in the drawings as controlled in protection circuits 14-1 and
14-2 by the respective protection control switches 32-1 and 32-2
which are operated when the premises are opened for business so
that armatures 34-1 and 34-2 make with front contacts 35-1 and 35-2
thereby connecting lines 12-1 and 12-2 to ground point 18 via
elements 30-1, 31-1, 44-1, and 30-2, 31-2, 44-2, respectively, and
removing protection devices 36-1, 37-1, 42-1 and 36-2, 37-2, 42-2
from the respective circuits. The devices 31-1, 44-1 and 31-2, 44-2
remaining in the respective protection circuits 14-1 and 14-2
ordinarily constitute the so-called "permanent" protection devices
such as foil or wire protected walls, ceilings, etc.
Adjustable resistors 30-1, 41-1 and 30-2, 41-2 are provided in
circuits 14-1 and 14-2 at the protected premises to limit the
current in each of the lines 12-1 and 12-2 to a selected nominal
value, e.g., fifteen milliamperes, during the period while the
entire protection system is in operation, usually termed the
"protection-on" or "night" period. The circuit components are so
selected and adjusted that operation of a break signal detection
device will cause the line current to fall to or below a first
selected value, e.g., nine milliamperes, while operation of a
ground signal detection device will cause the line current to rise
to or above a second preselected valve, e.g., twenty-one
milliamperes. During the "protection-off" or "day" period, the
adjustments are such that the system is in the grounded condition.
While specific values of currents and voltages are referred to
herein for convenience in explaining the operation of the
invention, it should be understood that these specific values are
given by way of illustration only and are not to be construed as in
any manner limiting the invention.
In supervisory circuit 13-1 at central station 10, the break relay
B-1 is arranged to be held energized by the normal current in line
12-1, while the ground relay G-1 is .[.de-energizing.].
.Iadd.de-energized .Iaddend.during the "protection-on" period. A
decrease in the line current to or below the first selected value
will cause the break relay B-1 to become de-energized and open the
normally closed contacts B1-1, while an increase of line current to
or above the second selected value will energize the ground relay
G-1 and close the normally open contacts G1-1. The contacts B1-1
and G1-1 are coupled to alarm annunciators 20-1 which are of
conventional design and serve to provide audible and visible
indications of the operation of the break and ground relays,
respectively, to the attendants at the central station. Operation
of supervisory circuit 13-2 corresponding to protection circuit
14-2 is similar in all respects to the above-described operation of
circuit 13-1.
The present invention improves the security of the system described
above during the protection-on condition by providing an
interconnection between circuits 14-1 and 14-2 in switching unit 15
so that operation of any device in either protection circuit which
interrupts the supervisory current in one circuit for more than a
predetermined time interval will not only cause an alarm signal to
be produced by the corresponding supervisory circuit at the central
station 10 but also cause an alarm signal to be produced by the
other supervisory circuit at the central station 10. Referring to
switching unit 15, conductor 17-1 from protection circuit 14-1 is
connected to the input terminal of winding 52-1 of relay R1 so that
relay R1 is energized by the flow of normal supervisory current to
ground at 18 supplied from battery 22-1 in supervisory circuit
13-1. Contact 50-1 of relay R1 is connected to conductor 16-2 from
protection circuit 14-2, and armature 51-1 of relay R1 is connected
to the input terminal of winding 52"2 of relay R2. In the energized
condition of relay R1, contact 50-1 is normally open so that
conductor 16-2 is not connected to ground point 18 through winding
52-2 of relay R2, but in the de-energized condition of relay R1,
armature 51-1 makes with contact 50-1 to provide a ground
connection for conductor 16-2 via winding 52-2 of relay R2. It is
therefore evident that by connecting conductor 16-2 to an
appropiate point 46-2 in protection circuit 14-2, a decrease in the
normal supervisory current flowing through protection circuit 14-1
to winding 52-1, which causes the current to fall to a level below
that required to energize relay R1, will release relay R1 and
thereby result in shunting a portion of protection circuit 14-2.
Since shunting of a portion of protection circuit 14-2 causes the
corresponding supervisory circuit 13-2 to produce an alarm signal,
protection circuit 14-2 monitors protection circuit 14-1 to provide
an indication of any decrease in supervisory current in circuit
14-1 which is sufficient to release relay R1, regardless of whether
this decrease is accompanied by the generation of alarm signals in
supervisory circuit 13-1 corresponding to protection circuit
14-1.
Relay R2 provides similar monitoring of protection circuit 14-2 by
protection circuit 14-1 and supervisory circuit 13-1. Conductor
17-2 from protection circuit 14-2 is connected to ground point 18
by way of winding 52-2 of relay R2 so that relay R2 is energized by
the flow of normal supervisory currents to ground at 18 from
battery 22-2 in supervisory circuit 13-2. Contact 50-2 of relay R2
is connected to conductor 16-1 from protection circuit 14-1, and
armature 51-2 of relay R2 is connected to the input terminal of
winding 52-1 of relay R1. In the energized condition of relay R2,
contact 50-2 is normally open so that conductor 16-1 is not
connected to ground point 18 through winding 52-1 of relay R1, but
in the de-energized condition of relay R2, armature 51-2 makes with
contact 50-2 to provide a ground connection for conductor 16-1 via
winding 52-1 of relay R1. It is therefore evident that by
connecting conductor 16-1 to an appropriate point 46-1 in
protection circuit 14-1, a decrease of sufficient magnitude in the
supervisory current flowing through protection circuit 14-2 to
winding 52-2 will release relay R2 and thereby result in shunting a
portion of protection circuit 14-1. Since shunting of a portion of
protection circuit 14-1 causes supervisory circuit 13-1 to produce
an alarm signal, protection circuit 14-1 monitors protection
circuit 14-2 to provide an indication of any decrease in
supervisory current in circuit 14-2 which is sufficient to release
relay R2, regardless of whether this decrease is accompanied by the
generation of alarm signals in supervisory circuit 13-2.
The present invention therefore provides a complementary monitoring
arrangement whereby attempts to defeat or compromise either one of
two protection circuits at a protected premises will result in the
production of an alarm signal by the other circuit. However,
although the invention has been illustrated and described in terms
of two protection circuits interconnected to provide complementary
monitoring, it is to be understood that if desired more than two
protection circuits may be provided at a protected premises, in
which case it is to be further understood that more than two
protection circuits may be interconnected in accordance with the
principles of this invention to provide complementary monitoring of
the type described.
FIGURE 2
Turning now to FIG. 2, this drawing illustrates an embodiment of
the principles of this invention designed for premises in which it
is either inconvenient or undesirable to divide the detection
devices between two separate protection circuits. For example, in a
location already provided with a protection circuit, it may be too
expensive or too time consuming to divide the existing protection
circuit into two circuits. In such a situation, complementary
monitoring may be provided by an arrangement of the type shown in
FIG. 2, where like reference numbers are used to identify
components which are similar to those illustrated in FIG. 1.
In FIG. 2 the block labelled protection circuit 14-2 at protected
premises 11 represents a circuit of the type shown by the like
numbered circuit in FIG. 1, in which circuit 14-2 typically
includes a number of detection devices. Protection circuit 14-2 is
provided with a direct wire connection 12-2 to corresponding
supervisory circuit 13-2 at central station 10, and circuit 13-2
functions in the manner previously described in connection with the
similarly numbered circuit in FIG. 1. In addition to protection
circuit 14-2, there is installed a protection circuit 200 which as
shown may include no detection devices such as foil, wire or door
switches, but only one or more current adjusting resistors 201 and
202. Protection circuit 200 is provided with a direct wire
connection 12-1 to corresponding supervisory circuit 13-1 at
central station 10, circuit 13-1 functioning in the same manner as
supervisory circuit 13-1 in FIG. 1.
Switching unit 15, which may be identical with the similarly
numbered component in FIG. 1, is connected to protection circuit
200 and protection circuit 14-2 by conductors 16-1, 17-1 and 16-2,
17-2, respectively. In this manner, a sufficient decrease in the
supervisory current flowing through either protection circuit 200
or protection circuit 14-2 will release the corresponding relay in
circuit 15, thereby to shunt a portion of the other protection
circuit and produce an alarm signal at central station 10 by the
supervisory circuit corresponding to the shunted circuit. As shown
in FIG. 2, the conductor 16-1 is connected to protection circuit
200 at point 203 to shunt resistor 202.
In FIG. 1 the connection of the shunting conductors 16-1 and 16-2
to the respective protection circuits 14-1 and 14-2 has been shown
at points 46-1 and 46-2, so that closure of either contact 50-1 or
50-2 will shunt all the protection devices in the respective
protection circuits which are subsequent to the points 46-1 and
46-2. It will be obvious to those skilled in the art that under
certain circumstances it may be desirable to locate the shunting
conductor connection at a different point in the protection circuit
or to provide means whereby the point of connection may be
selectively varied. For this reason, supplementary conductors 18-1
and 18-2 are shown connected to circuits 14-1 and 14-2 by way of
switches 47-1 and 47-2, respectively. The closure of either of
these switches 47-1, 47-2 while the protection control switches
32-1 and 32-2 are in the "protection-on" conditions will have the
effect of including the associated protection elements 36-1, 36-2
in the shunted portion of the circuits. Also, it will be understood
by those skilled in the art that the electromechanical relays shown
at various points in the drawings may be replaced by electronic
devices wherever desired.
While the invention hsa been described in connection with specific
embodiments thereof and in specific uses, various modifications
thereof will occur to those skilled in the art without departing
from the spirit and scope of the invention as set forth in the
appended claims.
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