Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to protection circuitry for
a cable transmission system and more particularly to an improved
system utilizing negative impedance switching devices such as gas
tubes for protecting line powered repeater amplifiers in such a system.
Background of the Invention
Cable transmission systems are widely used to transmit
carrier signals in the communications field. Typically, such a
system utilizes a multipaired cable having repeater amplifiers
spaced at periodic intervals along the system. These amplifiers
are often line powered amplifiers wherein a constant current, which
is fed longitudinally down one cable pair and back along another from
a main terminal, is used to power the repeater amplifiers connected
thereto. Each of the amplifiers must be protected from hazardous
voltages which originate from three primary sources. (1) Lightning
strikes - generally caused when a bolt of lightning elevates the
local ground potential which in turn causes a large potential
gradient between the grounded cable sheath and the cable core
resulting in arc-over to one or more cable pairs thereby causing
high voltages to strike one or more of the amplifiers. (2) Induced
power line surges - generally result when the telephone transmission
cables and the power line cables are carried on parallel (e.g. the same)
poles. These surges may be continuous where there is an imbalance
on the power transmission system, or momentary when a break,
arc-over or short occurs in the power system. (3) Power line
conduction - generally results when a break in the power cable
causes a conductor to fall across the telephone cable causing ~-
direct conduction through the cable sheath to ground. This in
; turn elevates the potential of a localized area of the cable
sheath~relative to its core.
yarious protection schemes have been used in the
past, generally involving the use of negative impedance switching
devices such as carbon protector blocks or gas tubes which have
been connected in a variety of configurations across the repeater
amplifier terminals. These devices present a very high impedance
to the circuit until their breakdown voltage is reached whereupon
they present a relatively low impedance until the hazardous voltage
is removed. With the introduction of data communication systems
utilizing higher carrier frequencies than the earlier analog systems,
gas tubes are generally preferred to carbon blocks as they do not
suffer from dust and moisture which can cause leakage and noise
problems. HoweYer, despite the use of gas tubes, problems
protecting the system have remained. It has been found that one
reason for this is that the requiremen-ts for protecting the repeater
amplifiers against lightning strikes are somewhat different from
those required to protect them against power line surges.
In the past, negative impedance switching devices have
been connected across both the input and the output terminals of
the repeater amplifiers as well as between both their input and
output terminals. At least one such device is also connected
between the terminals of each amplifier and a common bus at each
of the repeater sites. In some applications, these buses have
been allowed to float thereby protecting the circuitry against
relatively low voltage power line surges. ~lowever, this arrangement
does not provide satisfactory protection agains-t lightning strikes
where exceedingly high voltages are introduced between the grounded
cable sheath and the cable core resulting in uncontrolled arc-over
within the system. In still other applications the common bus has
been grounded. With this arrangement, the uncontrolled breakdown
is minimized. However, the surge currents are now coupled through
specific components, often resulting in their destruction.
Statement of the Invention
It has been found that if the common bus at each
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station is neither connected directly to ground nor allowed to
float but is connected to the grounded cable sheath via a negative
impedance switching device having a voltage breakdown higher than
that of the devices connected directly to the terminals and generally
greater than the voltages introduced by power line sur~es but less
than those introduced by lightning strikes, a significant reduction
in the number of failures encountered in the system can be realized.
Thus, in accordance with the present invention there
is provided a cable transmission system comprising a multipaired cable
which includes a core having a plurality of cable pairs and a
conductive sheath surrounding the core. In addition, a plurality of
line powered amplifiers are located at discrete locations along the
system, each connected in series with an individual cable pair via
a pair of input and output terminals. The system includes a plurali-ty
of negative impedance switching devices connecting each of the
terminals to a common bus. The system also includes a further
negative impedance switching device connected between the common bus
and the conductive sheath9 which has a switching voltage greater
than that of the devices connected to the terminals.
Brief Description of the Drawings
Example embodiments of the invention will now be
described with reference to the accompanying drawings in which:
Figure 1 is a schematic circuit diagram of an
intermediate repeater in a cable transmission system utilizing
protection circuitry as known in the art,
Figure 2 is a schematic circuit diagram of a pair
of intermediate rep~aters in a cable transmission system utilizing
protection circuitry in accordance with the present inventioni and
Figure 3 is a schematic circuit diagram of a pair
of power loopin~g repeaters in a cable transmission system also
utilizing protection circuitry of the present invention.
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Description of the Preferred Embodiments
Referring to the prior art schematic circuit diagram
illustrated in Figure l there is shown a typical intermediate
repeater in a cable transmission system which includes a typical
repeater amplifier lO connected in series with the input tip (T)
and ring (R) terminals ll and output tip (T) and ring (R) terminals 12
of a cable pair, forming part of the core of a multi-paired cable
surrounded by a conductive sheath 13 which is grounded at periodic
intervals along the cable.
Power for the amplifier 10 is obtained from across
a zener diode 14 which is supplied from a constant current source
fed serially to both T and R of the cable pair fro~ an end or
main terminal (not shown) in a well known manner. The zener diode 14
is connected between the center taps of input and output transformer
windings 15 and 16 respectively. Small voltage dropping
resistors 17, 18, 19 and 20 (generally in the order of 5.6 ohms)
are connected in series with each lead of the cable pairs ll and 12.
Two pairs of carbon blocks 21, 22 and 23, 24 are connected in series
across the input and output terminals 11 and 12 respectively. The
center points of these carbon protector blocks 21 - 2~ are connected
to a common bus 25. The resistors 17 20 are required in order
that sufficient voltage drop can develop across the protector
blocks 21 - 24 to fire them. In a typical system, a number of
such amplifiers (not shown), which are connected to different cable
pairs in the multi-paired cable, will be located at the intermediate
repeater. Each of the amplifiers will have their own carbon
protector blocks connected in the same configuration to the bus 25.
In some prior art applications, this bus 25 has been left Floatiny
while in other iapplications it has been grounded to the cable
sheath 13 as indicated by the dotted line. However, both of these
conFigurations have not proved satisfactory as will now be explained.
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Assume initially that the bus 25 is le~t floating.
If an inductive or conductive power surge occurs, portions o~ the
cable sheath 13 will be elevated above ground (even though it is
grounded periodically) due to the inherent impedance of the cable
sheath 13 itself. However, except under the most extreme
circumstances, such a surge will not cause the sheath voltage to
rise above about 2,000 volts relative to the cable core. Since
the normal breakdown voltage between the core (i.e. the cable
pairs 11 - 12) and the sheath is in the order of 30,000 volts
or greater, no breakdown will occur between the bus 25 or the
cable pairs 11 and 12 and the sheath 13. ~lowever~ assume now that
lightning strikes the cable sheath 13 or the ground immediately
surrounding it. The ground potential can be momentarily elevated
anywhere from a few kilovolts to 100 kV or higher. Generally the
median voltage will be in the 10 - 20 kV range. This exceeds the
breakdown voltage of the cable and causes pinholing or arcing-over
between the conductive sheath 13 and one or more of the cable pairs.
This causes uncontrolled breakdown to ground in the apparatus case
of the repeaters if their dielectric properties are not sufficient.
This uncontrolled breakdown eventually deteriorates if not destroys
a portion of the system so that future lightning strikes can cause
major system failures.
; Assume now that the bus 25 is grounded to the
cable sheath 13. When either a conductive power surge or lightning
strikes the cable sheath l39 a potential gradient commences to
build between the sheath 13 and the cable pairs 11 and 12. In
a practical sysl;em, the breakdown voltage of the devices 21 - 24
cannot be made exactly the same. Assume therefore that carbon
; block 22 arcs-over before the others,21, 23~ 2~, have reached
3Q thelr breakdown potential. As a consequence, the impedance of
block 22 is sharply reduced thereby causing a high current to
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flow from the sheath 13 through the bus 25, the block 22 and a
plurality of paths back to ground. The sharply reduced voltage
across protector block 22 now prevents protector blocks 21, 23 and 24
from firing. However, due to the resistance along the cable
pairs ll and 12, the potential gradient on R of lead ll and T - R
of lead 12 continues to rise until the carbon protector blocks fire
at the two remote terminals in either direction (not shown). As a
result, there are now four paths to ground (through llR, llT and
12R and 12T) for current flowing through block 22. The ground
currents flowing through resistors 17, 19 and 20 are conducted
through windings 15, 16 and diode 14 to resistor 18 resulting in
three times the current through this resistor relative to
resistors 17~ l9 and 20. This heavy surge of current through
the resistor 18 generally causes it to fail. A slightly different
action takes place when an inductive rather than a conductive power
surge is impressed on the cable, often resulting however ln a
similar failure of one or more of the resistors 17, 18, 19 or 20.
The substitution of gas tubes For the protector
blocks as well as the addition of gas tubes across the amplifier
does little by itself to improve the system protection, the result
of which is that neither grounding nor floating the bus 25 has
provided satisfactory protection for the various types of hazardous
voltages introduced in the system.
In the embodiments illustrated in Figures 2 and 3 of
the invention many of the components are identical to and perform
the same function as those illustrated in the prior art of Figure 1.
.
These components have the same base reference numeral with an
additional reference character being added to distinguish them from
each other. In~the following detailed description~ only the
30 reference numeral will be referred to where it is unnecessary to
distinguish between the various elements.
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ReFerring again to Figure 2, there is illustrated
two repeater amplifiers 10 connected in opposite directions in the
t~ cable at an intermediate repeater station. In a practical-e~b~me~
however, many more such repeater amplifiers would be utilized at the
same location. Power which is being fed serially along the cable
pairs llA~ 12A, for both amplifiers lOA, lOB, is obtained ~rom
across a single zener diode 14A. As a result, the mid-points of
transformer windinys 15B and 16B are connected directly to each
other to conduct power back along the associated cable pair in a
well known manner. To provide improved protection, four gas tubes
are utilized with each amplifier 10. These include three
two-element ~0 volt gas tubes 30, 31 and 32 connected T to R,
R to R and T to T respectively. In addition, a three-element
350 volt gas tube 33 is connected be-tween the output T and R
with its commo~ electrode being connected to the common bus 25A.
Full protection can be obtained utilizing only two of the three
two-element gas tubes 30, 31 and 32 since each of the input and
output terminals will still be connected through one or more gas
tubes to the common bus 25A. However, utilizing the additional
gas tubeprovides a measure of redundancy and hence added protection.
Alternately, two separate two-element tubes can be substituted
for the three-element tube 33. Generally one is a 90 volt tube
connected between T and R, while the other is a 350 volt tube
connected from either T or R to the common bus 25A.
Xn addition to the gas tubes 30, 31, 32 and 33,
a single two-element 3 kV gas tube is connected between the common
bus 25A and the sheath 13A. This additional protective element
provides a marked decrease in the number of failures resulting
from lightning surges striking the system. This can be better
illustrated frorn the following examples.
Assume a voltage surge strikes the cable sheath 13A
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at a point remote from the repeater. A voltage gradient will
commence to build up between the wires and sheath 13A primarily
across the gas tube 35A. If the voltage gradient remains below
about 3 kV, the tube 35A will not fire. However, no damage results
to the repeater circuitry as it is capable of withstanding such
gradients. However, should the gradient exceed 3 kV, the tube 35A
will then fire resulting in a low impedance between the bus 25A and
the sheath 13A. Since the breakdown point of the gas tube 35A
is at least several times that of the rest of the gas tubes, once
it fires the high voltage is immediately impressed across all of
the low voltage gas tubes 30, 31, 32, 33 causing many of them to
fire as well. As a result/ there is controlled breakdown between
the cable sheath 13, the common bus 25 and the core of cable pairs.
Thus, a particular advantage of utilizing the gas
tube 35A between the bus 25 and the sheath 13 is that it causes a
number of the low voltage gas tubes to fire once it is fired
thereby distributing the current throughout the cable core.
Alternately, unlike the prior art where the bus 25A floated relative
to the sheath 13A, the use of the gas tube 35A provides a controlled
breakdown so that excessively high potential differences such as
caused by lightning strikes cannot exist resulting in uncontrolled
breakdown across various elements of the repeater.
In applications involving the transmission of digital
signals additional circuitry is utilized to test the performance
of the amplifiers lOA, lOB. To achieve this, a single pair of ~- -
wires known as the fault locate pair 36 is dedicated to this
function. At each repeater station, a single filter 37 having a
unique band-pass characteristic is lightly coupled to the output -~
transformers 16 of each of ~he amplifiers 10 in a well known mannPr. ;
; 30 Utilization of t:his fault locate filter 37 was not a problem in the
prior art where the bus 25A was grounded to the cable sheath.
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However, in prior art applications where the bus 25A was permitted
to float relative to the sheath 13, high voltage build-up on a
single cable pair 36 would be coupled through the common fault
locate filter 37 to each of the amplifiers at that location thereby
causing multiple failures of the ampliFiers 10 themselves. Since
the bus 25A is not connected directly to the sheath 13A, this
condition is avoided by utilizing a single three-element 350 volt
gas tube 38 connected across T and R of the fault locate pair 36
with its common element connected to the bus 25A. This limits
the voltage potential between the fault locate pair 36 and the
balance of the components at the repeater station to a safe value.
Figure 3 is a power looping repeater rather than an
intermediate repeater where power is fed from each end of the system
towards a central point. The majority of the circuit elements are
identical to those shown in Figure 2 and have therefore been
assigned the same basic numerals. The major difference is that
protection now is placed between the input and output paths of
the power feed, i.e. cable pairs llC to llD and 12C to 12D. The
basic function of these elements is the same as that described
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All two-e1ement gas tubes have a 90 volt breakdown
while the three-element tubes are of the 350 volt type. This
higher voltage is necessary since the standard power fed to the
system at the main terminals is + 130 volts which can result in
a 260 volt potential occurring near the power feed points or
should an open circuit develop somewhere in the power feed.
Without these higher breakdown 350 volt gas tubes, this could
result in their firing causing current to flow between cable pairs.
While two separate 350 volt gas tubes can be substituted for ~ -
three-element tubes 33, the latter are preferred since once
breakdown of one half occurs, the ionized gas within the tube
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lowers the firiny point of the other gas tube thereby increasing
its chances of firing as well.
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