Note: Descriptions are shown in the official language in which they were submitted.
CA 02298308 2000-02-09
IMPROVED COAXIAL CABLE PROTECTION DEVICE
FIELD OF THE INVENTION
The present invention relates to the protection of conductive cables and
more particularly to the protection of cables carrying both signal traffic and
AC power.
BACKGROUND
Coaxial cable has been installed extensively by cable television (CATV)
operating companies to bring multi-channel wideband television services to
homes.
These services have provided almost exclusively downstream signal transmission
to
the customer with little or no upstream or interactive communication from the
customer to the head end. With deregulation of the cable television and
telephone
industries, opportunities exist to provide two way data and telephone services
over
the cable television coaxial cable network.
To provide new services over the coaxial cable distribution network,
power for operating the circuits at the customer end is required. Power could
be
derived from the electric power system, but this has the disadvantage of
connection to
mains power and the need for a backup battery in case of power failure. It is
therefore preferable to power the home equipment independently from the drop
cable
to the house.
As the majority of homes in suburban areas have a coaxial service cable
already provided it is economically preferable to use the existing drop to
also provide
the home circuit powering. It has been determined that the power supplied over
the
coaxial cable should be in the at voltage levels of 90 V AC or more to ensure
adequate powering over a typical CATV distribution system. A this voltage
level,
CA 02298308 2000-02-09
_2_
electrical codes require that a buried service entrance cable be buried at a
depth of 18
inches or more to limit exposure to hazardous voltages. A significant portion
of the
buried CATV coaxial cable drop cables are buried at depths less than the
minimum
requirement.
A ground fault protection device is required to ensure safe operation
without having to replace the existing drop cable. This poses a unique problem
as the
outer conductor of the coaxial cable is normally grounded and there is no
simple
means to distinguish between a power load from the centre conductor to ground
and
an unintended fault to ground along the drop cable.
One solution to this problem is disclosed in the applicant's prior United
States patent 5,793,590, the disclosure of which is incorporated herein by
reference.
That patent discloses a coaxial drop cable safety system in which tap end and
premise end units are connected to the drop cable. The tap end unit applies a
DC
voltage to the centre conductor of the drop cable. A monitoring circuit in the
tap end
unit monitors the centre conductor DC voltage to a fault from the centre
conductor to
ground along the drop cable. The tap unit will immediately remove the powering
voltage In the event of either an open or faulted condition on the centre
conductor
along the entire length of the drop cable.
In the prior art system, each of the tap end and premise end units
includes a DC blocking capacitor in the AC conducting path. These must be of
sufficiently large value that the AC impedance is low. This will keep the AC
voltage
drop across the capacitors small so as not to reduce significantly the
supplied AC
powering voltage at the premise end.
CA 02298308 2009-08-03
With this system, in order to detect a resistive fault to ground, the
voltage to ground must fall to a determined minimum trip level. The DC
blocking
capacitors must discharge through a fault resistor before the trip level can
be
reached and the disconnect activated. The larger the value of the capacitors,
the
longer it will take to discharge them and drop the DC voltage to the trip
level.
This may be a problem with an application requiring the powering of
a Multiple Distribution Unit where several telephones and other powered
devices
are connected to a single drop. In this case, the DC blocking capacitor must
be
of a larger value than is required for a single distribution unit. The
requisite large
capacitors must discharge through the fault resistance. For fault resistances
in
the order of 20 to 30 thousand ohms, it can take one half second or more to
discharge the capacitors to the trip level for the monitoring circuit.
The present invention addresses the question of providing a shorter
disconnect time.
SUMMARY
According to the present invention there is provided a cable
protection system for a cable having first and second conductors for carrying
an
electric signal and AC power from a source to a destination, the system
comprising:
a first circuit component for passing the signal to the first conductor;
a second circuit component with an active state for applying the AC
power voltage to the first conductor and a blocking state for blocking the
application of the AC power voltage to the first cond uctor;
monitoring means including:
-3-
CA 02298308 2009-08-03
means for applying a DC reference voltage to the first
conductor; and
means for monitoring the DC current in the first conductor;
and
actuation means for actuating the second circuit component to the
blocking state in response to detection by the monitoring means of a value of
said DC current representing a faulted condition of the first conductor.
The system thus removes the powering voltage In the event of a
faulted condition on the first conductor, which with a coaxial drop cable will
be the
centre conductor. The actuation to a blocked state is based on a rapid current
change rather than a change of voltage that will be delayed by the discharge
of
large blocking capacitors. The preferred system has a range or "window" of
acceptable DC current levels. Currents outside the window, either above or
below the acceptable range, trigger the blocking state.
The monitoring means may include means for applying a DC
vonage to a resistive circuit including resistances at opposite ends of the
centre
conductor. This establishes the reference DC current in the centre conductor.
Variations from this reference value indicate a faulted or open circuit
conditiowof
the conductor. The DC circuit is limited to the drop cable by the blocking
capacitors so that the system can distinguish between a normal end located
load
and a fault from the centre conductor to ground along the drop cable.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, which illustrate an exemplary
embodiment of the present invention:
-4 -
CA 02298308 2000-02-09
-5-
Figure 1 is an illustration of a system according to the present invention;
Figure 2 is a generalized schematic of a tap unit circuit;
Figure 3 is a generalized schematic of a premise unit circuit;
Figure 4 is a detailed schematic of the tap unit circuit; and
Figure 5 is a graphic comparison of the disconnect time versus fault
resistance characteristics of the prior system and the present system.
DETAILED DESCRIPTION
Referring to the drawings, and especially Figure 1, there is illustrated a
coaxial drop cable safety system 10 for cable television, data and telephony
applications. The system is used on a buried drop cable 12 where 60 Hz
powering
voltages applied to the drop cable exceed low voltage safety limits and where
the
drop cable is buried at depths which is less than the minimum required to meet
electrical safety codes.
The system is housed in two separate units, a tap unit 14 and a premise
unit 16. The tap unit has an input 18 connected to a main distribution cable
20 at a
tap 22. The output of the tap unit is connected via a coaxial cable connector
24 to the
coaxial drop cable 12. At the opposite end of the drop cable, the premise unit
16 has
an input 28 connected to the drop cable and an output 30 for connection to the
premise equipment (not illustrated).
Referring to the tap unit basic circuit in Figure 2, this includes a centre
conductor 32 connected by input 18 to the signal and AC power carrying
conductor 34
of the tap. An outer conductor 36 is connected to a grounded outer conductor
38. At
the output 24, the centre conductor 32 and the outer conductor 36 are
connected to
CA 02298308 2000-02-09
-ti-
the centre conductor 40 and the outer conductor 42 respectively of the drop
cable 12.
A radio frequency (RF) coupling capacitor C1 is connected in centre conductor
32.
Between input 18 and capacitor C1, the centre conductor is connected to an RF
coil
L1 which is in turn connected to a capacitor C2 for AC power coupling and DC
blocking, and to a monitoring circuit 44. The other side of capacitor C2 is
connected
through a triac TR1, a sensing resistor RO and a second RF coil L2 to the
centre
conductor 32 between the output 24 and capacitor C1. The monitoring circuit 44
is
connected across the sensing resistor RO and to the trigger of triac TR1.
The premise unit circuit 16 includes a centre conductor 46 connected by
input 28 to the centre conductor 40 of drop cable 12. An outer conductor 48 is
connected to the outer conductor 42 of cable 12. The centre and outer
conductors 46
and 48 are connected to the output 30. An RF coupling capacitor C9 is
connected in
centre conductor 46. Between input 28 and capacitor C2, the centre conductor
is
connected to an RF coil L3 which is in turn connected to a capacitor C10 for
AC
power coupling and DC blocking, and a resistor R25. The other side of
capacitor C10
is connected through a second RF coil L4 to the centre conductor 46 between
the
output 30 and capacitor C9.
The operation of the circuits is as follows:
The AC power and RF signals are applied to the input connector 18 at
the tap side.
The RF signal is applied to the centre conductor via coupling capacitor
C1.
The AC power is applied to one side of C2 and to the monitoring circuit
CA 02298308 2000-02-09
-7-
via coil L1.
The monitoring circuit develops a DC supply voltage which is applied to
the inner conductor 32 of the drop cable through RO and L2.
The termination resistor R25 at the premise end circuit forms one part of
a voltage divider which places a reference voltage Vpc on the centre
conductor. This
produces a nominal monitoring current Vpc/R25 which is detected by the
monitoring
circuit.
With no fault to ground from the centre conductor, the monitoring circuit
applies a triggering signal to the triac which conducts, connecting capacitor
C2 in
parallel with capacitor C1 in the centre conductor path. The AC power current
is
passed through L1, C2 and L2 thereby applying AC power to the drop cable.
A fault from the centre conductor to ground will result in an
instantaneous DC current of V~c/Rf to ground where Rf is the fault resistance.
As
capacitors C2 and C10 are substantially the same value, the instantaneous
fault
current through RO will be approximately'/2 Vp~/Rf.
The monitoring circuit input is connected across RO and detects the
voltage drop (%2 Vp~Rf) R0, which is directly proportional to the
instantaneous current
through R0. The triggering signal to the triac is turned off, which opens the
powering
circuit.
The value of RO is selected such that the AC voltage drop is negligible.
The DC gain of the monitoring circuit is chosen to provide sufficient
amplification and
detection of the instantaneous and steady state fault currents.
This provides rapid fault detection and AC power disconnect on the
CA 02298308 2000-02-09
_$_
order of a few milliseconds.
In the case of an open circuit due to a break in the cable or tampering,
the nominal monitoring current Vo~/R25 will fall with a proportional decrease
in the DC
voltage drop across R0. When the voltage across RO drops below a lower limit,
the
monitoring circuit removes the triggering signal thereby turning off the triac
which
opens the powering circuit.
With more specific reference to the basic circuit in Figure 3, the
operation of the premise unit circuit is as follows:
The DC monitoring, 90 V AC powering and RF signals are applied to the
input connector at the drop cable side.
The RF signal is passed through via coupling capacitor C9.
The 90 V AC power is passed through L3, C10 and L4. and applied to
the premise side connector 30.
As discussed above, the termination resistor R25 from the junction of L3
and C10 to the outer conductor forms one part of a voltage divider circuit
which is
powered by the monitoring circuit to maintain a nominal monitoring current
V~c/R25 in
the centre conductor 40 of the drop cable 12.
Figure 4 illustrates a detailed schematic of the tap unit. The input 18 is
connected to the tap 22 on the distribution cable 20 (Figure 1 ). The RF
signal is
coupled through coupling capacitor C1 to the output connector 24. The 60 Hz
power
current passes through L1 to the junction of capacitor C2 and a resistor R1.
Diode D1
is connected in series with R1. This rectifies the powering voltage which is
applied to
a zener Z1 and a capacitor C3, which are connected in parallel to form a
regulated
CA 02298308 2000-02-09
_g_
negative voltage DC power supply 52. The power supply is limited to the zener
clamping voltage. The DC supply voltage is applied to a conductor 51. From
conductor 51, the supply voltage is applied to the inner conductor 32 through
the
series combination of a resistor R3, resistor RO and inductance coil L2. RO is
selected to be less than one ohm in value. The DC supply voltage is also
connected
to the op amps OA1 and OA2 to power the monitoring circuits.
At the premise end, resistor R25, which is of equal value to resistor R3,
shunts the centre conductor to ground thereby forming voltage divider with R3
and
R0. For a no fault condition the voltage divider produces a centre conductor
to
ground DC reference voltage Vp~ equal to one half of the DC supply voltage.
A passive low pass filter 54 has its inputs connected to the junction of
coil L2 and resistor RO and to the junction of resistors RO and R3. The filter
removes
the 60 Hz power voltage allowing only the DC monitoring voltage to reach the
summing inputs to amplifier OA2. This amplifier amplifies the small DC voltage
across RO to a useful value. The gain of the amplifier OA2 is determined by
resistor
R4. The output of amplifier OA2 is connected to the input of a level detector
circuit 56
which is designed to sense a high current limit in a resistive fault condition
and a low
current limit in an open circuit condition. The output of the level detector
56 is applied
to the positive summing input of an amplifier OA1. The output of amplifier OA1
is
applied to the input diode of an optical coupler OP1 through resistor R5.
In normal, non-faulted operation, a nominal DC current of Vp~/R25 is
detected through R0. The output of the level detector is in the high state and
coupler
OP1 is ON causing the triac to trigger, which applies the 60 Hz powering
voltage to
CA 02298308 2000-02-09
-10-
the centre conductor. A resistive fault to ground from the centre conductor
with a
resistance low enough to exceed the maximum current level through RO will
cause the
level detector to switch to a low state, which in turn causes OA1 to go low,
shutting off
OP1. The triggering to the triac is turned of and the triac stops conducting,
removing
the 60 Hz power from the centre conductor. This action happens almost
instantly as
the discharge of C2 through RO is detected immediately.
The same shut-off procedure will occur if the centre conductor is opened
or the premise unit is disconnected. An open circuit to the premise unit will
cause the
current through RO to decrease below the minimum set value, which in turn
causes
the level detector to go low, shutting off the triac and removing the power
from the
centre conductor.
The system thus monitors the drop cable, between the tap unit 14 and
the premise unit 16 and removes AC power from the cable whenever an electrical
fault occurs in the cable. At the same time, RF signals will continue to be
passed
through to the premise end of the system.
The relative speed of the disconnect compared to the prior system is
illustrated in the graph of Figure 5. The prior system disconnect time relates
to a
system using DC blocking capacitors suitable for a single distribution unit.
Substituting the larger capacitors allowed by the present invention in that
prior system
would significantly increase the disconnect time. As can be seen, for a fault
resistance of about 20,000 ohms, the prior system disconnects the power in
about
one half second, while the present system produces disconnect in just over 20
milliseconds.
CA 02298308 2000-02-09
-11-
While one particular embodiment of the invention is described in the
foregoing, this is by way of example only and is not to be construed as
limiting. The
invention is to be considered as limited solely by the scope of the appended
claims.
