U.S. patent application number 09/766254 was filed with the patent office on 2002-07-18 for ac phasing voltmeter.
Invention is credited to Bierer, Walter S..
Application Number | 20020093326 09/766254 |
Document ID | / |
Family ID | 25075881 |
Filed Date | 2002-07-18 |
United States Patent
Application |
20020093326 |
Kind Code |
A1 |
Bierer, Walter S. |
July 18, 2002 |
AC phasing voltmeter
Abstract
A phasing voltmeter having a high impedance AC voltmeter in
series with two high impedance probes. Shielding surrounds and
electrically isolates the voltmeter and probes. In parallel with
the voltmeter and connected electrically with the shielding is an
electrical circuit designed to add the capacitive current to a
current detected by the probes. The capacitive current is added in
such a way that the net effect on the measured current is zero. The
electrical circuit comprises two impedance elements, such as
resistors, that meet at a junction where they are connected to the
shielding. The impedances on either side of the junction are
matched either by careful selection of the elements or by selection
of adjustable elements so that the junction is a null point.
Inventors: |
Bierer, Walter S.;
(Blythewood, SC) |
Correspondence
Address: |
MICHAEL A MANN
NEXSEN PRUET JACOBS & POLLARD LLP
PO DRWR 2426
COLUMBIA
SC
29202-2426
US
|
Family ID: |
25075881 |
Appl. No.: |
09/766254 |
Filed: |
January 18, 2001 |
Current U.S.
Class: |
324/72.5 |
Current CPC
Class: |
G01R 1/06777
20130101 |
Class at
Publication: |
324/72.5 |
International
Class: |
G01R 031/02 |
Claims
What is claimed is:
1. A device for use in measuring voltage, said device comprising: a
voltmeter; a first probe having a first resistor in series with
said voltmeter; a second probe having a second resistor in series
with said voltmeter, said voltmeter being located electrically
between and in series with said first probe and said second probe;
electrical shielding surrounding but electrically isolated from
said voltmeter, said first probe and said second probe; an
electrical circuit in electrical connection with said shielding and
in parallel with said voltmeter for adding capacitive current in
said shielding to current running between said first and said
second probes.
2. The device as recited in claim 1, wherein said electrical
circuit adds said capacitive current in such a way that said
capacitive current has no net effect on measurement of said current
running from said first to said second probe.
3. The device as recited in claim 1, wherein said electrical
circuit includes a pair of impedance elements that meet at a
junction and wherein said shielding is electrically connected to
said junction.
4. The device as recited in claim 3, wherein said impedance
elements are selected from the group consisting of resistors,
capacitors and inductors.
5. The device as recited in claim 1, wherein said electrical
circuit includes a gain resistor, a null resistor and a balance
resistor in series with each other, and said shielding is connected
to said null resistor.
6. The device as recited in claim 5, wherein said gain resistor is
adjustable.
7. The device as recited in claim 5, wherein said shielding is
attached to said null resistor at a point and wherein said point of
attachment is adjustable.
8. A device for use with a voltmeter having a first probe and a
second probe, said device comprising: an electrical shield
surrounding and electrically isolated from said voltmeter and said
first and said second probes; an electrical circuit arranged to be
electrically in parallel with said voltmeter and in electrical
connection with said shield so that electrical currents in said
shield can be added to electrical currents flowing between said
first and said second probe.
9. The device as recited in claim 8, wherein said electrical
circuit further comprises two impedance devices meeting a junction
and wherein said shield is electrically connected to said
junction.
10. The device as recited in claim 9, wherein said impedance
devices are selected from the group consisting of resistors,
capacitors, and inductors.
11. The device as recited in claim 8, wherein said electrical
circuit adds said capacitive current in such a way that said
capacitive current has no net effect on measurement of said current
running from said first to said second probe.
12. The device as recited in claim 8, wherein said electrical
circuit includes a gain resistor, a null resistor, and a balance
resistor in series with each other, and said shielding is connected
to said null resistor.
13. The device as recited in claim 8, wherein said gain resistor is
adjustable.
14. The device as recited in claim 8, wherein said shielding is
attached to said null resistor at a point and wherein said point of
attachment is adjustable.
15. A device for use in measuring voltage, said device comprising:
an alternating current voltmeter; a first probe having a first
resistor in series with said voltmeter; a second probe having a
second resistor in series with said voltmeter, said voltmeter being
located electrically between said first probe and said second
probe; electrical shielding surrounding but electrically isolated
from said voltmeter, said first probe and said second probe whereby
said electrical shield carries a capacitive current; an electrical
circuit in electrical connection with said shielding and
electrically in parallel with said voltmeter for adding said
capacitive current in said shielding to current running between
said first and said second probes, said electrical circuit having a
junction and said electrical shielding being connected to said
junction.
16. The device as recited in claim 15, wherein said electrical
circuit includes two resistors in series and connected at said
junction.
17. The device as recited in claim 16, wherein the resistance of at
least one of said two resistors is adjustable.
18. The device as recited in claim 15 wherein said electrical
circuit includes means for selecting a range of voltage to be
displayed on said voltmeter.
Description
1. FIELD OF THE INVENTION
[0001] The present invention relates to voltmeters generally and to
voltmeters for use in electrical power transmission line servicing
and maintenance in particular.
2. BACKGROUND OF THE INVENTION
[0002] Electricity transmitted through power lines destined for
commercial, industrial and residential use can involve hundreds of
thousands of volts and high currents. Inevitably, there is an
element of danger in measuring the voltage on a transmission line
because of the need to make contact with the line. Indeed, even the
proximity to a high voltage line may be sufficient to cause a spark
to jump through the air to the nearest object. Nonetheless, in
installing, servicing and repairing power lines, there are various
occasions when contact is made, such as when the voltage carried by
a line must be measured.
[0003] The circumstances and equipment used for measurements of the
voltage of transmission lines varies considerably. For example, the
absolute voltage carried by a line may be measured by a "high line
resistive voltmeter." As another example, in servicing or repairing
voltage regulators, an "off neutral detector" is used to determine
if the regulator is passing current or has been effectively
isolated from the power source. In still another application, a
"phasing voltmeter" is customarily used in connecting individual
lines of the multi-phase transmission power lines. The phasing
voltmeter helps to prevent two lines that are not in phase from
being connected inadvertently. Phasing voltmeters are not very
accurate and generally do not need to be in order to indicate which
lines are in phase and which are not. For accuracy, high line
resistive voltmeters are used.
[0004] However, if a phasing voltmeter were accurate, it may have
additional uses, such as replacing the high line resistive
voltmeter or the off neutral detector. In order to be accurate, an
alternating current phasing voltmeter should be capable of making
four very distinctly different voltage measurements: phase to phase
(FIG. 2A), phase to ground (FIG. 2B), ground to phase (FIG. 2C),
and zero reference test (FIG. 2D). This last measurement should
indicate very nearly zero volts when measuring the voltage
difference between two conductors of the same phase and voltage or
between two points on the same conductor.
[0005] Presently, high voltage phasing voltmeters use two high
voltage resistors in series with each other and a meter and a
cable. The resistors are housed in two insulated holders that are
connected to the series cable and the series meter. The holders
will have metal hooks or other fittings on their ends for good
electrical contact with transmission lines. Often the meter is
mounted to one of the two insulated holders (see FIG. 1) and
oriented so that the electric utility worker can read the voltage
displayed on the meter. "Hot sticks" may be used to hold and
elevate the entire assembly. The meter may be designed to measure
either voltage or current, but its display indicates voltage.
However, the indicated voltage is not always the true voltage
difference for the four types of measurements listed above.
[0006] High voltage measurements are plagued with inaccuracies
stemming from stray capacitive charging currents. At high voltages,
these stray currents emanate from the surface of every component of
the measuring device including the cab le. The capacitive current
is related to the capacitive reactance, Xc, which can range from
several thousand ohms on up, depending on the position of the meter
and cable with respect to the ground. Under extreme conditions,
such as when the series cable is lying directly on the ground
between two pad-mounted transformers, the value of the capacitive
reactance can be very low. The resulting capacitive current can
then equal or exceed the measured current. Moreover, the voltage
measured by the meter varies depending on the location of the meter
and cable.
[0007] To demonstrate the theoretical limits of the inaccuracy of
the prior art phasing voltmeter, assume that the capacitive current
goes to a maximum (capacitive reactance goes to zero). This
situation would electrically ground the series cable. The
inaccuracies for the four basic measurements would be 15% too high
for phase to phase, 100% too high for phase to ground, zero for
ground to phase, and 200% of line to ground voltage for the zero
reference test.
[0008] If the inaccuracies in phasing voltmeters attributable to
capacitive currents could be eliminated, phasing voltmeters could
be used to make measurements currently made by high line resistive
voltmeters and off neutral detectors. This would eliminate the need
for these additional voltmeters and provide measurements in which
electrical utility employees can have greater confidence.
[0009] The problem of the inaccuracies introduced by the capacitive
currents is known, and there have been attempts to address this
problem. One solution is to keep the current actually measured by
the phasing voltmeter relatively large compared to the capacitive
current so the error introduced by the latter is relatively small.
However, larger currents carry with them heat that can affect the
resistance of the high voltage dropping resistors, which introduces
another source of error if the phasing voltmeter is kept in contact
with the line too long. Bevins in U.S. Pat. No. 3,392,334 teaches
that the cable must be kept as short as possible and that use of
two conductors in the interconnect cable can nullify the effects of
capacitive current. However, neither of these steps corrects the
error. For example, a phase to ground reading with a phasing meter
having dual conductors in the cable will be the same as the ground
to phase reading, but both will understate the actual voltage.
Keeping the cable short certainly helps with accuracy but makes the
phasing voltmeter less useful than one with a longer cable.
[0010] Thus there remains a need for a phasing voltmeter that is
accurate regardless of the capacitive current.
SUMMARY OF THE INVENTION
[0011] According to its major aspects and briefly recited, the
present invention is a phasing voltmeter where the capacitive
currents are combined with the primary voltage measurement of the
electrical transmission lines in such a way that the capacitive
current has no net affect on the voltage measured regardless of the
magnitude of the capacitive current. Thus, the meter and cable can
be moved about without affecting the primary voltage measurement,
and the cable can be of any length.
[0012] The present phasing voltmeter includes a pair of high
impedance resistors in series with a cable and a high impedance
alternating current (AC) voltmeter, essentially similar to the
prior art phasing voltmeters, but also having a low impedance
electrical circuit in parallel with the meter and that is tied at a
single point of contact to electrical shielding provided for the
resistors, cable and AC voltmeter. This shielding picks up the
capacitive currents in the vicinity of the phasing voltmeter.
[0013] By this arrangement, the present phasing voltmeter
establishes three voltage divider networks. The first voltage
divider network divides the source voltage by an exact amount and
provides a precise voltage to the AC voltmeter. The second voltage
divider network divides the voltage between the first high
impedance resistor and the single point shield attachment. The
third voltage divider network divides the voltage between the
second high impedance resisotr and the single point shield
attachment. The vector sum of the three voltages thus established
across the AC voltmeter input from these three voltage divider
networks is exactly proportional to the source voltage. Thus, the
second and third voltage divider networks allow the capacitive
current to be coupled to the measured voltage in such a way that it
has no net effect on the measured voltage.
[0014] A number of different embodiments of this electrical circuit
are described herein, but a preferred one includes three resistors
in series. The first is an adjustable gain resistor. The second is
a null resistor tied to the shielding. The third is a balance
resistor. Before the first use of the present phasing voltmeter,
the gain and null resistors are set so that the particular system
is balanced. Thereafter, no further adjustments are needed and the
voltmeter will read true.
[0015] By adding switches and additional gain, null and balance
resistors in accordance with the present invention and replicating,
the electrical circuit can be established to provide range
selection so that the displayed voltage will fall into one of a
plurality of ranges selected by the user.
[0016] A feature of the present invention is the electrical circuit
in parallel with the AC voltmeter. This simple circuit allows the
capacitive current to be combined with the measurement of the
voltage in such a way that there is no net effect regardless of the
magnitude or changes in the capacitive current. This result has a
number of advantages.
[0017] First, it eliminates what is potentially a large source of
error in the measurement of voltages using phasing voltmeters, thus
making much more accurate measurements possible. An accurate
phasing voltmeter has other uses, as mentioned above. Moreover, an
accurate phasing voltmeter helps electrical utility employees
engaged in the servicing and repair of power lines to avoid
injuries.
[0018] Second, it allows the measurement to be done anywhere and
with any length of cable. A longer cable increases capacitive
currents but as long as there is no net affect on the voltage
measurement, it does not matter how long the cable is.
[0019] Third, the present invention allows the voltmeter to make
its measurements on much smaller currents because the capacitive
current is not a factor. Prior art meters reduced the impact of
capacitive current by keeping the current running through the high
voltage resistors and the meter relatively higher. Because lower
currents can be used with the present device, resistors are kept
cooler and can be used longer without heating up and causing the
electrical resistence coefficient to drift.
[0020] These and other features and their advantages will be
apparent to those skilled in the art of transmission line voltage
measurement from a careful reading of the Detailed Description of
Preferred Embodiments accompanied by the following drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] In the figures,
[0022] FIG. 1 illustrates a phasing voltmeter in use.
[0023] FIG. 2A-2D illustrates the four basic types of measurements
for which a phasing voltmeter may be used.
[0024] FIG. 3 is a schematic illustration of a phasing voltmeter
according to a preferred embodiment of the present invention.
[0025] FIGS. 4A-4F illustrate six alternate embodiments of the
electrical circuit used in a phasing voltmeter according to
preferred embodiments of the present invention.
[0026] FIG. 5 is a schematic illustration of an electrical circuit
arranged to operate also as a range selector, according to a
preferred embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0027] The present invention is a phasing voltmeter that is an
improvement over existing phasing voltmeters. Externally, and to a
significant extent internally, the present phasing voltmeter has
many of the same features as prior art voltmeters and operates in
much the same way.
[0028] Referring now to FIGS. 1 and 3, the present invention is
illustrated schematically and generally indicated by reference
number 10. The circles designated A, B, and C represent electrical
power lines carrying alternating current where each line is 120
degrees out of phase with the other two lines. The transmission
lines are of course not part of the present invention. The
measurement of the voltage is being made across the A and B lines
in FIG. 1 and across the A and C lines in FIG. 3.
[0029] A first probe 12 is shown in contact with the A line; a
second probe 14 is shown in contact with the B line. First probe 12
includes a resistor R1; second probe 14 includes a resistor R2.
Ideally, these two resistors have large resistances and are matched
so that the resistance of R1 and R2 are the same or very close in
magnitude. The resistances of R1 and R2 are preferably tens of
millions of ohms, such as, for example, 50,000,000 ohms.
[0030] Between first and second probes 12, 14, and electrically in
series with them, is a voltmeter 16, which, by itself and in prior
art phasing voltmeters is well known in the art. In the present
invention voltmeter 16 must be AC, and the impedance of voltmeter
16 should be large compared to the balance of the electrical
circuit, preferably on the order of 10,000 times larger than the
impedance of the balance of the electrical circuit in parallel with
it and to be described presently. The combined impedances of
voltmeter 16 and Resistors R1 and R2 should be sufficient to keep
the current very low, on the order of a milliamp or preferably
less, with approximately 0.5 milliamp being most preferred, in a
phasing voltmeter 10 that can be in contact with transmission lines
continuously without resistive heating of resistors R1 and R2 or
the components of voltmeter 16 affecting the measurement
significantly.
[0031] Connecting resistors R1 and R2 and voltmeter 16 is a cable
20. Voltmeter 16 is encased in housing 22 and mounted to either
first probe 12 or second probe 14 so that cable 20 may run from
voltmeter 16 mounted on first probe 12 to second probe 14 rather
than to both first and second probes 12, 14, from voltmeter 16, as
illustrated.
[0032] Electrical shielding 24 comprises shielding on cables inside
of first and second probes 12, 14, shielding on cable 20 and
shielding on the aluminum or other non-ferrous metal of housing 22
of voltmeter 16. Resistors R1 and R2 are also preferably shielded
as part of shielding 24 by, for example, the use of coaxial
resistors. Shielding 24 is electrically isolated from probes 12,
14, cable 20 and voltmeter 16 except for a single point of contact
at 26. Because shielding 24 extends over substantially the whole of
phasing voltmeter 10, it helps to assure that the capacitive
reactance, Xc, between the ground and every part of phasing
voltmeter 10 is the same.
[0033] The capacitive reactance will vary depending on the physical
relationship between phasing voltmeter 10 and the ground, but
wherever phasing voltmeter 10 is, Xc will be the same throughout
shielding 24.
[0034] In parallel with voltmeter 16 is an electrical circuit 28
that couples the capacitive current transmission lines A and C (or
other source voltage) across probe 12 and probe 14 by an exact
amount (such as by a factor of 1,000,000) and provides a precise
voltage to voltmeter 16. Resistors R1 and R2 serve as the two
primary voltage dropping resistors in this first precision voltage
divider network. Resistors R3 and R4 serve as metering resistors in
this first voltage divider network.
[0035] The second of the precision voltage divider networks divides
the voltage between first probe 12 and contact 26 (also by a factor
of, say 1,000,000) to produce a voltage that is precisely
proportional to the charging current supplied by first probe 12 and
delivers that voltage to voltmeter 16. R1 serves as the primary
voltage dropping resistor of this network and R3 serves to as a
metering resistor in this network. R3 also serves as an extremely
efficient (99.9999%) delivery path from R4 in the third precision
voltage divider network to voltmeter 16.
[0036] Finally, the third precision voltage divider network divides
the voltage between second probe 14 and contact 26 (by the same
factor as in the second voltage divider network. R2 is the primary
resistor in this third network and R4 is the metering resistor. R4
in the third network, as R3 was in the second network, is an
extremely efficient delivery path for the second voltage divider
network to voltmeter 16. These three voltages are summed vectorally
and fed to voltmeter 16. The voltages of the second and third
voltage divider network add or subtract to the voltage of the third
voltage divider network, leaving the voltage displayed by the
voltmeter 16 equal to the voltage across transmission lines A and
C.
[0037] Electrical circuit 28 can take a variety of forms other than
that illustrated in FIG. 3. FIGS. 4A-4F illustrate six more forms
for electrical circuit 28. In each of the illustrations of FIGS.
4A-4F, the reactance, Xc, is tied to shielding 24 at contact 26,
but that is not shown to simplify the illustrations. Similarly, in
each of the illustrations, the same precision voltage divider
networks that include contact 26 as a terminal are established that
permit the voltage measured by these networks to be added
vectorially to the source voltage by voltmeter 16.
[0038] In FIG. 4A, gain resistor R3 is adjustable so that the
combined impedance of R3, R4 and R4 can be adjusted to more easily
form a precision voltage divider network with RI, R2, R3 and R5. In
FIG. 4B, balance resistor R4 and gain resistor R3 are both made
adjustable.
[0039] In FIG. 4D, a transformer 36 is used to determine voltage.
The capacitive reactance is coupled to an intermediate point, via
contact 26, on transformer 36 so that impedances on either side of
the intermediate point balance, thus canceling the effect of the
capacitive current.
[0040] FIG. 4E illustrates yet another embodiment of the present
invention. Between R1 and R2, and in parallel with voltmeter M are
two inductors 38, 40, one on either side of the point of contact
with capacitive reactance. When inductors 38, 40 are balanced, the
capacitive current is added to the current in such a way that it
has no net effect. FIG. 4F illustrates that capacitors 42, 44, can
be substituted for inductors 38, 40 (FIG. 4E), or resistors R3 and
R5 (FIG. 4C).
[0041] Importantly, a circuit component pair that uses impedance
elements, such as a pair of resistors, capacitors or inductors, or
a combination of these, to create electrical circuit 28 in parallel
with voltmeter 16. By connecting the shielding 24 to a point in
this electrical circuit via contact 26 where the impedance on
either side of the contact point is balanced or can be balanced by
adjustment, the capacitive current in the shielding can be added to
the current in the voltmeter so that there is no net effect
regardless of how large or small the capacitive current is. The
pair of components that make up the electrical circuit can be
chosen to match each other impedance or be adjustable so that the
impedance on either side of the contact point can be balanced and a
null current point established.
[0042] FIG. 5 illustrates an embodiment for electrical circuit 28
in the form of a range selector 18 to the current running through
phasing voltmeter 10 from lines A to C (FIGS. 3-5), so that
voltmeter 16 measures both. However, the nature of electrical
circuit 28, as will be explained, is such that the net effect of
the capacitive current is zero. The term "electrical circuit" will
be understood to mean impedance elements in electrical connection
with each other and that achieve a particular function as described
herein and many examples of which are illustrated in FIGS. 3,
4A-4F, and 5. Other impedance elements may be arranged to perform
the same function. In particular, electrical circuit 28 is adapted
to be connected electrically in parallel with voltmeter 16 and to
have a junction between its components that, because of the
selection of components can be, or be made to be, a null point for
connection of contact 26 to shielding 24 so that the capacitive
current, whatever it may be, can be added to the current flowing
through phasing voltmeter 10 with no net increase or decrease in
displayed voltage. To achieve this null point, the voltage divider
networks on either side of the junction must be essentially
identical.
[0043] Electrical circuit 28 in the embodiment illustrated in FIG.
3 includes two resistors: R3, and R4. The magnitude of resistors R3
and R4 should be as similar as possible and relatively small, on
the order of 50 ohms for example.
[0044] Phasing voltmeter 10, as described, is thus a five terminal
device consisting of two high impedance resistors, R1 and R2, and
one low impedance element, electrical circuit 28, that together
form three precision divider networks. The first and second
terminals, T1 and T2, are the points of contact between first probe
12 and transmission line A (or other source voltage) and between
second probe 14 and transmission line C (or other source voltage),
respectively. The third and fourth terminals, T3 and T4, are the
points of attachment to the inputs of voltmeter 16. Contact 26 is
where shield 24 meets electrical circuit 28.
[0045] The first of these precision voltage divider networks
divides the voltage between connected in parallel with voltmeter
16. Each range is selected by closing two switch pairs, 44 and 46,
48 and 50, or 52 and 54 which allows current to flow through an
electrical circuit 56, 58, or 60, respectively, of the type just
described. Each electrical circuit, 56, 58, and 60, includes three
resistors in series: a gain resistor 62, 68 and 74; a null resistor
64, 70, and 76; and a balance resistor 66, 72, and 78,
respectively. Gain resistors 62, 68, and 74 are adjustable to
establish a precision voltage divider network. Null resistors 64,
70, and 76 are adjustable to "zero out" the effect of the
capacitive current.
[0046] In a preferred embodiment, these ranges would include
voltages up to 2 kV, to 20 kV and to 50 kV. The resolution at each
of these levels is 1 volt, 10 volts and 100 volts. The number of
ranges that can be selected and the ranges are somewhat arbitrary,
and those skilled in the art of voltmeters for electrical power
transmission will recognize that different ranges may be
selected.
[0047] The present phasing voltmeter 10 is capable of accuracies of
0.0001% or better with resistors of 2%. Even when the resistors do
not have the precise resistance they are rated to have, accuracy
can still be obtained. As long as the impedance of the AC voltmeter
16 is high with respect to electrical circuit 28, the balance of
the ratios of the networks is more significant than the balance of
the resistance of the networks. In practice, a small AC voltmeter
12 can be preset to display a voltage 1,000,000 times greater than
it detects on its input terminals. This ratio has a tolerance of
slightly less than 1%, so that the voltage may actually range from
990,000 times to 1,010,000 times greater.
[0048] As an example of the present invention, suppose the desired
voltage divider ratio is 1,000,000 to one and R1's nominal
resistance is 50,000,000 ohms but its actual resistance is
48,000,000. R2's nominal resistance is also specified as 50,000,000
but its actual resistance is 49,000,000 ohms. The gain resistor,
R3, has a resistance that can range from zero to 50 ohms; the null
resistor, R4, has a resistance that can range from zero to 10 ohms.
The balance resistor, R5, has a nominal resistance of 45 ohms but
actually has a resistance of 46 ohms.
[0049] The sum of the resistances of R1 and R2 is 97,000,000 so the
resistance of the electrical circuit should be 97 ohms at 1,000,000
to one. Thus, the resistances of R3, R4, and R5 should sum to 97
ohms. R5 is fixed at 46 ohms; R4 is set to 10 ohms because
adjusting it will not change the through resistance presented to
the other components in the series string because of the way it is
configured in the circuit. R3 is adjusted to 41 ohms (97 ohms less
10 ohms less 46 ohms).
[0050] Some portion of R4 and all of R3 will form a precision
voltage divider network with R1. Because R1 is actually 48,000,000
ohms, this network should have a resistance of 48 ohms (again, at
1,000,000 to one). Since R3 is now set to 41 ohms, the portion of
R4 must be 7 ohms.
[0051] The remainder of R4 (3 ohms) and R5 (46 ohms) must also
equal one millionth of R2, or 49 ohms.
[0052] It will be apparent to those skilled in the art of
electrical power transmission line servicing and maintenance
equipment design that many modifications and substitutions can be
made to the foregoing preferred embodiments without departing from
the spirit and scope of the present invention.
* * * * *