Buried power line contact alert

A method and a device for use with horizontal boring equipment or excavating equipment while making an underground tunnel or digging near known or unknown electric power lines. The device indicates contact with the electrical conductors, whether the contact is momentary or continuous, by monitoring voltage gradients in the earth and activating an alarm if voltage is sensed.

BACKGROUND 
1. Field of Invention 
This invention relates to safety devices for horizontal boring or 
excavating operations, specifically to an alerting or warning method and 
device that indicates that an electrical power source has been contacted 
and its insulation broken. 
All equipment used to excavate ground cavities have a potential hazard of 
damaging buried electrical cables, and as more cables are buried the 
probability increases. The process of horizontal boring is particularly 
hazardous because it is usually employed to make a tunnel or conduit for 
installation of lines under an existing structure, road, or private lawn 
where power lines are common. 
2. Discussion of Prior Art 
It has been an erroneous general feeling and teaching that a metal rod 
buried in earth will be grounded and will cause circuit protection devices 
to open if the rod contacts a power transmission line. OSHA, UL, and NEC 
codes and standards have indicated an acceptable ground rod will have an 
impedance of 25 ohms or less. At 25 ohms, a 115 volt house line will only 
draw less than 5 amps. Field measurements on horizontal bore strings 
buried over 25 feet in length show impedances back to utility power 
returns of up to several hundred ohms. Vertical rods usually have lower 
impedances. 
Earth has a very high impedance to the flow of electricity compared to the 
low impedance of most metals. Any metal that contacts an electric 
transmission line even if on top of or buried in earth will attain the 
voltage level of that line. 
If the earth impedance is high, only small currents will flow and the 
hazard potential will be great. Fuses and circuit protection may not turn 
the power off. Also large voltage gradients through the earth may present 
hazards to bystanders and workers standing on the ground some distance 
from the equipment. 
If the earth impedance is low, large currents will flow and fuses, 
breakers, or switch gear will trip. On-site workers may not feel or see 
any sign of the contact, or conversely it is possible the jolt will be 
lethal. If switchgear was tripped, it may automatically reclose after a 
few seconds. 
With either high or low impedance earth, there will be a voltage difference 
or gradient between the line-contacting metal and nearby earth 
irrespective of any attempts to `ground` the equipment. There will also be 
voltage gradients through the earth as a function of the space or distance 
from the equipment to line contact point. Therefore, sensing the gradient 
from the equipment to earth or from close earth to farther earth serves 
the same purpose and provides a reliable indication of contact or breeched 
insulation. 
The current sensing alarm in U.S. Pat. No. 4,755,805 to Chau, 1988 Jul. 5, 
requires an equipment grounding point sufficient to reduce voltages at the 
equipment to a negligible level. Since switchgear typical in shopping 
malls are designed to open lines drawing 80,000 amps, an adequate ground 
point and wire would have to have much less than 1 milliohm for a 
negligible voltage at the equipment. A sensing coil used to detect the 
current would in itself defeat the ability to obtain a low resistance 
ground. 
Frequently, when boring or excavating, a power line will be hit a momentary 
glancing blow removing insulation but not causing major burns or other 
indication of contact. Also, a line may be protected with fuses or 
switchgear that trip quickly. In either case, Chau's horn, of the above 
referenced patent, would not sound. A subsequent movement, return of the 
bit, or closing switchgear might cause a hazard. In the device of the 
present invention, the horn is latched in its activated state with even a 
momentary voltage and must be manually reset after all hazards are known 
to be cleared. 
Therefore, it is an objective of this invention to provide a method and a 
device for announcing contact of a boring bit or excavation tool with an 
in-ground source of electricity such as a buried power line. 
An additional objective is to provide a continuing signal if the contact 
with an electrical source is only momentary. 
Another objective of this invention is to provide a signaling device that 
does not require on-site measurement, calibration or provision of a 
special equipment ground. 
A further objective of this invention is to provide a conveniently small 
and portable device that can be mounted on excavating or boring equipment 
or that can be carried separately as an independant device that may be 
used with portable tools such as digging bars, shovels or any other earth 
penetrating equipment. 
Other objects and advantages of this invention will be apparent after 
consideration of the drawings and descriptions. 
SUMMARY OF THE INVENTION 
As will be more completely described, the invention disclosed herein is of 
a method and a device that senses voltage gradients within the earth near 
boring or excavating equipment and sounds an alert signal when the earth 
voltage gradients exceed a predetermined level. Voltage is sensed because 
any current flowing through the earth will cause voltage gradients for 
sufficiently large distances from the source, an no other requirements 
need be placed on the boring or excavating equipment being used. sound 
off. In actual practice, either one or both methods of mounting the 
invention may be used at an excavation site. 
FIG. 2 shows an equivalent combined schematic and block diagram of the 
system of FIG. 1. An electrical power source 40 such as a generator or 
substation transformer has an equivalent nominal output impedance Z.sub.t 
and is connected to earth ground through some non-zero impedance Z.sub.g. 
A fuse 44, or any other protection device, connects the power source 40 to 
the buried power line 30 which has a distributed characteristic impedance 
Z.sub.L. If a drill bit 26 contacts the power line 30 at contact point 46 
current will flow through the variable contact resistance R.sub.v, the 
distributed resistance R.sub.i of the iron drill tube 18, the drill tube 
joint resistances R.sub.j, and multiple, interchangeable earth path 
resistances R.sub.s. It is obvious now that this current path from the 
contact point 46 through the drill bit 26 and the bore tube 18 is 
representative of any current path through any metal member of any type of 
excavating equipment forced into the earth contacting a power line. 
Any representative earth path resistance R.sub.s1 is broken into three 
series resistances R.sub.a, R.sub.b, and R.sub.c. The resistance from the 
equipment to the electronics box 14 may be negligible if the box is 
mounted on the equipment or a finite value if used independently. The 
earth resistance R.sub.b is between the box contact point 60 and the 
The output voltage of amplifier A2 is fed to the positive input of 
operational amplifier A3 which is connected with positive feedback to act 
as a comparator. When ever the positive input voltage V1 exceeds the 
reference negative input voltage V2, the output of A3 will be its positive 
rail voltage. Input resistor R11 is selected high enough in relation to 
feedback resistor R12 so that if the output of A3 is high the positive 
input voltage V1 will be higher that the reference negative input voltage 
regardless of the output voltage of amplifier A2, thereby creating a latch 
condition. (If the reference input voltage is selected higher than the 
feedback voltage, the circuit will not latch and the system may be useful 
on construction equipment, that may potentially contact overhead power 
lines, to warn operators of a contact and allow them to move away from the 
wire before disembarking or otherwise jeopardizing their safety.) The 
entire unit is powered by a battery 56 which when disconnected by the 
reset switch 58 allows amplifier A3 to return to its initial relaxed 
condition, and when power is restored, the reference voltage V2 will come 
up faster than the feedback voltage V1 allowing the condition of amplifier 
A2 to control. 
The output of amplifier A3 drives a conventional MOSFET switch circuit 52 
which when activated completes a return circuit for a conventional loud 
horn 54.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As FIG. 1 depicts, the signaling device of this invention 8 comprising a 
ground spike 10, a connecting wire 12 and an electronics box 14 may be 
used with a horizontal boring machine 32 or any other machine or tool that 
bores or excavates the earth. In this example, the horizontal boring 
machine 32 is comprised of a power driven carriage 20 which rotates and 
pushes a bore tube 18 to which is attached a boring bit 26 to make a 
tunnel 28. An operator controls the boring operation from a remote control 
stand 22 with attached control leads 24 to keep him away from any 
potentially electrified equipment. If the boring bit 26 contacts and 
damages a hidden buried power line 30, voltage gradients in the earth will 
cause the signaling device 8a to sound off, and the voltage gradient from 
the horizontal boring machine 32 to earth at some distance will cause the 
signaling device 8b to ground spike contact point 62, and the leftover 
earth resistance R.sub.c will be a reasonable finite value. It can be seen 
that for any non-zero soil resistance and non-zero line voltage there will 
be a voltage across R.sub.b if the contact resistance R.sub.v is finite. 
The circuit in the electronics box 14 includes a voltage detection function 
48, a comparator/latch circuit 50 to compare the detected voltage level to 
a set reference level and latch the result that turns on the driver 52 and 
activates the horn 54 or any other warning device. The entire unit is 
powered by a battery 56 which when disconnected by the reset switch 58 
allows the latch 50 to return to its initial relaxed condition. 
The schematic in FIG. 3 shows operational amplifier A1 connected with 
resistances to act as a differential amplifier sensing the voltage 
difference between the box contact point 60 and the ground spike contact 
point 62. The output of operational amplifier A1 is amplified and isolated 
by operation amplifier A2. The inputs to A1 are protected from over 
voltages by zener diode D1. The resistors R1 and R2 in combination with 
diode D1 are selected to burn open before any other part of the circuit is 
damaged if the sensed voltage is extremely high for a relatively long 
period of time. (Voltages as high as 120,000 volts are known to be buried 
with plans for much higher voltages being made.) 
Note that input noise protection to avoid false firing is not needed 
because voltage gradient noise in the earth is extremely low except in 
extraordinary circumstances such as thunderstorms. 
OPERATION 
A worker or crew about to dig or bore a hole would first follow all normal 
safety procedures of calling utilities to mark line locations and of 
wearing protective apparel, etc. Then the device of this invention would 
be placed near the hole, with the box and the spike separated by a few 
feet. Boring or digging would then begin. If an unknown or incorrectly 
located power line is damaged, the horn will sound and workers can leave 
the location until experienced electrical crews remove the hazard. Without 
the alarm, unaware workers may move to hazardous locations or attempt to 
bare hand the now possibly energized equipment. Also, if the workers do 
not know a line has been damaged, they might leave the damaged wire and 
cause a future power failure or more serious accident.