Electric field detector

THE UNIVERSITY OF LAHORE.
NAME: JAWAD ALI (BSET01111037)
SYED IMRAN ALI (BSET01113054)
SUBMIT TO: SIR HISHAM KHALIL.
PROJECT TOPIC: ELECTRIC FIELD
DETECTOR.
CLASS: EE-5
SUBJECT: ELECTROMAGNETIC FIELD THEORY.

CONTENTS:
1) Abstract
2) Background
3) Disclosure in Invention
4) Construction
5) Working
6) Weak, medium and stronge field
measurements
7) Amplified E-Pod 3 Color Static Field Proximity
Range Detection Description
8) Advantages
9) Applications
10) Conclusion
11) Refrences

ABSTRACT:
The detector utilizes the microstructure
and effects integration of an electric field over the
volume of ferrite core.
BACKGROUND:
In the past, electric fields were detected
using free-body electric field meters.
These detectors were typically of spherical or cubic
geometry and were constructed from conductive
material.
When placed in a electric field a charge will oscillate
between two electrically isolated halves of the
detector.
Mathematically this charge can be described by:
Q=A·εo·E
Where:

εo=permittivity of free space, E=electric field
strength to be detected, A=a constant proportional
to detector surface area, Q=charge on detector
To achieve useful detector sensitivity the detector
dimensions are typically in the order of 10 cm (4
inches.)
Due to the large surface areas of these detectors
they are very prone to stray capacitive coupling to
other bodies in their proximity.
This can modify the capacitance of the detector
assembly, and the above equation can be re-
arranged to:
Q=C·d·E
Where:
C=total capacitance of detector d=spacing between
detector halves Q & E as above
Hence it can be observed that the output of the
detector is directly related to the capacitance of the
detector.
So any modification of detector capacitance by stray
capacitive coupling will modify the detector output,
thus giving false readings.

DISCLOSURE OF THE INVENTION:
It is the aim of the present invention to eliminate or
at least minimise the foregoing disadvantages and
also to enhance certain desirable characteristics of
free body electric field detectors.
If a stable known capacitance can be
intrinsically added to the detector then the output of
the detector is proportionally increased.
If this can be achieved with a physically smaller
detector then the effects of stray coupling
capacitance are reduced by two mechanisms.
Firstly the area of the detector is reduced thus
directly reducing coupling capacitance. This is
evident from the basic capacitor equation:
C=(ε.A)/D
Where:
C=capacitance ε=permittivity A=area of conductive
plates D=plate separation
Secondly, if the intrinsic capacitance of the detector
is large in relation to the coupling capacitance, then
the effect of the coupling capacitance is minimised.

This occurs because the coupling capacitance can
now only make a small percentage change in the
total detector capacitance.
ILLUSTRATION:
If we consider two cube shaped conductive boxes
separated by a small distance, it can be shown that
relatively large dimensions are required to achieve
sufficient detector capacitance.
Further, only the outer surface of the cubes are
significant because the conductive material acts as a
Faraday shield, hence excluding any electric field
from their internal volumes.
However, ferrites have some interesting properties in
this regard. Most ferrites are relatively poor
conductors and allow electric fields to penetrate into
their internal volumes, hence minimizing the Faraday
shield effect and allowing the sensor to detect
electric field in a space volume. Preexisting designs
only detected electric field over the surface area of
the detector.
At this stage it is convenient to consider a ferrite
structure being composed of metal particles
entrapped in a ceramic substrate.

Referring to FIG. 1 we can propose an electrical
equivalent circuit for such a model. The metallic
particles act as capacitor plates with the ceramic
substrate acting as a dielectric.
Ferrite has volume resistivity and is modeled by
parallel resistances. As shown a network of
resistor/capacitor elements can be built up. The
ferrites proposed are the MnZn type which have a
classic spinel atomic lattice structure.
At a microscopic scale the resistivity of this structure
is not homogenous, with the resistivity of the grain

boundaries being typically a million times that of the
ferrite within the grains. Typical grain sizes range
from 5 to 40 um, with the grain boundaries having an
enrichment of Ca, Si and Ti ions which produces a
high resistivity boundary of approximately 10
angstrom units width.
This grain structure has a dominant influence on the
effective permittivity of the ferrite. Such ferrites are,
in effect, compound dielectrics composed of very
thin high resistivity grain boundaries separating
semi-conducting grains of low resistivity, with a
resulting effective permittivity as high as 100,000.
Remember that capacitance is directly related to
permittivity. FIG. 1 shows a conceptual view of four
grains in a ferrite structure, and indicates the
associated resistivity and capacitance between the
grains as R′ and C′ respectively.
From this it can be deduced that ferrite has both
volume resistivity and volume capacitance.

FIG. 2 shows a macroscopic equivalent circuit for a
volume of ferrite. The values Cv and Rv are the
algebraic addition of all the R′ and C′ values for all
grains in the ferrite volume.
Now referring to FIG. 3 if the ferrite equivalent circuit
is arranged so it is in parallel with the detector plate
capacitance then the total capacitance is increased.

That is Csensor=Cp+Cv
But as shown previously the charge Q generated in
a given field is directly proportional to the
capacitance.
The volume resistivity of the ferrite and the
increased inductance of the assembly improves
output stability and discriminating high frequency
noise.
The sensor plates have a fiberglass dielectric which
increases the value of Cp by a factor of εr for
fiberglass.
It is also important to correctly condition the sensor
output signal with suitable electronics. By monitoring
current output from the sensor rather that voltage,
some loss of sensitivity occurs but there is a marked
improvement in detector output stability and
discrimination of stray effects.
The current out of the sensor is equal to the time
derivative of the charge, and for electric fields it can
be written:
l=jωAEε
j = complex operator E = electric field ω = angular
frequency A = area ε = permittivity

A JFET is used to sense the electric field generated by high voltage electric
line; the JFET amplifies the signal very little, but it lowers the impedence and
provide current to a level suitable for transistor amplification. The two
transistor can be any low power NPN scavenged from anywhere. The two-
transistor are configured as a sort of thresholded amplifier: when the voltage
at R2 rises at above 3V approx, Q1 starts pumping current into the LED with a
step curve providing a better go/no go rensponse.
A low current LED could have been connected directly between V+ and the
Drain of the JFET and removing TR1, TR2 and R3 through R7: in this case the
LED would light up in a linear way, no threshold.
Stray charge may escape from the tip of R1 for tip-effect letting the intrinsic
capacitance at the gate of the JFET charge positively giving a positive read
after a little while. So, a curled tip and a very high resistance path to ground in
the form of a few turns of thin wire around resistor R1, help keep things in
balance. In case, a very very high resistor can be connected between gate and

ground (R8), but this will limit sensitivity very much.
Resistor R1 is there to provide a protection to the delicate gate of the JFET and
the delicate heart of the operator in case of contact to a live line. Here, the
higher the value of the resistor, the better. R1 also provides for the sensing
tip.
Electric Field Measurement:
Electric fields surround every energized conductor. All things being equal, electric
field strength is directly proportional to voltage magnitude. The higher the
voltage, the stronger the field and the greater distance from which it can be
detected. The measure of an electric field is in voltage over a unit if distance,
typically volts/meter. For a typical overhead distribution line at 7,200 volts and
12.2 meters (40 ft) up in the air, the average electric field strength beneath the
line would be 7,200 volts / 12.2 meters or about 590 volts/meter. This is an
average field strength however and electric fields are generally not uniform in
strength over their distance. In this case, as shown in figure , the local field would
be much stronger directly adjacent to the conductor but would fall off rapidly
with distance and be barely detectable down near the ground.
For line conductors suspended above ground, an inverse relationship generally

prevails; twice the distance from the conductor results in one half of the electric
field strength. This can also be illustrated as equipotential lines as shown in this
figure .
In a typical underground URD cable, the electric field is contained completely
within the cable between the inner conductor and the outer shield. The very high
electric field inside the cable is not detectable outside the cable however due to
the outer grounded shield.
Weak Field Measurements:
Weak electric fields are in the range of magnitude from a few volts/meter to tens
of volts/meter. These weak fields exist either a short distance from a low voltage
source or a long distance from a higher voltage source. Both situations have
application in voltage detection tools.
Recent emphasis on what is frequently called urban stray voltage has placed a
premium on the ability to easily detect voltages in the range of 6 to 10 volts on
objects in the public right of way. Light poles, traffic signal controls, power
pedestals, manhole covers, etc. are all subject to the scrutiny of inspectors
looking for low AC voltage, usually caused by compromised insulation and/or

defective grounds. A compact voltage detector with a single point of contact and
a reliable detection threshold allows for quick and efficient inspections.
Such a detector is shown here in figure .
Designed to make direct contact with the metallic tip to equipment to be tested,
it indicates the presence of voltage with visual and/or tactile alarms. The hand of
the user forms a virtual ground around the handle of the device and collapses the
electric field into the small distance between this hand and an inner electrode.
Collapsing the very weak electric field in this manner increases the field strength
and allows for a high degree of sensitivity and the detection of very small voltages
including those resulting from equipment that is intact and working properly but
simply ungrounded.
A second application for weak electric field measurement is in personal voltage
detectors. These devices, typically worn on the body of the user, detect very weak
electric fields as an indication of the presence of distribution voltage conductors
in the general area of the wearer.
Following storms or other accidents, downed medium voltage conductors are a
persistent hazard to inspectors and work crews making repairs. A warning of the
presence of nearby downed energized conductors can be helpful and even life
saving but only if that warning is a useful indication of a potential hazard and not
simply another reminder of known live conductors such as intact overhead lines.

Devices such as that shown in following figure are highly directional to the
electric field vector with strong preference given to optimum sensitivity in the
direction forward from the wearer and reduced sensitivity in the directions of the
rear, far sides and overhead.
Internal sensing electrodes exactly parallel to the plane of the body of the wearer
result in optimum sensitivity in the direction of both work and movement of the
user. Further shaping of these electrodes fine tunes sensitivity where it is desired
and attenuates sensitivity in directions where it is not as shown in graph.
The concentric ovals signify three distinct warning levels.

The presence of live conductors in the zone of optimum sensitivity will provide an
audible and visual warning to the user from a distance of 2 to 5 meters (6.5 to 16
feet).

Medium Field Measurements
Medium strength electric fields in the range of hundreds of volts/meter to
thousands of volts/meter are found in the general vicinity of energized
distribution and transmission voltage conductors. Measurement of medium
strength fields has application in a wide variety of voltage detectors and
indicators.
While the original live line detector was a pair of pliers or wire cutters used in a
gloved hand to touch or “fuzz” the line as a crude indication of the presence of
voltage, OSHA and other regulations now require a detector with both audible
and visual indications of voltage.
Direct contact voltage detectors are those designed to make direct electrical
contact to an energized conductor and to indicate the presence of AC voltage on
that conductor. Often, these devices have a fixed or variable voltage threshold
setting allowing the user to selectively detect only voltages of interest.
Proximity voltage detectors are those designed to indicate the presence of AC
voltage on a nearby conductor from a short distance away, generally within one
half meter distance. Like
their direct contact cousins, these devices have a fixed or variable voltage
threshold setting allowing the user to selectively detect only voltages of interest.
Most important to the mission of these devices is their ability to discriminate
directionally. The typical work environment does not always consist of only a
single conductor as shown in figure .

Multiple three phase feeders often with intersecting overbuilds or underbuilds
make for an electrically complex environment. An electric field detector that can
discriminate.
More sophisticated types of detectors have more sophisticated sensing
electrodes and sensing circuitry to measure the electric field strength in more
than one direction. These multiple measurements in combination with a well
designed algorithm can make the detector respond differently to different
conductor geometry. The purpose of this increased sophistication is to make the
voltage detector more likely to detect voltage on conductor configurations that
are harder to test with simpler detectors.
Bends and corners in stranded conductors and buss bars result in significant
electric field changes as shown by the equipotential lines in figure .

Outside corners concentrate electric fields while inside corners spread them out.
The result is a change in electric field strength for a given voltage that can
challenge voltage detectors to provide consistent results. Detectors that make
multiple electric field measurements will be able to detect these unusual field
gradients and make adjustments in sensitivity accordingly.
Strong Field and Other Measurements
Electric field measurements in the close vicinity to transmission voltage
conductors provide special challenges. At the upper range of transmission
voltages the conductor diameter increases to minimize discharge and these larger
diameter conductors result in different electric field gradients for a given voltage.
The electric field near the surface of a 138kV buss bar of 2 in. diameter will mimic
the electric field near thesurface of a 345kV buss bar of 4 in. diameter and an
unsophisticated voltage detector will not know the difference. Further, at
voltages of 345kV and up intense ionization at the surface of the conductor
breaks down the surrounding air thus increasing the effective electrical diameter
of the conductor.

Fortunately, it is difficult to fail to recognize that a conductor is energized at a
transmission voltage. Proximity to these conductors, even well outside the
minimum working distance, results in tingly skin and unmistakable audible
discharges. The frequent challenge at these higher voltages is to distinguish
conductors energized at nominal line voltage from those that are de-energized
but ungrounded. Crowded and long transmission corridors can result in
substantial voltages on conductors that are not intentionally energized but are
simply left ungrounded. These voltages, though well below nominal voltage, can
still be lethal. Projects such as reconductoring or other line maintenance place a
premium on knowing the difference between energized lines and those running
parallel with resultant induced voltages.
Accurate measurement of electric fields in combination with intelligent
compensation for the transmission voltage effects mentioned above allows not
just detection of voltage but a display of the measure of the voltage. Seen
in figure ,
this new ability in a tool with a single point of contact allows users to easily
distinguish nominal line voltage from induced voltage and to do so on systems up
to 765kV.

Electric field measurements in a device of this type undergo further digital
processing to compensate for conductor diameter, local ionization and other
corona effects. The displayed number gives the user far more information than
voltage detection alone, does so with ease and convenience and improves safety
by providing a clearer picture of the status of the circuit being tested.
Amplified E-Pod 3 Color Field Proximity Range
Detection Description:
E-Pod-Amp
Keep in mind that during use, Static electricity is formed much better when
the air is dry or the humidity is low. The E-field Pod can detect static up to
several feet when the air is very dry. When the air is humid, water
molecules can collect on the surface of various materials and this can
prevent the buildup of electrical charges and minimize the detection range
considerably. The E-Pods simply detect the presence of the E-Field around
the static, it does not measure the Static Charge. You would need a very
expensive Electrostatic voltmeter for that.
1) With a Negative charged condition, the lights will remain ON and as a
charged object slowly approaches the E-Pod-AMP, the GREEN, BLUE and
RED lights will begin to dim in relationship to the e-field strength and
proximity to the static source. Eventually all of the the lights will go off when
the charge is within a few inches of the E-Pod. The lights will slowly come
back on again when the charged object moves away from the E-Pod-AMP.
2) With a Positive charged condition, the light will remain OFF and as a
charged object gets close to the E-Pod, the light will come on. It then turns
OFF when the charged object is removed.
Important: The E-field Pod Amplified 3-Zone Device has been designed to
assist a Paranormal Investigator make an informed decision based on EMF
and Electro-Static evidence collected during an investigation..

Amplified E-Pod 3 Color Field Proximity Range Detection Feature
>Detects E-Field (Static) Energy Charges up to 12' away at 35% relative
humidity.
>Easy to View Bright RED, GREEN, BLUE Indication
>E-Field Detection Range: 500mV to 700+Volts
>E-Field Sensitivity: Positive and Negative Charges
>Mini Telescopic Antenna provides 360 Degree Coverage

CONCLUSION:
More sophisticated electric field measurement when combined
with digital processing can provide new capabilities for voltage
detection and indication.
More sophisticated field measurements include sensing in
multiple directions, field gradient testing and compensating for
geometry and other high voltage field effects. Users reap the
benefits of these advances with a new generation of tools that
is smaller, lighter, more accurate and easy to use..
REFRENCES:
1)MARK TRETHEWEY ”ELETRIC FIELD DETECTOR” USA
PATENT APPLICATION PUBLICATIONS.
2)Alston, L.L. High Voltage Technology. Oxford
University Press 1968..

Electric field detector

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