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

Constrution:

WORKING:

BAND PASS FILTER:

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 MeasurementsMedium 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 MeasurementsElectric 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

ADVANTAGES:

APPLICATIONS:

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..