Geography of Microwave Survey-20080226-A

HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential

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FIELD TELECOMMUNICATION SURVEY

Geography of Microwave Survey

HUAWEI TECHNOLOGIES CO., LTD. Page 2Huawei Confidential

FIELD TELECOMMUNICATION SURVEYFIELD TELECOMMUNICATION SURVEY

ContentsContents1) Topography

- Map representation

- Geographical coordinates

- Altimetry

2) Terrain profiles

- Propagation

- Profiles / Clearance criteria guidelines

- Reflection

3) GPS

- The various types of GPS

- GeoExplorer user's guide

4) Grounding system






- Altimetry

2) Terrain profiles

- Propagation


- Reflection

3) GPS



4) Grounding system


Map reading

• Collect all the maps available, preferably 1/50 000 (1/24 000 for North America).

• In some circumstances 1/200 000 maps will also be useful.

• Scale

1/200 000 1 mm = 200 m1/50 000 1 mm = 50 m1/24 000 1 mm = 24 m1

1/10 000 1 mm = 10 m1/ 5 000 1 mm = 5 m

TOPOGRAPHYTOPOGRAPHY

Map representationMap representation



• Image of the earth surface, watched from particular point of view and projected on to a tangent plane on the earth surface

• Types of projections– MERCATOR (Gérard Kremer alias)– LAMBERT



• Cylindrical projection.

• The projection surface is a cylinder which is tangential or secant to the earth's model.


• Example: UTM projection is divided of 60 zones of 6° in longitude– France is on 3 zones: zone 31 30 32


Direct cylindrical projection Oblique cylindrical projection

Transverse cylindrical projection


• Conical projection.

• The projection surface is a cone tangent or secant to the earth's model.


• Example: LAMBERT projection is divided into 4 zones– LAMBERT I - II - III - IV or LAMBERT II extended which covers all the

country.


Tangent conic presentation Secant conic presentation



• For map representation, the ellipsoid closest to the Geoid of the area to be represented will be used. For France, we'll use clarke 1880 ellipsoid lambert conformal map projection. For Africa we'll use clark 1866 ellipsoid

• Ellipsoid is a mathematical model to define the earth surface

• Geoid is a mathematical model to which coincides with mean sea level extended to all continents.(Geoid conventionnally coincides with altitude zero)

• Datums: NAD 27, NAD 83, Old Hawaiian (for North America)



Geoid

Earth surface

Ellipsoid


• Date of creation and update

• Ellipsoid




Magnetic variation• Magnetic North is the only one which can be measured (compass).

• Geographic north is the north on the map.

• An angle measured from magnetic north can be reported on a map if the variation is known.

• It’s possible to Check the magnetic variation via the internet:

www.geolab.nrcan.gc.ca/geomag/e_cgrf.html

www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl



• When Magnetic variation decreases, it means it's getting closer to geographic North - it varies according to geographic areas on earth - it is more important near the poles.

2°47'

Magnetic North Geographic North

Example of indication on IGN France map

Magnetic variation coincides with the middle of

the sheet on 1st January 1990 it decreases

every year by 8'.


Magnetic variationMagnetic variation



MNGN

7°

237°/MN

230°/GN


Magnetic variationMagnetic variation







- Altimetry

2) Terrain profiles

- Propagation


- Reflection

3) GPS



4) Grounding system


Latitude and longitudeLatitude and longitude

• On a single map, we can have two different projections.– The projections can be found in the map’s legend.

• Each map provides a legend that must be read.

• Contour lines are essential to draw a profile.

• Systematically check the contour intervals.


Geographical coordinatesGeographical coordinates


Each index contour line is accentuated Contour intervals: 10m

112m

100m

50m





Latitude and longitude

• Enable to calculate geographic coordinates of any point on earth thanks to abscissa and ordinate report

• Earth circumference 40000 km40000 km / 360° = 111.111 km = 1°

1° = 60' => 1.852km = 1' = 1 mile1' = 60" => 31 m = 1"

AT EQUATOR LEVEL




Origin of any map

W

E

LONG ELAT S

LONG WLAT S

LONG ELAT N

LONG WLAT N

GREENWICH MERIDIAN OR MERIDIAN 0 (zero)

N

S

Equator





1cm

5mm

Cross with soft lead pencil

Pointing with hard pencil


• Site pointing



Latitude and longitude

• Coordinate calculation– 1) Write out the point on the X and Y axes with a square and HARD PENCIL

– 2) Measure the mm. value of 300" and deduce the value of 1 second in xmm longitude and ymm latitude

– 3) Measure the variation between the point written out and the origin selected

– 4) Apply the rule of three

– 5) Add the calculated values to original values

DO NOT HESITATE TO DETAIL YOUR CALCULATIONS

°degrees, ' minutes, " seconds






48°05'

ymm

Latitude

y'mm

48°00

Origin

2°00' 2°05'X'mm

longitudexmm






- Altimetry

2) Terrain profiles

- Propagation


- Reflection

3) GPS



4) Grounding system


• ALTITUDE MEASUREMENThpa (hectopascal) = Meteorologymmhg (millimeter of mercury) = Medical unitmbar (millibar) = Old unitbar = Industrial pressure

unit

===>Altitude Pressure


AltimetryAltimetry


• The altimeter is a barometer


AltimetryAltimetry

²/1²1

11 mN

m

NEWTONpa


• Normal atmospheric pressure measured at standard point Altitude m

7000

6000

5000

4000

3000

2000

1000

0

Pressure hpa

410

472

540

616

701

795

899

1013

Temp °C

-30.5°

-24°

-17.5°

-11°

-4.5°

2°

8.5°

15°


AltimetryAltimetry


Map information

• Origin of altitudes measured

Altimetry

• The altimeter under forcasts altitude deviations under warm temperatures and overforcats them under cold temperatures.In winter it gives too high altitude and too low in summer


AltimetryAltimetry


• Summer– 4000 m measured in a summer day with a temperature of 0°C:

+11°C deviation compared with standard atmosphere– Applicable correction is: +11x4x4m = 176m Real altitude is 4176m

• Winter– 4000m measured in a winter day with a temperature of -20°C: -9°C

deviation compared with standard atmosphere– Applicable correction: -9x4x4m = -144m Real altitude is 3856m

Correction is of 4m per 1000m, and per deviation degree compared with standard temperature at reading altitude with deviation sign


AltimetryAltimetry


• It is thus necessary to apply the so-called double measurement procedure2 altimeters are requiredMark a point on the mapCalibrate both altimeters on itReference altitude will be ckecked out at regular time slots, every 10' for example

• Altitude measurements will be performed during that time with the 2nd altimeter, indicating each time the measurement time.Altitudes will be compared and correction applied


AltimetryAltimetry


Can be A geodetic point with hub333

132 An altimetry point

Alt A

Reference

Alt B

Measurement


AltimetryAltimetry


Alt A

Reference

Alt B

Measurement

Report variationsevery 10' or moreFill in a sheet

Measure the pointsand report measurementtimes


ReferenceReference

AltimetryAltimetry


Alt A Alt B

Origin

- 8:00 333- 8:10 333- 8:20 334- 8:30 335- 8:40 336- 8:50 337- 9:00 338- 9:10 338- 9:20 339- 9:30 340

= +1

= +2

= +3

= +4

SITE A = 234m.

SITE B = 198m.

SITE C = 182m.

SITE D = 206m.

SITE ASITE B

SITE C

SITE D

8:20235m. 8:30

200m.

8:40185m.

8:50210m.

Correction

Area limited to 10 to 15km around the point with altitude deviation <500m


ReferenceReference

AltimetryAltimetry






- Altimetry

2) Terrain profiles

- Propagation


- Reflection

3) GPS



4) Grounding system


• What is propagation ?– Energy transfer with no physical transportation

• Line of sight propagation– Propagation between 2 points for which the direct ray is sufficiently clear

of obstacles for diffraction to be a negligible effect.

TERRAIN PROFILESTERRAIN PROFILES

PropagationPropagation


(A) antenna supplied by P power (transmitted power) will create in the wholespace an E magnetic field and (B) antenna introduced in this space willcollect a part of E field (Received power).

A B

E R




• Line of sight links– Link in which diffraction effects are minor

• What is diffraction ?– Diffraction is a phenomena which tends to modify radio wave path

nearing on obstruction




• Radio waves are related to 3 phenomena

Diffraction

Refraction

Reflection




Let's examine the following diagram: it seems there is no diffraction

E R

B

P




• Maxwell equations indicate that:

– The field in R point can be calculated with the field created in E in any point of P Plane

– P plane will be separated in concentric rings

– If P plane is moved in parallel to itself, B creates a revolution ellipse harring E and R as centers.

– The main part of the Energy is concentrated along the ER line.The first ellipsoid along this line concentrated along the ER line.The first ellipsoid along this line concentrates the main part of the energy.

– The first ellipsoid is called Fresnel Ellipsoid




• The ellipsoid shall be cleared from any obstruction. But the energy radiated in E will suffer from attenuation when reaching point R. This attenuation is the ratio of transmitted power to received power. It is called propagation loss (Diffraction loss)

)(

)(4log20

m

mdAdB




• Definition of CLEARANCEC = 1 means that 100% of Fresnel zone is cleared from any obstruction. Only 60% of the first ellipsoid shall be cleared of obstructions to have a received level equivalent to the level of free space.

• For Microstar (Short High Frequency Hops), 100% of clearance (C = 1) will be required.

• Please, refer to the next page for the Main (Top-to-Top) Path Clearance Rules





PropagationPropagationClimate-Terrain Factor c

Band <2 (good to average)

>2 (moderate to very difficult)

Above 3 Ghz 0.6F1 @ k = 1 F1 @ k = 4/3 and 0.3F1 @ k = 2/3*

Below 3 Ghz 0.6F1 @ k = 1 0.6F1 @ k = 1

* If 0.3F1 @ k=2/3 clearance is controlling, diversity protection is usually required

Diversity (Top-to-Bottom) Path Clearance Rule

All bands 0.6F1 @ k = 4/3 0.6F1 @ k= 4/3

« Blackout » Area Main Path Clearance Rule

Above 3 Ghz N/A K = 1 grazing over a 150 ft ABL


In reality, atmosphere has an influence. It is not homogeneous.

Instead of being straight, the wave will be bent in relation to the

atmosphere's refraction index.

Air n index is written n=1+N 10-6 and is close to the unity




n0

n1n2n3n4

n5

n6

n0 sin io = n1 sin i1 = n2 sin i2 ............... = etc

n2

n1

n1 sin i1 = n2 sin i2

1 i

2

Refracted ray

Reflected rayIncident ray

n1 and n2 indexes are linked to the environment


DESCARTES SNELL's refraction lawDESCARTES SNELL's refraction law



• Propagation of a radius in an atmosphereof which index depends on the altitudeRo is the Earth's radius, that is to say 6400 km

nk

ni

nm

k

K'k

hk

1

L 1

Earth

Ro

0

h1


• With fundamental relation that rulespropagation in this kind of atmosphere

n (Ro+h) cos = Cte



• A few definitions:– Troposphere: the lower layers of the atmosphere just above the

Earth's surface in which temperature decreases with height. This portion extends from the surface up to 9km at the poles to about 17 km at the equator

– There can be temperature inversion in the troposphere– Refractive index n: ratio between wave speed in vaccuum and wave

speed in the environment consideredN refractivity = one million times the amount by which the refractive index n exceeds unitySpheric atmosphere with constant vertical gradient of

dn

dhN per km 40 /




• Empirical formula for N

T is the temperature in KELVIN (Degree in celsius + 273.15)p is the air pressure (hpa in mbar)e is water vapor pressure

p, e and t depend on the height, therefore.N depends on the height

NT

peT

77 6

4810.

( . )

dn

dhN per km 40 /




Let's use the formula (Ro+h) cos = Cte

With successive deviation, we have a relative curvature of

rays compared with Earth's surface

We suppose that the index is approximatively a linear

function of the height, therefore:

dn

dh Ro

1



dh

dnconstant


• 3 cases:

Casedndh

1 0

Casedn

dh2 0

highly positive

Casedndh

3 0

highly negative

ray path is straight

ray path is downtilted: there is a subrefraction

ray path is uptilted: there is a superrefraction




• The most frequent case is n°3 rays ondulate further than if their propagation is in straight line they have thus better clearing above the ground.It is difficult to define a project by taking into account the rays' curvature. The important element in calculation being the rays' relative curvature compared with real earth, we'll replace the real earth by a fictive one on condition that the rays' relative curvature remains constant.




Let's use a fictive earth enabling wave propagation to be in straight line

that is to say

The fictive earth's radius is given by:

using a k coefficient, we have

written:

0dh

dn

1 1

R Ro

dno

dh

kRo dno dh

1

1 ( / )

kR

Ro

8500

64004 3 1 33/ .




• We replaced real case by equivalent case in which propagation is straight

Fictive earth's radius varies with propagation in compliance with R Law = k Ro

Parameter k being defined, paragraph a) of CCIR 338-5 Rec for profile definition will apply




• In the case of super-refraction– Fictive ray is highly superior to standard ray k>1 and R>Ro and there will

be visible lowering of obstructions. This is case number 3– In the case of sub-refraction– Fictive ray is highly inferior to standard ray k<1 and R<Ro and there will

be visible raise of obstructions. This is case number 2Case number 2 is important for project definitionA curve defined by BOITHIAS and BATTESTI based on measurements carried out under continental temperate climate shows the values under which coefficient k = R/Ro does not decrease during 10-4 of the time, depending on the hop length.




Path length (km)Figure 2 - ke value exceeded during about 99.9% of the worse month

(continental temperate climate)


PropagationPropagation1.1

1

0.9

0.8

0.7

0.6

0.5

0.4

0.310 20 30 40 50 60 70 80 90 100 200






- Altimetry

2) Terrain profiles

- Propagation


- Reflection

3) GPS



4) Grounding system



ProfilesProfiles

• Terrain profiles are necessary to determine antenna heights.

• The following criteria must be observed to select the various sites of a Telecom network:

• Line of sight between them according to the respect of the clearance rules.



Path profile

• Example of the path profile, plotted with the application software recommended by Harris MCD:

Fresnel zone Line of sight

Tower 2

Tower 1

Altitude + vegetation

ProfilesProfiles



Path profile

• This figure shows the path profile with first fresnel zone and terrain

that varies with k value.

The line of sight is drawn as a straight line and the ray bending due to

variation of k value is added to the terrain elevation.

There must be 60% clearance of first Fresnel zone to avoid diffraction

loss in addition to the free space loss.

Earth bulge

• In order to draw the line of sight in a path profile, the ray bending due

to variation of the k value is added to the terrain heights.

ProfilesProfiles






- Altimetry

2) Terrain profiles

- Propagation


- Reflection

3) GPS



4) Grounding system



Ground reflectionsGround reflections

• This figure shows a typical signal reflection. The more conductive the

ground, the stronger the reflection is.

A B

Reflection point

Reflective surface

ReflectionReflection


• Reflections from sea, ponds etc … are more critical than reflections from terrain with vegetation.The reflection coefficient is dependant of the type of terrain.Generally the reflection coefficient decreases with the frequency.On the other hand a larger area is required to reflect a signal at a lower frequency.

• The effective reflection coefficient is also a function of the path's grazing angle and the curvature of the earth (the k value).Generally vertical polarization gives reduced reflection, especially at lower frequencies.




• The received signal is the combination of the direct signal and the reflected signal.




• Adding these two signals will give a signal strength that is a function of the height at the receiver site as indicated in the figure below


OptimumAntennaSeparation

Field strength

Height



• To counteract the effect of ground reflections space diversity arrangements with two receiver antennas with a vertical separation are widely used.The antenna separation should give maximum received signal level at the space antenna when the main antenna is at a minimum and vice versa vice versa.

• The optimum antenna separation may be found using one of 2 different methods.

– 1 Geometrical method using Fresnel zone– 2 Analytical method using services expansions




• 1 Geometrical methodA geometrical property of the ellipsoid is that the angle of incidence equals the reflection angle at the circumference. This property may be used to find the reflection point.


Tangent



• When fresnel ellipsoid tangents the reflection plan can be calculated.Consequently the reflection point may be found by increasing the fresnel zone until it touches the terrain. If the ellipse tangent is parallel to the terrain, there is a reflection point.








- Altimetry

2) Terrain profiles

- Propagation


- Reflection

3) GPS



4) Grounding system


• Models used by Harris MCDThe various types of GPSThe various types of GPS

LIST OF TOOLS USED DURING THE SURVEYLIST OF TOOLS USED DURING THE SURVEY

Garmin …GPS 12XLGarmin …GPSMAP 76S (WAAS)Garmin …GPS III / III Plus






- Altimetry

2) Terrain profiles

- Propagation


- Reflection

3) GPS



4) Grounding system


What is GPS

A super accurate system

• Developed and maintained by Dept. of Defense

• Nuclear subs needed positioning

• Satellite-based

• Sold Congress on the idea that other applications would follow

GPS user's guideGPS user's guide

GPSGPS


Status of GPs

• In development since 1973

• First satellite launched in 1978

• All GPS satellites built and tested

• Next generation of satellites (Block IIR) are already on contract

• Managed by the Department of Defense

GPSGPS



Navstar satellite constellationNavstar satellite constellation24 Sat6 planes20.200 km Orbit

GPSGPS



Space segment descriptionSpace segment description

• 24 satellites in final constellation– 6 planes with 55° rotation– Each plane has 4 satellites

• Very high orbit– 12,600 miles– Approximately 1 revolution in 12 hours– For accuracy– Survivability– Coverage

GPSGPS



Satellite-basedSatellite-based

Uses trilateration from satellites

• 24 satellites in final constellation– 21 operational, 3 spares

• Satellites in very high or bit (12,600 miles)– for accuracy– survivability– coverage

• Only possible with today's technology– computers and clocks

GPSGPS



Satellite-basedSatellite-based

• Weight when the satellite is lauched- 3855 kg

• Weight in final ORBIT - 816 kg

• Power - 700 w

• 2 frequency bandwiths L and S

• S1 2227,5 MHz

• S2 1783,74 MHz

• L1 (1575 MHz)

• L2 (1227 MHz)

• L1 an L2 are generated by a Ref frequency 10,23 MHz given by a ATOMIC reference clock (cesium)

GPSGPS



GPS segmentsGPS segments

MASTER

CONTROL SEGMENT

CONTROLSTATIONS

SPACE SEGMENT

USER SEGMENT

GPSGPS



How accurate is it ?How accurate is it ?

That depends:

• Depends on some variables– Time spent on measurements– Design of receiver– Relative positions of satellites

• Sub-centimeter accuracies from survey products

• Fifteen to fifty meters with non-differential GPS

• One to five meters with differential GPS

• Gouvt. can degrade accuracy if they want to

GPSGPS



How does GPS work ?How does GPS work ?

2GPS measures distancefrom the satellitesusing speed of light.

3To measure the distanceGPS needs good clocksand a fourth SV

4Once GPS knows distance,it needs to know satellite'sposition

5Then correct forionospheric andtropospheric delays.

1 Trilateration from satellitesis basis of system

GPS receiver

GPSGPS



1 trilateration from satellites1 trilateration from satellites

• By measuring distance from several satellites you can calculate your position thru mathematics

GPSGPS



TrilaterationTrilateration

• One measurement narrows down our position to the surface of a sphere

11,000 miles• We're somewhere on the surface of this sphere.

GPSGPS




• Second measurement narrows it down to intersection of two spheres

11,000 Miles

12,000 Miles

Intersection of twospheres is a circle

GPSGPS




• Third measurement narrows to just two points

Intersection of threespheres is only twopoints.

GPSGPS




• Fourth measurement will decide between two points.

Fourth measurementwill only go throughone of the two points.

GPSGPS




• In practice 3 measurements are enough

• We can discard one point

• One point will be a ridiculous answer– Out in space– Or moving at high speed

• We still need the 4th measurement because there are four dimensions to solve for (X,Y,Z and Time)

GPSGPS



2 satellite ranging2 satellite ranging

Measuring the distance to a satellite

• Done by measuring travel time of radio signals

GPSGPS



Speed-of-light measurementSpeed-of-light measurement

Measure how long it takes the GPS signal to get to us

• Multiply that time by 186,000 miles/sec– Time (sec) x 186,000 = miles

• If you've got good clocks, all you need to know is exactly when signal left satellite

GPSGPS



GPS community base stationGPS community base stationGPS community base stationGPS community base station

Mounting Pole*

PC*

Data/PowerCable

GPS Receiver

120V Uninterruptible PowerSupply *

* Supplied by Customer

120VAC to12VDCPower Supply

GPS Antenna

Antenna Cable

GPSGPS



Line of sight

Why we use satellites for mappingWhy we use satellites for mappingWhy we use satellites for mappingWhy we use satellites for mapping

TRIMBLE NAVIGATION

GPSGPS



PDOPPDOPPDOPPDOP

PDOP BAD PDOP Good

GPSGPS



Dilution of precision (DOP)Dilution of precision (DOP)

Can be expressed in different dimensions

• GDOP - Geometric dilution of precision

• PDOP - Position dilution of precision

• HDOP - Horizontal dilution of precision

• VDOP - Vertical dilution of precision

• EDOP - East dilution of precision

• NDOP - North dilution of precision

• TDOP - Time dilution of precision– GDOP² = PDOP² + TDOP²– PDOP² = HDOP² + VDOP²– HDOP² = EDOP² + NDOP²

GPSGPS



Altitude referenceAltitude reference

• Ellipsoid– A smooth, mathematically defined model of the earth's surface

• Geoid– A surface of equal gravitational pull (equipotential) best fitting the average

sea surface over the whole globe

Geoid

MSLHAE

Earths Surface

Ellipsoid

GPSGPS



DatumDatum

• There are many regional datums that are chosen so that the ellipsoid could conform as closely as possible to the geoid over the region rather than the whole globe.

EllipsoidfittingNorthAmerica

EllipsoidfittingEurope

Geoid

GPSGPS



DatumDatum

• A datum is a specifically oriented reference ellipsoid defined by 8 elements

– Position of the network (3 elements)– Orientation of the network (3 elements)– Parameters of the reference ellipsoid (2 elements)

GPSGPS



Datum (WGS 84)Datum (WGS 84)Datum (WGS 84)Datum (WGS 84)

GPSGPS



Datum (NAD 27)Datum (NAD 27)Datum (NAD 27)Datum (NAD 27)

GPSGPS



DatumDatum

• One point can have different sets of coordinates depending on the datum used.

X

GPSGPS



Projection typesProjection typesProjection typesProjection types

Reference to WGS72Designation Ellipsoid Diameter 1/f

y x zZone

WGS72 6378135 298.26 0 0 0 World

Europ 50.ED Mayford 1924 6378388 297 103 84 127 Europe

NAD27 Clarke 1866 6378206 294.98 -157 22 -176 USA

IGN (NTF) Clarke 1880 6378249 293.47 66 170 -311 France – NorthAfrica

Wake-Eniwetok1960

Hough 6378270 297 -68-62-62

-112-121-144

442238

KwajaleinWakeEnnwetok

Guam 1963 Clarke 1866 6378206 94.98 235 89 -254 Iles Marianes

Arc 1950 (CAPE) Clarke 1880 6378249 293.465 131 129 292 South Africa

Adindan Clarke 1880 6378249 293.47 26 152 -212 Egypt

GPSGPS



2d versus 3d data2d versus 3d data

• 3d needs 4 SV's (X, Y, Z and Time)

• 2d needs 3 SV'S (X, Y, Time and user entered Z)

GPSGPS



2d versus 3d data (Contd.)2d versus 3d data (Contd.)

Inputting a poor elevation will give a poor horizontal position.

Line of position

GPSGPS



2d versus 3d data2d versus 3d data

Inputting the correct elevation will result in the correct position.

Line of position

Correct Latitude

Correct Elev.

GPSGPS



2d versus 3d data (Contd.)2d versus 3d data (Contd.)

Inputing the wrong elevation will result in the wrong position.

Line of position

Wrong Latitude

Wrong Elev.

GPSGPS







- Altimetry

2) Terrain profiles

- Propagation


- Reflection

3) GPS



4) Grounding system


• Roles of the grounding systemWhat is it used for ?The only use of a ground system is to handle into the ground

the currents entering or leaving the location in common mode.

A ground system is only a "waste receptacle" but shall be a good quality system.

• It ensures:1) electrical protection / lightning2) technical quality

GROUNDING SYSTEMGROUNDING SYSTEM


• Protective ground– The primary role of a ground connection is to protect people against

electrocutions risks. Electric shock risk depends on the strength of the electric current flowing through the body, and on the part of the human body touched.

– Human body resistance is not linear.1mA current is hardly detected by the hands. 10mA current causes strong shock and a 30mA current can tetanize muscles and cause heart fibrillation.

– Rules for protection against electric shocks can not take into account variable resistance of human body. They reduce contact voltage to conventional protection value in order to prevent fatal shock.

– The concept of protective ground is not standardized. It's important to know that it's the correct ground equipotentiality which protects installation, and not the grounding system.

– Manhole bond around residential housing is more efficient than a single ground post, whatever its resistance.



Equipment leakage current

• Leakage currents are handled via grounding conductors grounding conductor is conventionally in green/yellow color. It connects equipment chassis to the ground system but current doesn't flow to the ground "fault currents" are closed by connecting neutral conductor to the ground, not in the ground. Grounding is strictly conventional. We may think that the role of a ground connector is to dry off the leakage current but it's no true


Conductor current doesn't flow into the groundUnavoidable leakage currents are looped byneutral's grounding conductor

leakage

230V

PHASE

NEUTRAL

Ground conductor



• A leakage current of some ten ampères is normal for large computer rooms

• Since leakage and fault currents are internal currents, they don't flow through the ground. Ground resistance is therefore different




• Static potential referent– Some moving vehicules isolated from the ground (trucks, planes ...) are

charged compared to the ground (i.e dry dusty wind). Electric charges carried by airborne particles settle on the vehicule of which potential difference compared to the ground can reach tens of kilovolts!During fuel tank filling, a spark may inflame vapors if the vehicule is not discharged first. Only a connector used to discharge a moving vehicule can called "protective ground" without making a mistake

i=xya

R<10k

Rising sand




• Ground connection is not used as protection it is the equipotentiality between chassis which is taken into account. It does not dry off the leakage currents (except for HV in TT configuration)

• A simple post can discharge an isolated moving vehicule. External currents, including lightning current, are dried off both through the ground and through other external cables.

• Ground resistance is not important in cable protection




• For people as well as for equipment, the risk lies under too important potential differences between near points. The most important thing is for people the equipotentiality between chassis simultaneously accessible and for equipment, the equipotentiality between interconnected equipment.

• The most important for equipment operation is the location equipotentiality



Ground resistance measurement

• Ground connection quality is measured by its resistance.

• Ground resistance is applicable only with low frequencies. Beyond several MHz (frequency range in which electronic systems are very sensitive), ground connector impedance can no longer be measured and has physically no sense any more.

• All the ground connectors on a single location shall be interconnected.



Importance of ground resistance measurement

• When the absolute value of ground resistance is unchanged, it's evolution in time is interesting (if the value falls, it means ground cables are deteriorating). Ground network resetting may be required.

STORYAutomatic switch's ground resistance value = the infinity war bomb had cut off the grounding conductor but no one had noticed it in the system operation for half a century



Ground resistivity

• Ground resistivity is measured via 4-wire ground current meter. The unit of measurement is the Ohm.meter

Resistivity < 300 is correct Resistivity > 500 is bad

• Contrary to ground network resistivity, ground resistivity is very important– Resistivity of soil layers varies significantly and ground network

conductors should preferably be burried at low resistance depth.– In soils with high resistivity, meshed ground networks should be made of

small size meshes for good horizontal equipotentiality.



Ground resistivity

• With average ground resistivity and the geometry of underground conductors, it is possible to assess the ground network resistance.

• It is not very important but evaluating the resistance with calculation and validating it with measurement may put the customer's mind at ease



Vertical post or horizontal cable ?Underground conductor may be rammed in vertically (post) or burried horizontally (manhole bond or bridles)Horizontal conductors are better for the location equipotentiality

man-hole

Ground post

min.2m

Building

Implementation of a ground network



Implementation of a ground network

• Horizontal conductors should be spaced by at least 20 cm from other metal cables to reduce corrosion rate (use preferably 50 mm² section copper or 35 mm² flat copper cable). Depth of lay: 1m. The trench will be refilled with low resistivity arable soil, but not with crusher - run stones. Underground cable connections should be brazed and welded. Underground network should be meshed



Raft foundation

10m

Bridle Bridle

Bridle Bridle

Manhole bond

Example of meshing



• 3 mistakes to be avoided

– Low ground resistance requirement• Only location equipotentiality is important, or at least the equipotentiality of

interconnected equipment (only the ground system of VHV station shall have low impedance rate) let's not spend money in ground resistance reduction

• Separated ground systems• They break the equipotentiality principle

– Star connection of chassis to the ground connector• Meshing is the only solution allowing chassis current division and equipotentiality

improvement


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Documents

Geography of Microwave Survey-20080226-A