تأريض2

التأريض
الدكتور المهندس محمد منذر القادري
munthear@gmail.com
11/22/2014 1

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ما الحاجة إلى نظام التأريض و لماذا نستخدمه ؟
إن أهم أسباب تأريض نظام كهربائي هي :
1 - حماية العاملين من أخطار الصدمة الكهربائية في حالة حدوث عطل ما .
2 - إيجاد وسيلة أو ممر لتيارات التسريب الكهربائي إلى الأرض .
3 - ضمان دقة عمل الأجهزة الكهربائية و سلامة استخدامها .

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التأريض الوقائي
1 - منع جهد جسم الآلة من الارتفاع إلى قيمة قد تشكل خطرا
على الإنسان.
2 - السماح بمرور تيار إلى الأرض له قيمة كافية لتشغيل أجهزة
الوقائية )فيوزات وقواطع( أو الأجهزة الخاصة بكشف أي خطأ
للأرض.
3 - إيجاد مسار سهل وقادر على تحمل تيارات ضخمة.
 القيمة المحددة في المواصفات بين 1 و 25 أوم.

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التأريض
 تأريض المنظومات. Working earth
 التأريض الوقائي protective earth
Lightning earth

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GROUNDING ضيرأتلا
حمل حساس
SENSITIVE LOAD

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قياس مقاومة الأرضي لوتد وحيد
d
L
L
R
3
ln
2


d
L
إن مقاومة الأرضي لا تتغير
كثير بتغير قطر الوتد بعد قيمة
معينة
عادة ما يتراوح قطر الوتد بين
25mm 16 و mm

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مقاومية التربة
أهم العوامل التي تعتمد عليها مقاومية التربة :
1 - نوع التربة.
2 - نسبة الرطوبة.
3 - نوع الأملاح المذابة ودرجة تركيزها )التركيب الكيميائي(.
4 - درجة حرارة التربة.
5 - حجم الحبيبات.

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المقاومة النوعية للتربة
 الحدود التقريبية للمقاومة النوعية لأنواع مختلفة من الأتربة:
مياه البحر 0.3 أوم . متر
طين 5 - 50 أوم .متر
خليط من طين ورمل 40 – 250 أوم .متر
رمل وحصى 60 - 100 أوم .متر
حجر رملي 10 - 500 أوم .متر
صخور بلورية 200 - 10000 أوم .متر

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تأثير الرطوبة
1000
2000
3000
4000
10
20
30
40
50
مستوى الرطوبة %
المقاومة أوم متر
تربة طينية

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العلاقة بين طول القضيب ومقاومة الأرضي
10
20
30
40
3
6
9
12
طول القضيب )متر(
المقاومة أوم
15
50
60
70

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التأريض باستخدام عدة قضبان

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تأثير المساحة على مقاومة الأرضي
2
4
6
8
100
200
300
400
500
المساحة )متر مربع(
المقاومة )أوم(
10

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تأثير طول السلك على مقاومة الأرضي
2
4
6
8
20
40
60
80
100
طول السلك )متر(
المقاومة )أوم(
10
تغير المقاومة مع طول
سلك قطره 2.5cm
ومدفون على عمق
60cm في تربة
مقاوميتها
100hom.m

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استخدام الألواح أو الشبكات كإلكترودات تأريض

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تأثير التشكيلات المختلفة على مقاومة الأرضي
أطول الأذرع
) متر(
60
43
30
30
30
30
التشكيل المقاومة
)أوم(
 سلك مستقيم 2.9
 زاوية حادة 2.25
 نجمة ذات 3 أذرع 2.2
مقاومة الأرضي للتشكيلات
المختلفة لسلك قطره 2.5cm
مدفون على عمق 90cm في
مساحة قطرها 60m
ومقاوميتها 100 ohm.m

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معالجة التربة
50 سم
30 سم
30 سم
ملح كبريتات المغنسيوم أو
ملح كلوريد الصوديوم

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قياس مقاومية التربة
L L L
L /20
v
S
  (V I )2L
I
wenner طريقة

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قياس مقاومة الأرضي
v
S
R V I
H
D
I
H =0.618 D
E P C
خمسة أضعاف قطر الأرضي E عن P يبعد
المتعدد الأوتاد

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طرق التأريض
هناك ثلاثة طرق للتأريض :
1 - التأريض التسلسلي أحادي النقطة .
2 - لتأريض المتوازي أحادي النقطة .
3 - التأريض المتوازي متعدد النقاط .

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تأريض النقطة الوحيدة
Signal CurrentLoop Control?
Signal CurrentLoop Control?
1A. Series
1B. Parallel
1
2
3
1
2
3
I1
I2
I3
I3
I2+I3
I1+ I2+I3

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Schematically Represented
The ground potential for each load at the connection point is thesame asthey are connected to the same common ground point.

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¥ The grounding of this circuit is wired in series and can beschematically shown as the following:
R 1, R2, R3 is the resistance of the conductor connectingthe loads to ground.
Any ground current flowing from the loads causes anincrease in potential along the ground wire connecting the loads.
Ground potential at points A, B and C by Ohms law is:
Potential A =(i1+i2+i3)xRl
Potential B = Potential A + (i2 +i3 x R2)
Potential C = Potential B + i3 x R3

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تأريض دا ا رت الإشارة
هناك طريقتان لتعريف الأرضي :
أولهما وفقاً لمبدأ " الكمون المتساوي"
ثانيهما وفقا لمبدأ " ممر التيار " .

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أرضي الأمان

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Grounding
¥ Grounding is one of the most important aspects of an electricaldistribution system but often the least understood.
¥ In Ontario the Electrical Safety Code sets out the legalrequirements for all electrical installations.
¥ Section 10 of the Electrical Safety Code deals with the variousaspects of grounding.
¥ The object of grounding is given in section 10-002:10-002
Object. Grounding as required by this Code shall be done insuch a manner as to efficiently serve the following purposes:
(a) To protect life from the danger of electric shock, and propertyfrom damage; and
(b) To limit the voltage upon a circuit when exposed to highervoltages than that for which the circuit is designed; and
(c) In general to limit ac circuit voltage-to-ground to 150 V orless on circuits supplying interior wiring systems; and
(d) To facilitate the operation of electrical apparatus andsystems; and
(e) To limit the voltage on a circuit which might otherwise occurthrough exposure to lightning.
¥ In order to serve the above, effective earth grounding andequipment grounding is required.

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Earth Grounding
¥ Earth grounding establishes the zero voltage reference for anelectrical distribution system and provides protection to the
electrical system and equipment from superimposed voltagesfrom lightning and contact with higher voltage systems. Theearthground also prevents the build-up of potentially dangerous staticcharge in a building.
¥ The grounding electrode is most commonly a continuouselectrically conductive underground water pipe running from thepremises. Where this is not available the Safety Code describesother acceptable grounding electrodes in section 10-700.
¥ Grounding resistances as low as reasonably achievable willimprove the grounding system performance of the electricaldistribution system and will give improved protectiontopersonnel.
¥ Connection of the electrical distribution system to the earthgrounding occurs at the service entrance.
¥ The neutral of the distribution system is connected to ground atthe service entrance.
¥ The neutral and ground are also connected together at thesecondary of a transformer in the distribution system.
¥ Systems must be grounded if, when grounded, their maximumvoltage-to-ground does not exceed 150v, i.e.120/208v,120/240vcircuits or if the system incorporates a neutral.
¥ The above are the only places where the neutral is connected tothe ground conductor and any other connections, eitherintentional or unintentional, can be dangerous and can lead topower quality problems.

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Equipment Bondingتقييد الأجهزة
¥ Equipment bonding effectively interconnects all non-currentcarrying conductive surfaces such as equipment enclosures,raceways and conduits to earth ground.
¥ The purpose of equipment bonding is:
1) To minimize voltages on electrical equipment, thus providingprotection from shock and electrocution to personnel in contact with the equipment.
2) To provide a low impedance path of ample current carryingcapability to insure the rapid operation of overcurrent devicesunder ground fault conditions.
¥ These two points are directed toward personnel safety and canbe illustrated as follows:
¥ If the equipment were properly bonded and grounded theequipment enclosure would present no shock hazard and the ground fault current would effectively operate the overcurrentdevice.

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Equipment Without Proper Equipment Bonding

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Equipment With Proper Equipment Bonding

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تأريض الإشارة Signal Grounding
¥ Signal grounding provides a common reference point for circuitsof an electronic system. It is not intended to carry fault current orprotect equipment or personnel.
¥ Because safety grounds and signal grounds perform quitedifferent functions they are ideally independent except at onepoint where they are interconnected for safety reasons.
¥ The AC power ground is a generally poor signal ground becausethe potential difference at points on the power ground is oftenseveral hundred millivolts or even several volts and can be quitenoisy. Also the AC power ground is generally of low impedanceonly at low frequencies whereas the signal ground system oftenoperates with signals in the tens of megahertz range and evenhigher.
¥ If the signal ground system is connected to the AC power groundsystem at more than one point, a ground loop is created andnoise from the AC power ground system can be coupled into thesignal ground system and cause interference. With computerequipment this interference can cause errors in datatransmission and read/write errors ondisk drives in addition tomany other problems that are often thought of as hardware orsoftware problems.
¥ The signal grounding system within a piece of equipment is anintegral part of its design.
¥ The signal ground is sometimes connected to the equipmentground within the equipment and is connected to the AC powersupply ground when its power supply is connected.

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¥ This arrangement is fine when only one piece of equipment isused, but when interconnected with other equipment the ACpower ground is relied upon for the signal ground path. This maycause operational problems for reasons already mentioned.
¥ The signal ground and safety ground connection to a piece ofequipment are often separate. With multiple equipmentinstallations the signal grounds of the various pieces ofequipment should be joined together at a single common groundpoint. It is at this common ground point that the signal ground isconnected to the AC power ground to fulfill safety requirements.
With this arrangement the possibility of AC power ground noisebeing coupled into the signal ground system is greatly reduced.
¥ Often it is not practical or possible to separate the signal andsafety ground systems for all equipment in an installation so theAC power system ground is relied upon for the signal groundingfunction. Whether or not this is the case there are sometechniques that should be used in the AC power groundingsystem that will improve its performance when supplyingsensitive loads.

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تأريض الأحمال الحساسة
¥ The neutral/ground connection at the service entrance providesthe ground reference point for the distribution system.
¥ Circuits are grounded to this point via the site distribution systemwiring.
¥ Depending on how the system is wired, it will form a series or aparallel connected ground system.
¥ To illustrate this, the following shows how a number of loads maybe connected to a single circuit.
أحمال متصلة على التسلسل

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¥ It can be seen that this system does not provide an equipotentialground to the loads.
¥ This usually provides no problem to most loads and is the usualway to wire circuits. However, this may pose problems tosomesensitive equipment operating with other equipment at a slightlydifferent groundpotential.
¥ In order to provide an equipotential ground, the circuitsshould bewired in parallel so they share a common ground point.
¥ With parallel grounding, any ground current arising from a loadwill not affect the ground potential of other loads to the sameextent.
¥ This method costs more because it requires more wire and hashigher installation costs. It is not practical to wire an entire sitethis way, but wiring sensitive circuits in this manner is practical.

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¥ When computer equipment is located in one room, all equipmentcould be grounded at one central point where the electricalsupply enters the room. This ensures that common equipment inthe room have a common ground connection at the samepotential.
Common Grounding Point for Equipment

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مخارج التأريض المعزولة
¥ An extension of the above grounding method is the isolatedground outlet.
¥ An isolated ground (IG) outlet as recognized by the ElectricalSafety Code is a receptacle, orange in color or with an orangetriangle and marked "Isolated Ground", that is wired as anindividual branch circuit outlet. See Figure 4-16 (pg. 31).
¥ This outlet has a separate green or green/yellow wire which runscontinuously from the ground conductor terminal to the firstpanelboard where it is connected to the ground bus.
¥ Bonding of the conduit, boxes, etc. of the circuit is accomplishedby ordinary means, i.e., conduit or a separate ground wire. Thetwo grounds are connected only at the panelboard.
¥ The advantage of this arrangement is that any ground currentarising from equipment other than the IG supplied equipment willnot cause as severe a potential rise on the ground to the IGsupplied equipment.

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ملاحظات حول مخارج الأرضي المعزول
¥ The outlet is grounded to the same ground as the electricaldistribution system. The only difference is that it isconnected to ground via a separate wire. The ElectricalSafety Code allows only one earthing ground.
¥ The orange isolated ground outlet is the only recognized isolatedground outlet.
¥ Isolated grounds cannot extend from equipment other thanisolated ground receptacles.
¥ The National Electrical Code in the United States allows thegreen ground wire of an isolated ground outlet to pass through apanelboard to be grounded at the main service panelboard. Thisis not allowed by the Canadian Electrical Code.
¥ The isolated ground wire must be terminated at the ground busof the first encountered panelboard.

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إعتبارات تأريض التردد العالي
¥ Equipment grounding and the grounding of the electricaldistribution system provide a low impedance path to ground forlow frequencies (mainly 60 Hz).
¥ Computers and microprocessor controlled equipment operate athigh frequencies (often over 10 MHz) and transfer data betweenvarious pieces of equipment at high frequencies and low signallevels. Because of this, they are very susceptible to electricalnoise and interference.
¥ Noise which can affect the operation of sensitive equipment canrange in frequency from dc to the GHz range. In order to operatecorrectly, these types of equipment need an effective means ofgrounding for both low and high frequencies.

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¥ Wiring systems used within a building generally have lowimpedance at low frequency, but as the frequency increases theimpedance increases.
¥ "Real" wiring can be modeled by a wire with resistance andinductance and stray capacitance to ground distributed along itslength.
¥ For a grounding conductor to be effective at high frequencies itmust be short to minimize the effects of stray capacitance anddistributed inductance along its length.
¥ A rule of thumb is that the conductor should be shorter than1/20th of the wavelength at that frequency. This means a lengthshorter than 1.4 m at 10 MHz.
¥ The single point, parallel path, ground which made for a goodequipment ground makes for a poor highfrequency signalground.
¥ In order to satisfy both equipment grounding and signalgrounding requirements, a hybrid system should be employedThis system is a combination of the parallel path single pointground combined with a multipoint ground for good highfrequency performance.
¥ One such method, described in FIPS Publication 94, is a zerosignal reference grid.
تأثيرات التردد على النواقل

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الدارة المكافئة للنواقل
Equivalent Circuit of a Wire

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Zero Signal Reference Grid
¥ A ground plane is a conducting surface that has low impedanceat high frequency. The ideal situation would be to have allcomputer equipment located on a ground plane so shortconnections could be made from the equipment to the plane.
¥ While it is not practical to have a true ground plane, an effectivealternative is a grid of conductors spaced on approximately 50cmcenters, bonded at their intersections on the subfloor of theroom's raised floor.
¥ The grid is grounded to the electrical system ground at the pointwhere the supply enters the room. All powered equipment is alsogrounded at this point making the equipment grounding a singlepoint system.
¥ Equipment is bonded to the reference grid via short conductorscreating a low impedance path to ground for high frequencies.
¥ This hybrid system satisfies both equipment and high frequencygrounding requirements and complies with the Electrical SafetyCode.

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Zero Signal Reference Grid

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Predicted RadiatedEmissions, PCB (based on EMCad)
Emissions
dBuV/m
Limit
Frequency, Hz

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Microstrip, for stripline deduct 14 dB
Amount of case shielding needed to contain the PCB emissions
Maximum single aperture for –14 dB at 10 GHz or 0 dB at 5 GHz is about 1.2 inches
Maximum single aperture for 36 dB at 800 MHz is 0.12 inches (Approximately 1/8 inch)
Based on 1”yielding 0 dB at 6 GHz.
Predicted Radiated Emissions, PCB (based on EMCad) Assumptions

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Typical DVI Commercial Cable

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DVI MIL Cable Twinax vs. Quadax

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DVI Signals from coax
Limit
dB uV/m
Emissions
Frequency, Hz

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RE102 Emissions from 200 MHz

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RS103 Hardening requirements for Firewire
Threshold, dBuV
Frequency, Hz

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RE102 vs. RS103
Above about 1 GHz, more cable shielding needed for RS103 than RE102
RS103 > 27 dB @ 200 V/m
EME ship > 63 dB @ 13,000 V/m (3- 4 GHz)

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DVI Radiated Emission Analysis– Based on EMCadAnalysis –SE # 2, Cable Required, Micro-Coax model UFF092F (100 dB @ 1 GHz)
Center Conductor
silver-plated copper wire
dielectric constant 1.4 to 1.7
Silver-plated copper tape –Inner shield
Silver-plated copper wire –outer shield
100 dB @ 1 GHz

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GROUNDING/REFERENCING ةيعجرملاو ضيرأتلا
A good ground system must be designed, not left to chance.
Digital systems because of their inherent high threshold levels (>100 mV) may be chassis referenced at both ends.
Analog systems that have operational bandwidths 0 -10 MHz or less benefit from circuit isolation from chassis.
Grounding schemes must maintain case shielding integrity.
The best approach is a hard reference to chassis at each unit with the interlacing I/O cable becoming the circuit isolator between same.
The exception to this rule would be for analog or signaling circuits which cannot be easily isolated by a common inductance between the center conductor and its return.
However, leaving a ground system to be developed by each design entity is not recommended.

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SINGLE POINT OR REFERENCE CONCEPT
مفهوم النقطة الوحيدة أو المرجعية
Single point grounding
cannot exist
No dimensions,
therefore no way
to terminate wires
together
Ground reference area
of diameter “D” I x Zd= V
that is below all
circuit threshold valuesreferenced to ground.
Point
D
BW = 
1
tr
300 MHz
trnsecs
V
Zd
ITotal

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A MINIMUM OF THREE SEPARATE GROUNDS SHOULD BE USED FOR LOW FREQUENCY CIRCUITS (<1 MHz)
* AC power ground (green wire) should be connected tohardware ground (within 1.5 inches of case entry)
Signal
Ground
Noisy
Ground
Hardware
Ground*
distributed capacitance between “ground” leads
distributed mutual inductance between “ground” leads

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تأريض النقطة الوحيدة
Loop Control?
1A. Series
1
2
3
Loop Control?
1B. Parallel
1
2
3

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POOR SINGLE REFERENCE GROUNDINGBECAUSE OF LARGE EXPOSED LOOP AREAS
Input
Signals
Input
Signal
Conversion
and
Filtering
Conditioned
Signal
Control
Logic
Logic
Ground
Output
Signals
Output Ground
To
DC
Input
No Decoupling
of Logic Ground
at Port of Exit
Logic
Packaging
Ground
To
Cabinet
AC Power Line
SGP Power Supply Ground
AC Power
Filter
Logic Power
Supply
DC Output
Power Supply
Filtered Power
Output
Drivers

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POOR SINGLE REFERENCE GROUNDING
This viewgraph was taken from literature and was titled "The Proper Way to Ground".
One can see that the conditioned signals transferring from the input conversion to the control logic could not possibly be transmitted if the only signal return was through the single ground point (SGP).
This is because each of the 3 lines shown would form 3 concentric loop areas that would crosstalk.
The AC powerline filter needs its chassis to be directly grounded to minimize the inductance in series with the line to ground filter capacitors.

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PROPER APPLICATION OFTRANSMISSION LINES TO SINGLE-ENDEDCIRCUITS
Bad
No Shield
Large Enclosed Area
Preferred
Shield Area
Both Ends of Chassis
Grounded -Significant
Reduction in Loop Area
Bad
Shield Added
One End Chassis
Grounded -No
Reduction in
Loop Area

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تأثير التحجيب SHIELD على مساحة الحلقة
VS
RL
I
VS
RL
I
VS
RL
I
f<
Rs
2Ls
VS
RL
I
f>
5Rs
2Ls
R
I1
Is = I1
No GroundConnections
Shield Added
Two Ends Grounded
Large Area
Shield Added
One End Grounded
Large Enclosed Area
No Shield
Large Enclosed Area
Circuit Not Grounded
At One End
Reduced Area
Shield Added
Two Ends Ground
Reduced Area
Notes:
Rs = Shield Resistance
Ls = Self Inductance of Shield
f (cut-off) =
or 2f =
c =
Rs
2Ls
Rs
Ls
Rs
Ls

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CABLE
IMPEDANCE
()
CUT-OFF
FREQUENCY
(kHz)
FIVE TIMES
CUT-OFF FREQUENCY
(kHz) REMARKS
Coaxial Cable
RG-6A
RG-213
RG-214
RG-62A
RG-59C
RG-58C
75
50
50
93
75
50
0.6
0.7
0.7
1.5
1.6
2.0
3.0
3.5
3.5
7.5
8.0
10.0
Double Shielded
Double Shielded
Shielded Twisted Pair
754E
24Ga.
22Ga.
1
125
---
---
0.8
2.2
7.0
4.0
11.0
35.0
Double Shielded
Aluminum-Foil
Shield
Shielded Single
24Ga. --- 4.0 20.0
1 One pair out of an 11 pair cable (Belden 8775).
From: “Noise Reduction Techniques in Electronic Systems”, by H.W. Ott, 2nd ed. (1988) by Bell
Telephone Laboratories Inc., Reprinted by permission of John Wiley & Sons, Inc.
MEASURED VALUES OF SHIELD CUT-OFF FREQUENCY (FC)

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تجزئ التيار بين التحجيب وصفيحة التأريض
Subscript SH= Shield
Mesh Equation Inside Loop
For >> c
0 = IS(jLS+ RL) -ISH(jm)
IS= ISH( ) = ( ) ISH
At >> call current returns on shield
At <<call current returns through ground
j
j+ RS/LS
j
j+ c
IS
ISH
IS
IG
B
A
IG= IS-ISH
c=
RL
LS
IS
RL
ISH
IG
LS
m
B
A

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حلقة التأريض
If shielded current is allowed to flow and the shield is one of the signal conductors, a noise voltage will be generated due to the I-R drop in the shield.
V12= -jMIS+ jLSIS+ RS IS
Since M = LS(self inductance)
Then V12= RSIS
Therefore: Avoid shield being a signal conductor1. Use shielded twisted pair2. Use triaxial cables3. At high frequencies (>1 MHz) coax acts as triax due to skin effects
}Low frequency
RS
Shield
Center Conductor
M
V
CRT
Vs
R
1
2
IS

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مخططات التأريض
 تصنف إلى :
AC power grounding:safety
DC power distribution:3.3V, +5, + 12V, etc.
Signal reference
•Analog
•Digital
•Discrete
•Video
AC power green/yellow isolated from DC by transformer
DC power isolated from signal in transmission bandwidth by transmission line effect of interconnect cable (i.e., coax, twisted pair, tri-lead, GSG ribbon cable, or 3M style ground plane ribbon cable

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الكبل المحوري COAX كمخمد لحلقة التأريض
Common made currentlimited by mutualinductance of COAX
2nd unit PCBPower Distribution
Coax CableIsolationof Signals
DC Power
PC BoardPower
Distribution
/ /
/ /
/ /
/ /

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الكبل المحوري COAX كمخمد لحلقة التأريض
This is a schematic of the isolation achievable through the signal cable.
The common mode current IC is inhibited by the mutual inductance, hence the isolation quality of the coax signal cable.

11/22/2014 75
ممانعة سطح الأرضي
Total Impedance Consists of:
1.DC resistance
2.Inductance effect
3.Skin effect
4.Standing wave impedance(transmission line effect)
Copper
Area = A
B
t

11/22/2014 76
إستنتاجات
 Single reference ground is the only
acceptable scheme
 This creates apparent ground loop problems
 Isolation is required if ground loop is
excessive
 All I/O cables routed between shielded
equipment enclosures must have outer
shield grounded at both ends preferably
symmetrically
.001 f .001 f
Symetric Case Shielded Cable
AC shield
termination

11/22/2014 77
مخططات العزل
Differential Circuits
Optic Couplers
Isolation Transformers
Coax Cables and/or Twisted Shielded Pair
Fiber Optics
Except for the last two schemes, the above concepts do not work for controlling emissions or rejecting ESD and E-Field radiation.

11/22/2014 78
القيادة التفاضلية
Good to about 5 MHz (i.e. >20dB).
Cause of high frequency common reduction is:
1.Unbalance in common mode Z in high frequency.
2.Unbalance in connectors.
3.Unbalance in twisted pairs in internal route.
4.Unbalance in PC board traces.
5.Unbalance in twisted shielded pairs.
Radiation from differential drivers is 2X single ended drivers (line to ground).
Major external threat is 5-200 MHz range where common mode isolation is least.
Line Z 1
Line Z 2
V2
V1
Frequency
Common Mode Reject.
100 kHz
~ 80 dB
~ 10 dB to
0 dB
H
L

11/22/2014 79
العزل الضوئي
Mutual
Couple
1.Isolation limited by harness and connector coupling
2.To work without external cable shield they need to be located within 1.5" of case entry
3.If used, then need separate PC board connectors
Re-radiated
E-Field
To motherboard
connector

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