Download - EE4503 Electrical Systems Design
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EE4503Electrical Systems Design
Grounding
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Topics
• Introduction to Grounding
• Grounding of Electrical Systems
• Grounding Systems
• Grounding Resistant
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Grounding
3
• Grounding or Earthing
• Make the potential difference become the same as ground/earth.
• For human safety and equipment safety
• Follow NEC Article 250
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GroundingCategory
4
• System Grounding: connect the “live” portion of the electrical system to ground, e.g. neutral
• Equipment Grounding: connect a part (having no current) of the equipment to ground, e.g. case
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GroundingComponents
5
• Grounding Electrode (System)
• Grounding Electrode Conductor
• Grounded Conductor
• Main Bonding Jumper
• Equipment Grounding Conductor
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GroundingComponents
6
Electrical Systems Design: ประสทิธิ ์พทิยพฒัน์
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GroundingComponents
7
http://cr4.globalspec.com/thread/40145/Grounded-Neutral
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GroundingCommon Grounding Systems
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• System Grounding
• Service Equipment Grounding
• Separately Derived System
• Equipment Grounding
• Computer Grounding
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GroundingAC System Grounding
9
“live” portions of the electrical system
• Protect Overvoltage: specially from lightning
• Stabilize voltage level referring to ground
• Be part of protection circuit: be part of the short-circuit current
50V, 50 to 1kV, more than 1kV
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GroundingAC System Grounding – 50V
10
• NEC Standard
• The source– is stepped down from more than 150V
– is stepped down from the ungrounded system
• Cable in over the ground outside the building
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GroundingAC System Grounding – 50 to 1kV
11
• Usually at the distributed transformer
• Neutral is grounded
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GroundingAC System Grounding – Electrode
12
• Grounding Electrode Conductor
• Connected to three parts– Grounded Conductor: usually neutral line
– Equipment Grounding Conductor
– Main Bonding Jumper
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GroundingAC System Grounding – Electrode
13
• Cable: continuous copper cable with insulator
• Installation: connect to a surface, recommend conduit
• EMC: metal component needed to be connect to ground
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GroundingAC System Grounding – Electrode
14
• Cable: continuous copper cable with insulator
• Installation: connect to a surface, conduit (recommended)
• EMC: metal component needed to be connect to ground
• Connection: Exothermic Welding, or other recommended methods
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GroundingAC System Grounding – Exothermic Welding
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http://www.youtube.com/watch?v=CqixbTQG13M
AllTec Corp Exothermic Weld.flv
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GroundingAC System Grounding – Cable Size
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http://www.kroo-suchat.com/File_data/ground.pdf
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GroundingAC System Grounding – Cable Size
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http://irrigation.rid.go.th/rid17/Myweb/machanical/type%20lamp.html
Meter(A) MCB (A)
Cable Sizing (sq.mm.)
Voltage (V)
Incoming Cable in Air (Grounding)
Incoming Cable in Conduit (Grounding)
5 (15) 1 16 4 (10) 4,10**(10) 30015 (45) 1 50 10 (10) 16 (10) 300
30 (100) 1 100 25 (10) 50 (16) 30050 (150) 1 125 35 (10) 70 (25) 30015 (45) 3 50 10 (10) 16 (10) 750
30 (100) 3 100 25 (10) 50 (16) 75050 (150) 3 125 35 (10) 70 (25) 750
200 3 250 95 (25) 150 (35) 750400 3 500 240 (50) 500 (70) 750
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GroundingService Equipment Grounding
18
• Usually two points of grounding: at the transformer and at the main distribution box
• “Main Bonding Jumper” connects the metal part of the main distribution box to ground
• “Main Bonding Jumper” (now) connects to the grounded conductor
• Same calculation as “System Grounding”
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GroundingSeparately Derived System
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• Electrical systems that are separated inside the facility (after the utility): e.g. from utility and generator
• Common Neutral: grounded only at the main distribution box
• Isolated Neutral: grounded separately and have the isolated ground rod
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GroundingEquipment Grounding
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Grounding for the “non-live” metal portion
• The enclosure has the same potential level as the ground (protected for human contact)
• Improve short-circuit protection: conductor touching enclosure
• Path for the leakage current
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GroundingEquipment Grounding – What to be grounded?
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• Metal enclosure & Protected Fence: metal conduit, panelboard enclosure, electric crane, elevator’s enclosure
• Protected fence: HV station
• Electrical Appliance specially with outlet: more than 150V, except having proper insulator (e.g. double insulator)
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22
• Fixed Equipment with hard wires specially able to be touched– Close contact to human: grounded within 2.4m from
ground and 1.5m from wall
– Electrical contact to metal
– Wet area
GroundingEquipment Grounding – What to be grounded?
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23
• Continuity
• Low Impedance
• High Ampacity
GroundingEquipment Grounding – Effective Grounding
• Copper: with or without insulator
• Busware: that specified for grounding
• Metal Armor of some cables: AC, MI, MC
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24
GroundingEquipment Grounding – Effective Grounding
http://www.iaei.org/magazine/2006/09/new-form-of-mc-cable-crosses-application-boundaries/
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GroundingEquipment Grounding – Sizing
25
• Tables (if common ground cable, take maximum one)
– In general: according to rating of CB
– For motor: according to the rating of overload protection.
http://www.cable.alcan.com/NR/rdonlyres/19FC4111-742D-47B7-9A28-06DF8AB4023D/0/EquipmentGroundingConductor.pdf
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GroundingEquipment Grounding – Sizing
26
Rated Circuit Breaker or Overload Protector(A)
Ground Cable Size (mm2)
16 1.5 (recommended 2.5)20 2.540 470 6
100 10200 16400 25500 35
Electrical Systems Design: ประสทิธิ ์พทิยพฒัน์
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GroundingComputer Grounding
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• Power Distribution Grounding– Single Ground Point to protect “Ground Loop”
• High Frequency Grounding– in the order of kHz or MHz
– 60cm x 60cm conductor grid
Recommend common ground electrode
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Grounding Electrode SystemEarth
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• Refered as 0
• Deeper in the ground (more humidity) lower resistance
• Good for make the reference voltage as 0
• ** not capable enough to conduct high current
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Grounding Electrode SystemElectrode
Existing Electrode• Underground metal pipes
• Building structures
• Pile
• Under ground metal structures
Made Electrode• Ground Rods
• Concrete Encased Electrode
• Buried Plate
• Ring
• Grid
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Grounding Electrode SystemGround Rod
30
• Copper rod with at least 16mm in diameter and 2.4 m in length
• Few longer rods are better than many short rods
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Earthing Scheme
31
• 2,3 or 4 characters
• 1st: relation between distribution system and earth
– T means single point connecting to earth
– I means isolated from earth
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Earthing Scheme
32
• 2nd: relation exposed conductive part and earth– T means no connection to the electrical system
– N means there is a connection between the exposed conductive and electrical system (neutral)
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Earthing Scheme
33
• 3rd(if any): arrangement of neutral (N) and protective conductor (PE)– S means separation between N & PE
– C means common between N & PE
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Type of Earthing SystemIT
34
http://www.electrical-installation.org/enw/images/5/5e/EIG_chap_E-2010_1.pdf
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Type of Earthing SystemTT
35
http://www.electrical-installation.org/enw/images/5/5e/EIG_chap_E-2010_1.pdf
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Type of Earthing SystemTN-C
36
http://www.electrical-installation.org/enw/images/5/5e/EIG_chap_E-2010_1.pdf
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Type of Earthing SystemTN-S
37
http://www.electrical-installation.org/enw/images/5/5e/EIG_chap_E-2010_1.pdf
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Type of Earthing SystemTN-C-S
38
http://www.electrical-installation.org/enw/images/5/5e/EIG_chap_E-2010_1.pdf
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Resistance to Ground
39
• By E.I.T. standard 5 Ohm
• Some area, authority might allow up to 25 Ohm
• If cannot achieve, try adding extra electode
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• Electrical Systems Design: ประสทิธิ ์พทิยพฒัน์
Reference
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