Assumption University Thailand EE4503 Electrical Systems Design Grounding 1.

Assumption University Thailand

EE4503Electrical Systems Design

Grounding

1


Topics

• Introduction to Grounding

• Grounding of Electrical Systems

• Grounding Systems

• Grounding Resistant

2


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


GroundingCategory

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• 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


GroundingComponents

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• Grounding Electrode (System)

• Grounding Electrode Conductor

• Grounded Conductor

• Main Bonding Jumper

• Equipment Grounding Conductor


GroundingComponents

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GroundingComponents

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http://cr4.globalspec.com/thread/40145/Grounded-Neutral


GroundingCommon Grounding Systems

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• System Grounding

• Service Equipment Grounding

• Separately Derived System

• Equipment Grounding

• Computer Grounding


GroundingAC System Grounding

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“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


GroundingAC System Grounding – 50V

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• 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


GroundingAC System Grounding – 50 to 1kV

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• Usually at the distributed transformer

• Neutral is grounded


GroundingAC System Grounding – Electrode

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• Grounding Electrode Conductor

• Connected to three parts

– Grounded Conductor: usually neutral line

– Equipment Grounding Conductor

– Main Bonding Jumper



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• 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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• 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


GroundingAC System Grounding – Exothermic Welding

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http://www.youtube.com/watch?v=CqixbTQG13M

AllTec Corp Exothermic Weld.flv


GroundingAC System Grounding – Cable Size

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http://www.kroo-suchat.com/File_data/ground.pdf


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) 300

15 (45) 1 50 10 (10) 16 (10) 300

30 (100) 1 100 25 (10) 50 (16) 300

50 (150) 1 125 35 (10) 70 (25) 300

15 (45) 3 50 10 (10) 16 (10) 750

30 (100) 3 100 25 (10) 50 (16) 750

50 (150) 3 125 35 (10) 70 (25) 750

200 3 250 95 (25) 150 (35) 750

400 3 500 240 (50) 500 (70) 750


GroundingService Equipment Grounding

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• 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”


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


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


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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• 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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• 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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GroundingEquipment Grounding – Effective Grounding

http://www.iaei.org/magazine/2006/09/new-form-of-mc-cable-crosses-application-boundaries/


GroundingEquipment Grounding – Sizing

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• 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


GroundingEquipment Grounding – Sizing

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Rated Circuit Breaker or Overload Protector(A)

Ground Cable Size (mm2)

16 1.5 (recommended 2.5)

20 2.5

40 4

70 6

100 10

200 16

400 25

500 35

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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


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


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

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• Copper rod with at least 16mm in diameter and 2.4 m in length

• Few longer rods are better than many short rods


Earthing Scheme

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• 2,3 or 4 characters

• 1st: relation between distribution system and earth

– T means single point connecting to earth

– I means isolated from earth


Earthing Scheme

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• 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)


Earthing Scheme

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• 3rd(if any): arrangement of neutral (N) and protective conductor (PE)

– S means separation between N & PE

– C means common between N & PE


Type of Earthing SystemIT

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http://www.electrical-installation.org/enw/images/5/5e/EIG_chap_E-2010_1.pdf


Type of Earthing SystemTT

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Type of Earthing SystemTN-C

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Type of Earthing SystemTN-S

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Type of Earthing SystemTN-C-S

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Resistance to Ground

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• By E.I.T. standard 5 Ohm

• Some area, authority might allow up to 25 Ohm

• If cannot achieve, try adding extra electode


• Electrical Systems Design: ประสิ�ทธิ์�� พิ�ทยพิ�ฒน์�

Reference

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Assumption University Thailand EE4503 Electrical Systems Design Grounding 1.

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