wiring and grounding for safety

8/13/2019 Wiring and Grounding for Safety

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1

Wiring, Grounding, and

Power DistributionEquipment

For safety

PCL


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Wiring & Grounding

Why is it important to be aware of building wiring,grounding, and power distribution equipment ?

Because:

Understanding the proper design and installation of buildingwiring, grounding, and power distribution equipment can

Assure a safe operating system

Help reduce power quality problems.

Understanding the most common wiring and groundingerrors can quickly help identify the source of power qualityproblems.


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The purposes of grounding are:

Source: National Geographic

1. Safety? reduce fire hazard

? prevent electric shock

? avoid equipment damage

2. Performance

? control stray currents

? create equal-potentialplane

?

reduce communicationnoise

3. Lightning and SurgeMitigation

?

control transients? remove static charge

? bond between all services

? equalize surge referencepoints


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NEC Code for Wiring and Grounding - Safety

The purpose of this Code isthe practical safeguarding ofpersons and property fromhazards arising from the useof electricity1 .....not

necessarily efficient,convenient, or adequate forgood service or futureexpansion of electricalservices.2

1. NEC 2002, 90.1(A), Pg

2. NEC 2002, 90.1(B)


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Power and Grounding Standards in IEEE Performance & Transient Protection

IEEE 141 - Industrial PowerDistribution (red)

IEEE 142 - Grounding (green)

IEEE 446 - Emergency &Standby Power (orange)

IEEE 1100 - Powering &Grounding Electronics(emerald)

IEEE 1159 - MonitoringPower Quality

IEEE 1250 - Service toCritical Loads

IEEE P1346 - SystemCompatibility in IndustrialEnvironment


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Grounding-Related Definitionsfrom National Electric Code

Ground:A conducting connectionintentional oraccidentalbetween an electrical circuit or equipment andthe earth, or some conducting body that serves in place ofthe earth.

Grounded conductor:A system or circuit (current carrying)

conductor that is intentionally grounded. The neutral wire.White Wire

Grounding conductor:A conductor used to connectequipment or the grounded circuit of a wiring system togrounding electrode(s). Not intended to conduct current.Green Wire

Premises wiring: Interior and exterior wiring, includingpower, lighting, . from service point of utility or aseparately derived power source.

(NFPA 70)


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480Y/277V 3, 4W

208Y/120V 3, 4WMain Service

Entrance Disconnect

N

G

A

C

B

GroundRod

Building Steel

GroundRing

ConcreteEncased Electrode

Underground

Water Pipe

Telephone

Cable TV

Solidly Grounded Wye System

Notes:

1. Effectively (Intentionally) Grounded System. Intentionally connected to earth through a ground

connection(s) of sufficiently low impedance and having sufficient current-c arrying capacity toprevent the buildup of voltages that may result in undue hazards to connected equipment or to

persons.

2. Grounded Conductor.A system or circuit conductor t hat is intentionally grounded.

3. Neutral.A grounded conductor intended to carry im balanced load current.

4. Bonded (Bonding). The permanent joining of metallic parts to form an electrically conductive path .

5. Grounding Electrode System (GES).A system c reated by intentionally bonding together G round

Rod(s), Underground Water Pipe, Concrete Encased Electrodes, Ground Ring and Building

Structural Steel. Provides a low impedance path which can carry lightning currents and system faultcurrents.

6. Grounding Electrode Conductor (GEC). Conductor used to connect the GES to the equipmentgrounding conductor. Keeps a;; equipment and utilities at a common ground potential .

7. Main Bonding Jumper. The connection between the grounded circuit conductor and the equipmentgrounding conductor at the service. Allows short circuit current to return to the source required for

protective device operation.

Grounded System1

Grounded Conductor2

(Neutral3)

Grounding Electrode System (GES)5

Main

Bonding

Jumper7

Grounded

Electrode

Conductor6

13,200-208Y/120VElectric Utility Transformer

13,200-480Y/277V


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8

Common Grounding Electrode ConductorFor Non-metallic Structure

NEC2002 HB,

Exhibit 250.15,Page189

Common GEC

sized based on

total secondary

cross-sectional

area and table

250.66


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PremisesGroundingTechniques(Service Entrance at 60 Hz)

250.58 Use one common grounding electrode for all enclosures and equipment within a building.

250.50 Bond all elements of grounding electrode system. water pipe, building steel, ground ring.

250.56 A grounding electrode of more than 25 ohms shall be augmented by one additional electrode.

Exhibit 250.29, NEC Handbook, Page 102


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10

Exhibit 250.30

Exhibit 250.31

Listed Bonding Devices

Exhibit 250.32 Bonding of ametal raceway that contains agrounding electrodeconductor to the conductor atboth ends, as required by

250.64(E).


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11

Steel Building Columns

? B vs. C: B = 80% C = 20%

? C vs. H: C = 90% H = 10%

? C vs. G: C = 95% G = 5%

100 FeetG

A

B

C

H

1 foot

KAUFMAN EXPERIMENT

Classic Kaufman Experiment in 1954 led to NEC 250.91, thou shall runthe equipment grounding conductor with or enclosing the circuitconductorsGround Fault Return Conductor

S

IAC

2.5-inch Steel Conduit

Ground Fault Return Comparisons


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12

Listed Bonding Devices

Exhibit 250.35 Grounding-and-bonding bushings used toconnect a copper bonding orgrounding wire to conduits.(Courtesy of Thomas & BettsCorp.)

Exhibit 250.36 A threadedgrounding bushing with setscrews used to ensure electricaland mechanical connection.

(Courtesy of Thomas & BettsCorp.)

Bonding Jumper Sizing

Service same as GEC

Branch Circuit same as EGC


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"NEC", Article 250.4(A)(5)

Basic Equipment Grounding

Effective grounding path. The path to ground from circuits,

equipment, and conductor enclosures shall:

a) Be permanent and continuous,

b) Have capacity to conduct safely any fault current likely to beimposed on it,

c) Have sufficiently low impedance to limit the voltage to ground

and to facilitate the operation of the circuit protective devices in

the circuit,

d) The earth shall not be used as the sole equipment ground

conductor.


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14

Types of Electric Service

UngroundedWye

480 V 3, 3W

Delta480 V 3, 3W

240 V 3, 3W

Grounded? Grounded Wye

480Y/277 V 3, 4W208Y/120 V 3, 4W

High-Impedance Grounded Wye480 V 3, 4W

Corner Grounded Delta480 V 3, 3W240V 3, 3W

Mid-Point Grounded Delta240 V 3, 3W &120/240 V 1,

3W

Single-Phase120/240 V 1, 3W


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480V 3, 3W

Main Service

Entrance Disconnect

N

G

A

C

B

GroundRod

BuildingSteel

Ground

Ring

Concrete

Encased Electrode

UndergroundWater Pipe

Telephone

Cable TV

13,200-208Y/120V

Electric Utility Transformer

GroundingImpedance

Impedance Grounded Wye System


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Corner Grounded Delta System

Corner

Ground

480V 3, 3W240 3, 3WMain Service

Entrance Disconnect

A

C

B

GroundRod

Building

Steel

GroundRing



Telephone

Cable TV

13,200-240VElectric UtilityTransformer

G


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120/240 1, 3W

Main Service

Entrance Disconnect

L1

L2

GroundRod

Building

Steel

GroundRing



Telephone

Cable TV

13,200-240V

Electric Utility

Transformer

Center-Tap

Ground

Grounded Split-Phase (Residential) System

G

N


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Entrance Disconnect

A

C

B

GroundRod

BuildingSteel

GroundRing



Telephone

Cable TV


Ungrounded

G

Ungrounded Delta System

General Notes:

1. Ungrounded systems typically consist of a 3-phase 3-Wire 480 Voltor a 240 Volt Delta system.

2. An ungrounded system is a system which does not contain agrounded conductor (neutral) .

3. An ungrounded system is generally considered more reliable than agrounded system because short circuit faults involving only onephase and ground will not cause the overcurrent protection tooperate. The system will continue to operate with no problems andthe fault can be located at a convenient time.

4. The grounding electrode conductor provides a low impedance pathwhich can carry lighting currents into the earth and is connectedbetween the equipment grounding conductor and the groundingelectrode system (NEC 250.24).

5. The grounding electrode conductor keeps all equipment and utilitiesat a common ground potential by connecting the equipmentgrounding conductors and the service equipment enclosures to thegrounding electrodes in the earth {NEC 250.24, .28}.


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Entrance Disconnect

A

C

B

Ground

Rod BuildingSteel

GroundRing



Telephone

Cable TV


GFDs

Notes:

1. The proper use of suitable ground fault detectors (GFDs) on ungroundedsystems can indicated when a single ground fault has occurred so it can becleared before a second ground fault occurs on another phase. Groundfaults on more than one phase can cause damage arcing overvoltages,equipment brownouts, and building fires [(NEC 250.21(3)(d)]

Ungrounded Delta System With GFDs

G


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480V 3, 3W

Main Service

Entrance Disconnect

G

A

C

B

Ground

Rod

BuildingSteel

Ground

Ring



Telephone

Cable TV

13,200-208Y/120V


Ungrounded Wye System

Ungrounded


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Ungrounded Wye System With GFDs

480V 3, 3W

Main Service

Entrance Disconnect

G

A

C

B

Ground

RodBuilding Steel

Ground

Ring

Concrete

Encased Electrode


Telephone

Cable TV

13,200-208Y/120V


Ungrounded

GFDs


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23

Load Current

Equipment Grounding Conductor

~0 Volts

HOT

NEUTRAL

120 Vac

0 Volts 0 Volts

Imok

leakag

e

curren

tonly

? 250.80 All metallic enclosures (conduit,

raceway, panel boards) must bebonded.

Correctly Grounded EquipmentNormal Operation

THE GREEN WIRE SHOULD

NORMALLY SEE NO CURRENT


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24

Source

LoadGround Fault

Grounding Conductor

A low impedance path for the return offault current to trip the circuit breaker.

Bonded

Metallic enclosureBreaker trips in 2-3 cycles (lessthan .05 seconds) maybe

Phase to Ground Fault Path


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25

Incorrectly Grounded Equipment andGround Fault Path

Source Phase A

Phase B

Phase C

Grounded Circuit Conductor

LoadGround Fault

Code violation, no grounding conductor!

10 ohms

20 A

120 V

This breaker wont trip, prolonging the fault,because: I = E/R = 120/10 = 12 Amps

This enclosure isnot effectivelygrounded!


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26

Load Current

Equipment groundingconductor disconnected

violates NEC!

Ground Fault

HOT

NEUTRAL

120

Vac

0Volts

Volts?

Current Flow

x

~ 120V

Unknownearth resistance

Im not

ok!

Unsafe Practice of Isolating Equipment


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27

? Improper N-G bonds can cause unsafe situations aswell as performance problems, violates 250-24

TransformerGround

N

G

Hot

GroundedConductor(Neutral) Ground

Wire

Main Electrical

Panel

Distribution

Panel Receptacle LoadEquipment

ConduitGround

ConduitGround

EquipmentGround

40 A

? A

? A

? AServiceGround

Misapplied Neutral to Ground Bonds

N

G

N

G


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28

? Neutral to ground bonds are required at main service (NEC 250-24) and at

any separately derived power sources (NEC 250-30) and shall not be made

at any grounded circuit conductor (neutral) on the load side of the service

disconnect.

Proper Neutral to Ground Bond

TransformerGround

Hot

GroundedConductor

(Neutral)

Ground Wire

Main ElectricalPanel

DistributionPanel Receptacle

ComputerSystem

Separately

DerivedSystem

N-G bond per 250-30

N-G bond

per 250-24

NN

GG

N

G


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Neutral

Ground

3-Wire

Input

Ground

Transformer

Per 250.50 form a common premisesgrounding electrode system by connecting to

nearest effectively grounded:1. Building Steel2. Metal Water Pipe, or

3. Other Electrode

Ground

Neutral

Phase A

Phase BPhase C

Load

Separately Derived ac Systemper NEC 250.30

A neutral to groundbond between thesource and the firstdisconnect.


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30

Isolation TransformerNeutral to Ground Bond


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Neutral

Ground

3 Wire

Input

Ground

Transformer

Grounded

Phase A

Phase B

Phase C

Load

Fault

Common Building GroundingElectrode System

Separately Derived ac SystemInput-side Ground Fault


32/47

Neutral

Ground

3-Wire

Input

Ground

Transformer

To Computer Room

Signal Reference Grid

Grounded

Phase A

Phase B

Phase C

Load

Fault

Common building grounding

electrode system

Separately Derived ac Systemwith External Ground Fault


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33

Grounding Application For SeparatelyDerived Systems

Exhibit 250.13, NEC Handbook, Page 187,

Neutral-ground bond established at transformer

Exhibit 250.14, NEC Handbook, Page 187,

Neutral-ground bond established at first disconnecting means


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N

G

ServiceEntrance

Grounding

Electrode

N

G

Local

Grounding

Electrode

G

Generator

3-PoleAutomatic

Transfer Switch (ATS)

3W+G Load

Grounding and Transfer withStandby Generator and 3-W Load


35/47

N

G

ServiceEntrance

Grounding

Electrode

N

G

Local

Grounding

Electrode

G

Generator

3-PoleAutomatic

Transfer Switch

4W+G Load

N

Incorrect Grounding with Standby Gen, 4-Wire Loadand 3-Pole ATS


36/47

N

G

ServiceEntrance

Grounding

Electrode

N

GG

Generator

4-PoleAutomatic

Transfer Switch

4W+G Load

Local

Grounding

Electrode

Correct Grounding with Standby Gen,4-Wire Load and 4-Pole ATS


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37

Wiring & Grounding Evaluation

? Check rms voltage levels

? Check for extra neutral-ground bonds

? Check for overloaded neutral conductors

? Check grounding electrode system

? Check isolated ground receptacle wiring

? Check overall circuit layouts

? Check for use of separately derived systems

?

Check for ground loops

? Apply protection to data lines and communications lines if ground loop

potential exists


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38

DETAILS, DETAILS

Neutral

Ground

3-Wire

Input

CASE #1

Where Is the Grounding Electrode Connection?


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39

GROUND, THERE IT IS !

HERES THE BLDG GRD?

Neutral

Ground

3-Wire

Input


40/47

Problem: Erratic computer operation occurred in a large office buildingduring lightning storms.

Findings: Isolated ground conductors from all computers wereconnected to an isolated grounding electrode system. Voltages werebeing developed between the isolated computer ground and the buildingground.

Improper Isolated Ground

G

N

Service

Entrance

Grounding

Electrode

ITE

Equipment

IsolatedGround

ReceptacleDistribution

Panelboard

N

IsolatedGrounding

Electrode

Ground

Fault

Look for special quiet isolated ground!

IG

N

IG


41/47

Problem: A new ten-story office building includeda 1/4x1 copper busbar installed from thebasement to the top floor. The busbar wasinsulated from the building and connected to a

ground rod driven through the basement floor. Allisolated ground receptacles throughout thebuilding were connected to this isolated groundbusbar.

Solution: The electrical inspector would not allowoccupancy of the building until the IG receptacleswere rewired.

Isolated Grounding System


42/47

Problem: Four separate nearby buildings containedcontrol for chemical processing. There wereinterconnecting data cables between buildings. Each

of the four computer systems was separatelygrounded to its respective buildings groundingsystem. Operation of the computer systems waserratic.

Solution: The interconnecting data lines werereplaced with fiber-optic communication links inburied plastic conduits.

Multiple Interconnected Buildings


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Supplemental

Ground Rod

Properly InstalledMay Still Invite

Stray Ground

Currents

OPEN

CONCENTRIC

ONE OF

WORSTCONDITIONS

POSSIBLE


44/47

CNCMachine

GroundingElectrodes

Building Premises

ServiceTransformer

Cable Tray (Raceway)

DedicatedGround Rod

ServiceEntranceSwitch

0A0B0C

Incorrect and Unsafe Installation of Isolated,Dedicated Ground Rod

Attempts

To IsolateFrame

Create anUnknownResistance

Path Backto Service

N

G

PVCIsolator

G


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SAFETYFIRST!


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?s

wiring and grounding for safety

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