electrical earthing · grounding methods 5 • petersen coil grounding (arc suppression) •...

ELECTRICAL

EARTHING

SPECIAL NOTE:

•THE APPROACH HERE IS BASED ON STANDARD IEEE 80

(SAFETY IN AC SUBSTATION GROUNDING)

•THESE DISCUSSIONS ARE FOR ILLUSTRATION ONLY

•GROUNDING PRACTICES ARE SUBJECT TO LOCAL

REGULATIONS/CODES WHICH WILL TAKE PRECEDENCE

OBJECTIVES OF GROUNDING:•PROVIDES AN ELECTRICAL SUPPLY SYSTEM WITH A

REFERENCE TO THE GROUNDMASS (SYSTEM GROUNDING)

•PROTECTIVE GROUNDING OF ELECTRICAL EQUIPMENT ENCLOSURES• MAKES THEM SAFE TO PERSONS WHO MAY COME INTO CONTACT WITH THEM • ENABLES THE FLOW OF FAULT CURRENT IN THE EVENT OF A FAILURE

•PROVIDES A LOW IMPEDANCE PATH FOR ACCUMULATED STATIC CHARGES AND SURGES (LIGHTNING PROTECTION GROUNDING)

•HELPS IN MITIGATING THE GENERATION AND PROPAGATION OF NOISE (GROUNDING OF SHIELDS AND SIGNAL REFERENCE PLANES)

EARTHING SYSTEMSHALL SATISFY SAFETY, FUNCTIONAL REQUIREMENTS OF

ELECTRICAL INSTALLATION

SHALL ENSURE

• PROTECTION AGAINST INDIRECT CONTACT

• PROPER FUNCTIONING OF ELECTRICAL PROTECTIVE DEVICES

• PROTECTIVE AND FUNCTIONAL REQUIREMENTS ARE MET UNDER EXPECTED

CONDITIONS

• EARTH FAULT, EARTH LEAKAGE CURRENTS CAN BE CARRIED SAFELY

• ADEQUATE STRENGTH APPROPRIATE TO EXTERNAL INFLUENCES

• ADEQUATE VALUE OF EARTHING RESISTANCE

BENEFITS (1)

•FAULT DAMAGE NOW MINIMAL

• REDUCES FIRE HAZARD (ESPECIALLY IN MINES)

•LOWER OUTAGE TIMES

• LESS LOST PRODUCTION, LESS LOST REVENUE

•TOUCH POTENTIALS KEPT WITHIN SAFE LIMITS

• PROTECTS HUMAN LIFE

BENEFITS (2)

•LOW FAULT CURRENTS REDUCE POSSIBILITY OF IGNITING GASES•MINIMIZES EXPLOSION HAZARD

•LOWER MAGNETIC OR THERMAL STRESSES IMPOSED ON PLANT DURING FAULT

•TRANSIENT OVERVOLTAGES LIMITED• PREVENTS STRESSING OF INSULATION, BREAKER RESTRIKES

FAULT IN AN UNGROUNDED SYSTEM:

EFFECT OF NEUTRAL (SYSTEM) GROUNDING:

GROUNDING METHODS 2

SOLID GROUNDING• NEUTRAL CONNECTION ON

GENERATOR / TRANSFORMERIS CONNECTED TO EARTH BY ASOLID CONDUCTOR

• COST REDUCTIUONS DUE TOAVOIDANCE OF SENSITIVERELAYS AND GROUNDINGDEVICE, GRADING OFINSULATION TOWARDSNEUTRAL END.

• BUT CIRCULATION OF THIRDHARMONIC CURRENTSBETWEEN NEUTRALS

GROUNDING METHODS 3

•RESISTANCE GROUNDING

• NEUTRAL CONNECTION ON

GENERATOR / TRANSFORMER IS

CONNECTED TO EARTH (0V)

THROUGH A FIXED RESISTANCE

TO LIMIT THE EARTH FAULT

CURRENT

• MAINLY USED BELOW 33 KV

• FULL LINE TO LINE INSULATION

REQUIRED TOWARDS NEUTRAL

GROUNDING METHODS 4

•REACTANCE GROUNDING

• NEUTRAL CONNECTION ON

GENERATOR / TRANSFORMER

IS CONNECTED TO EARTH (0V)

THROUGH A FIXED REACTANCE

TO LIMIT THE EARTH FAULT

CURRENT

• CAN BE CHEAPER COMPARED

TO RESISTANCE

GROUNDING METHODS 5

•PETERSEN COIL GROUNDING

(ARC SUPPRESSION)

• NEUTRAL CONNECTION ON

TRANSFORMER IS CONNECTED TO

EARTH (0V) THROUGH A VARIABLE

REACTANCE TO NEUTRALISE THE

CAPACITIVE EARTH FAULT CURRENT.

RESULTS IN ARC EXTINCTION

GROUNDING METHODS 6

• NEC GROUNDING (WITH AND WITHOUT RESISTANCE)

• IN HV DELTA SYSTEMS NO EARTH CONNECTION IS AVAILABLE. A 3 PHASE NEUTRAL GROUNDING COMPENSATOR IS CONNECTED TO ALLOW EARTH FAULT CURRENTS TO FLOW - ALLOWING DETECTION OF THESE FAULTS

PROTECTIVE GROUNDING

•PROTECTS PERSONNEL AGAINST SHOCKS

•PERSONNEL DO NOT EXPERIENCE DANGEROUS HIGH VOLTAGES

WHEN CONTACTING ENCLOSURE ACCIDENTALLY CONNECTED

TO LIVE PARTS

•PROVIDES A LOW IMPEDANCE PATH FOR ACCUMULATED

STATIC CHARGES AND SURGES (LIGHTNING PROTECTION

GROUNDING)

•HELPS IN MITIGATING THE GENERATION AND PROPAGATION OF

NOISE (GROUNDING OF SHIELDS AND SIGNAL REFERENCE

PLANES)

IMPORTANCE FOR EARTHING

An Electrical equipment is considered dead when

• At or about zero potential

• Disconnected/ Isolated from live system

• Disconnection alone not adequate

• Can retain stored charge

• Can acquire a static charge

• Can accidentally be made alive

• Nearby live conductors may induce voltage

IMPORTANCE OF EARTHING! Ensure earthing before working on electrical equipment

Earthing

• Connect apparatus electrically to general mass of earth in such a

manner as will ensure at all times an immediate safe discharge of

electrical energy

• Connect to earthed metal earth bar or spike with good metallic

conductor

Earthing by

• Closing of earthing links

• Attaching of fixed earthing devices

• Affixing of portable earthing straps

IMPORTANCE OF EARTHING

Ensure before applying earth

• Earthing connection is mechanically, electrically in

good condition

• No broken strands

• Clamps should be rigid and without defect

• Applied properly in intimate contact with conductors

and earth-bar/ spike

• Earthing cable tails as short as possible

• Connect to earth first when installing earthing,

disconnect earth last while removing earthing

HAZARDS OF IMPROPER EARTHING

• Electrocution

• Burns from arcing

• Electric shock leading to falls

BONDING

•CONNECTING OF VARIOUS GROUNDING SYSTEMS AND

NON CURRENT CARRYING PARTS

•TO ACHIEVE POTENTIAL EQUALIZATION BETWEEN

DIFFERENT ACCESSIBLE CONDUCTING SURFACES

•POTENTIAL DIFFERENCE BETWEEN DIFFERENT

ACCESSIBLE CONDUCTING SURFACES, DIFFERENT

GROUNDING SYSTEMS HAZARDOUS

TYPICAL EARTHING SYSTEM

TOUCH, STEP, TRANSFERRED VOLTAGES

An Earth Leakage Circuit Breaker (ELCB) is a

device used to directly detect currents

leaking to earth from an installation with high

earth impedance to prevent shock.

Typical RCB circuit(Three

Phase):

This relay causes tripping of the associated circuit breaker to disconnect the power supply to the

equipment under protection

CONDUCTORS

-

--

-

High Resistance

-

-

-

-

-

-

- -

--

-

-

-

Low Resistance

A conductor has many free electrons so is good at transferring electrical

current

Good Conductor Bad Conductor

Conductance is the opposite of resistance

It is measured in ‘Mho’s (ohm backwards) ℧

DIFFERENT TYPES OF CONDUCTORS

Material Used

Copper

High conductivity

Easily soldered

Heavier & more expensive

than aluminum

Copper used in house

wiring

1mm2, 1.5mm2

4mm2 , 6mm2

Aluminum

60%

conductivity of

copper

Cheap &

lighter than

copper

Lowest

conductivity

Heavier than

aluminum

Galvanized Iron (GI)

Used in

overhead lines

DIFFERENT TYPES OF CONDUCTORS

Respective of their property

Good Conductors Bad Conductors

Medium

resistance

Used for

converting

electrical

energy into

heat, light &

sound

PVC, glass

High

resistance

Non Conductors

Carry current

Low resistance

Copper & Aluminum

Tungsten & Nichrome

Insulators

Wires & cables

use conductors

& non

conductors to

their advantage

DIFFERENT TYPES OF CONDUCTORS

Physical Appearance

Solid Conductor

Used in cables.

e.g. copper,

aluminum, steel

Stranded Conductor

Flexible

1.13 to 3.73 mm diameter

1, 7, 19, 37 stands

Multi stranded Conductor

0.2 or 0.3 mm diameter

14, 22, 24,84 strands

Flexible Conductor

14, 23, 40 strands

<0.2 mm diameter

WIRES & CABLES

Wires

Domestic & small industry wiring

In appliances

Cables

Small & big industries

Distribution Lines

Transmission lines

The size & type of wire/cable must suit the power rating required for their

use. The higher the power the thicker the wire/cable

Wires & Cables are purpose built conductors

TYPES OF WIRES

Vulcanized India Rubber (VIR)

suitable for: low &

medium voltage supply

only

tinned copper/ aluminumCotton tape & cotton

braiding

Bitumen

Vulcanized India Rubber (VIR)

To protect against corrosion

from the VIR

Old type: not

readily available

to purchase

TYPES OF WIRES

Cabe Tyre Sheath wire (CTS)tinned copper

Rubber/plastic

Thicker

Rubber/plastic

Don’t absorb moisture

Available in 250/440V only

Old type: not

readily available

to purchase

TYPES OF WIRE

PVC Wire

copper/ aluminum

Polyvinyl chloride (PVC)

Widely used

Long life

Durable against

water, heat, oil, UV

light

Available in 600,

660, 1100 Voltage

WIRING APPLIANCES

Earth

Takes current to ground if

appliance has fault

Live

Provides current to

appliance

Neutral

Returns current to

power source

What wire is missing and

why?

What do each of these

wires do?

STANDARD WIRE GAUGE & CURRENT CARRYING CAPACITY

Higher current will heat the

cable and damage the

insulation and may result in

short circuit

Each number

represents a size of

wire

Maximum safe

current that can flow

through a cable

CABLESLarger sized conductors

Type of insulation

Types of cable are sorted by:

Cotton covered

Silk coated

Asbestos covered

Rubber coated

PVC coated

Type of conducting material

Their shape

Unarmored

Armored

Voltage Grade

Low

High

Copper

Aluminum

Mechanical protection

Flat

Round

2. UNIT PRICE ESTIMATE

UNIT PRICES ARE OBTAINED FROM DATA ON

PROJECTS ALREADY PERFORMED.

COST OF LABOR, MATERIAL, AND EQUIPMENT FOR ALL

UNITS OF WORK ARE ADDED TOGETHER AND DIVIDED BY

THE NUMBER OF UNITS INVOLVED.

PROF AWAD S. HANNA

Elemental Estimate Analysis

Gross floor Area = 250,000 ft2

4. DETAILED (DEFINITIVE )ESTIMATE

PREPARED AFTER DRAWINGS AND SPECIFICATION

ARE COMPLETED.

REQUIRES A COMPLETE QUANTITY TAKEOFF BASED

ON DRAWING AND THE COMPLETE SET OF

CONTRACT DOCUMENTS

NEED INFORMATION ON LABOR RATE

"PRODUCTIVITY", MATERIAL COST, COST OF

RENTING OR PURCHASING EQUIPMENT

VARIATION FACTORS IN ESTIMATING1. TIME

WE BASE OUR ESTIMATE ON THE COST OF EXISTING

PROJECTS THAT WERE BUILT IN THE PAST

PRICE-LEVEL CHANGES OVER TIME

WE NEED TO PROJECT COSTS OF FUTURE PROJECTS

MANY ORGANIZATIONS PUBLISH CONSTRUCTION COST DATA

ON REGULAR BASIS:

US DEPARTMENT OF COMMERCE

US DEPARTMENT OF LABOR

ENR

TURNER CONSTRUCTION COMPANY

HANDY-WHITMAN UTILITIES

1. TIME (CONT.)

COST INDICES

PUBLISHED BY R.S. MEANS AND ENR

USED TO UPDATE OLD COST INFORMATION

USES

1. TO UPDATE KNOWN HISTORICAL COSTS FOR NEW ESTIMATES

2. TO ESTIMATE REPLACEMENT COST FOR SPECIFIC ASSETS

3. TO PROVIDE FOR CONTRACT ESCALATION

LIMITATIONS

1. THEY REPRESENT COMPOSITE DATA, AVERAGE OF MANY PROJECTS.

2. THEY FAIL TO RECOGNIZE TECHNOLOGICAL CHANGES.

3. THERE IS A REPORTING TIME LOG.

P

r

o

f

A

w

a

d

S

.

H

a

n

n

a

3. SIZE

Proposed Size

Comparison Size

E - 1

PROF AWAD S. HANNA

TABLE FORUNIT AND TOTAL COST MULTIPLIERS

Buildings

Complex

Project s

Project

Size

Fact or Buildings

Complex

Project s

1.175 1.904 0.2 0.235 0.381

1.128 1.619 0.3 0.338 0.486

1.096 1.443 0.4 0.438 0.577

1.072 1.320 0.5 0.536 0.660

1.052 1.227 0.6 0.631 0.736

1.036 1.153 0.7 0.725 0.807

1.023 1.093 0.8 0.881 0.875

1.011 1.043 0.9 0.910 0.939

1.000 1.000 1.0 1.000 1.000

0.991 0.963 1.1 1.090 1.059

0.982 0.930 1.2 1.178 1.116

0.974 0.900 1.3 1.266 1.170

0.967 0.874 1.4 1.354 1.224

0.960 0.850 1.5 1.440 1.275

0.954 0.829 1.6 1.527 1.326

0.948 0.809 1.7 1.612 1.375

0.943 0.780 1.8 1.697 1.423

0.938 0.774 1.9 1.782 1.470

0.933 0.758 2.0 1.866 1.516

0.928 0.743 2.1 1.950 1.561

0.924 0.730 2.2 2.033 1.605

0.920 0.717 2.3 2.116 1.648

0.916 0.705 2.4 2.199 1.691

0.912 0.693 2.5 2.281 1.733

0.909 0.682 2.6 2.363 1.774

0.905 0.872 2.7 2.445 1.815

0.902 0.662 2.8 2.526 1.855

0.899 0.653 2.9 2.607 1.894

0.896 0.644 3.0 2.688 1.933

0.893 0.636 3.1 2.768 1.972

0.890 0.628 3.2 2.849 2.010

0.887 0.620 3.3 2.929 2.047

0.885 0.613 3.4 3.008 2.084

0.882 0.606 3.5 3.088 2.121

0.880 0.599 3.6 3.167 2.157

0.877 0.593 3.7 3.246 2.192

0.875 0.586 3.8 3.325 2.228

0.873 0.580 3.9 3.404 2.263

0.871 0.574 4.0 3.482 2.297

Use This Side for

Unit Cost Mult ipliers

(UCM Met hod)

Use This Side for

Tot al Cost Mult ipliers

(TCM Met hod)

PROF AWAD S. HANNA

CHART TO CONVERT TOTAL PROJECT COST

0.000

0.500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

0.1 1.0 10.0

Size Factor

To

tal

Pro

ject

Co

st

Mu

ltip

lier

(TC

M)

Buildings Complex Projects

MINIATURE CIRCUIT BREAKERS (MCB)MULTI 9

• PROTECT CABLES AGAINST

OVERLOADS AND SHORT

CIRCUITS

• LET THE CURRENT FLOW

WITHOUT OVERHEATING ON

CABLES

• OPEN AND CLOSE A CIRCUIT

UNDER RATED CURRENT

• PROTECT AGAINST

INSULATION

FAULTS (SEE EARTH LEAKAGE

PROTECTION MODULE)

DEFINITIONMain functions

Miniature circuit = switch + trip unit breaker (MCB) 4

Symbol

• A MECHANICAL SYSTEM TO OPEN AND

CLOSE THE CIRCUIT

• A BIMETAL STRIP TO ACT AGAINST

OVERLOADS

• A MAGNETIC TRIP TO ACT AGAINST SHORT

CIRCUITS

DEFINITIONA Multi 9 circuit breaker has:

Miniature circuit = switch + trip unit breaker (MCB) 4

THE TECHNOLOGYActive parts

C60N NC100 / NC125

terminal

fixed contact

moving

contact

strip braid

magnetic

trip

load side

terminal

terminal

bimetral strip

strip braid

moving

contact

magnetic trip

load side

terminal

strip braidbimetal strip

4

OVERLOAD PROTECTION

bimetal

tripping barbimetal

tripping bar

screw

calibration

strip

braid

NC45 NC100 / NC125

• A BIMETAL STRIP IS USED:

O TO PROTECT THE CABLE AGAINST OVERLOADS

O SYMBOL:

• PRINCIPLE OF THE BIMETAL: IN A MULTI 9 CIRCUIT

BREAKER4

• THERMAL TRIPPING

CURVE

• THE "THERMAL" PART

OF THE

TRIPPING CURVE MUST

BE IN

ACCORDANCE WITH

THE STANDARD

OVERLOAD PROTECTION

Legend:

In = rated current

Ind = no tripping current

Id = tripping current 4

tripping

no

tripping

destruction

of bimetalaccuracy

t (s)

I/InIn Ind Id