28
Types of Overcurrent & Calculations

A - Types of Overcurrents & Calculations

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Page 1: A - Types of Overcurrents & Calculations

Ty

pe

s o

f O

ve

rcu

rre

nt

&

Ca

lcu

lati

on

s

Page 2: A - Types of Overcurrents & Calculations

NE

Art

icle

10

0 –

De

fin

itio

ns

Overcurrent

An

ycu

rre

nt

ine

xce

sso

fth

era

ted

curr

en

to

f

eq

uip

me

nt

or

the

am

pa

city

of

aco

nd

uct

or.

It

ma

yre

sult

fro

mo

ve

rlo

ad

,sh

ort

circ

uit

,o

r

gro

un

dfa

ult

.

Page 3: A - Types of Overcurrents & Calculations

NE

Art

icle

10

0 –

De

fin

itio

ns

Overload

Op

era

tio

no

fe

qu

ipm

en

tin

exc

ess

of

no

rma

l,

full

-lo

ad

rati

ng

,o

ro

fa

con

du

cto

rin

exc

ess

of

rate

da

mp

aci

tyth

at,

wh

en

itp

ers

ists

for

a

suff

icie

nt

len

gth

of

tim

e,

wo

uld

cau

sed

am

ag

e

or

da

ng

ero

us

ove

rhe

ati

ng

.A

fau

lt,

such

as

a

sho

rtci

rcu

ito

rg

rou

nd

fau

lt,

isn

ot

an

ove

rlo

ad

.

Page 4: A - Types of Overcurrents & Calculations

No

rma

l Lo

ad

Source

RR

RR

R

R

Load

R

I L=

VS

/R

T

VS

I L I L

Page 5: A - Types of Overcurrents & Calculations

Ove

rlo

ad

Source

Wit

hin

th

e n

orm

al

path

RR

RR

R

R

Load

RV

S

I OL=

VS

/R

T

Page 6: A - Types of Overcurrents & Calculations

Sh

ort

Cir

cuit

Source

Ou

tsid

e th

e n

orm

al

path

RR

RR

R

R

I SC=

VS

/R

T

VS

Load

R

Ou

tof

the

circ

uit

Page 7: A - Types of Overcurrents & Calculations

Ty

pe

s o

f Fa

ult

s

•B

olt

ed

Fa

ult

s

•A

rcin

g F

au

lts

Page 8: A - Types of Overcurrents & Calculations

Bo

lte

dS

ho

rt

Cir

cu

it

AB

Arc

ing

Fa

ult

AB

Curr

ent

Thru

Air

Page 9: A - Types of Overcurrents & Calculations

Bo

lte

d F

au

lts

–V

ari

ou

s Ty

pe

s A B C Gro

und

N

480Y

/277 V

/ 4W

Soli

d G

rd

Page 10: A - Types of Overcurrents & Calculations

Bo

lte

d F

au

lts

–T

hre

e P

ha

se

A B C Gro

und

N

480Y

/277 V

/ 4W

Soli

d G

rd

Typic

ally

consi

der

ed t

he

“wors

t

case

” or

hig

hes

t m

agnit

ude

Page 11: A - Types of Overcurrents & Calculations

Bo

lte

d F

au

lts

–Li

ne

to

Lin

e A B C Gro

und

N

480Y

/277 V

/ 4W

Soli

d G

rd

87 %

of

the

thre

e phas

e

bolt

ed f

ault

Page 12: A - Types of Overcurrents & Calculations

Bo

lte

d F

au

lts

–Li

ne

to

Gro

un

d

A B C Gro

und

N

480Y

/277 V

/ 4W

Soli

d G

rd

Typic

ally

much

low

er t

han

fault

, but

can b

e >

fault

nea

r X

fmr

term

inal

s

Page 13: A - Types of Overcurrents & Calculations

AB

Arc

ing

Fau

lts

–M

any

Var

iab

les

Eff

ect

Cu

rren

t &

Wh

eth

er S

ust

ain

able

Sy

stem

Vo

ltag

e

Gap

sp

acin

g

Av

aila

ble

Sh

ort

Cir

cuit

Am

ps

Am

ou

nt

of

Co

pp

er V

apo

rize

d

Deg

ree

of

Co

nta

inm

ent

Co

nfi

gu

rati

on

of

Eq

uip

men

t

Ty

pic

ally

do

es n

ot

sust

ain

on

20

8Y

/12

0V

Page 14: A - Types of Overcurrents & Calculations

Arc

ing

Fa

ult

s –

Pro

gre

ssio

n

A B C Gro

und

N

480Y

/277 V

3�

/ 4W

Soli

d G

rd

Page 15: A - Types of Overcurrents & Calculations

Arc

ing

Fa

ult

s –

Th

ree

Ph

ase

A B C Gro

und

N

480Y

/277 V

/ 4W

Soli

d G

rd

Can

var

y w

idel

y p

oss

ibly

up t

o

89%

of

bolt

ed f

ault

Page 16: A - Types of Overcurrents & Calculations

Arc

ing

Fa

ult

s –

Lin

e t

o L

ine

A B C Gro

und

N

480Y

/277 V

/ 4W

Soli

d G

rd

Can

var

y w

idel

y p

oss

ibly

up t

o

74%

of

bolt

ed f

ault

Page 17: A - Types of Overcurrents & Calculations

Arc

ing

Fa

ult

s –

Lin

e t

o G

rou

nd

A B C Gro

und

N

480Y

/277 V

/ 4W

Soli

d G

rd

Can

var

y w

idel

y

Page 18: A - Types of Overcurrents & Calculations

Arc

ing

Fa

ult

s –

Su

sta

ina

bil

ity

A B C Gro

und

N

480Y

/277 V

/ 4W

Soli

d G

rd

Rule

of

thum

b:

Arc

ing f

ault

s w

ill

typic

ally

not

sust

ain

at l

ess

than

38%

of

bolt

ed f

ault

Page 19: A - Types of Overcurrents & Calculations

Ho

w d

o y

ou

kn

ow

wh

at

the

sh

ort

-cir

cuit

cu

rre

nt

is t

hro

ug

ho

ut

a s

yst

em

?

an

d

Wh

at

are

so

me

ty

pic

al

va

lue

s

Page 20: A - Types of Overcurrents & Calculations

MM

MS

B

Short

Cir

cuit

Curr

ents

Var

y D

epen

din

g o

n M

any

Fac

tors

Tra

nsf

orm

er S

ize

& %

Z

Volt

age

Conduct

or

Siz

e &

Len

gth

Page 21: A - Types of Overcurrents & Calculations

MM

MS

B

Short

Cir

cuit

Curr

ents

Var

y D

epen

din

g o

n M

any

Fac

tors

60,0

00 A

27,0

00 A

40,0

00 A

60,0

00 A

18,0

00 A

9,0

00 A

Page 22: A - Types of Overcurrents & Calculations

500 K

VA

480/2

77V

1

5 %

Z

500 K

VA

480/2

77V

1A

2 %

Z

1500 K

VA

480/2

77V

2

5%

Z

1500 K

VA

480/2

77V

2A

2 %

Z

Sh

ort

Cir

cu

it C

urr

en

t E

xam

ple

s #

1

1500 K

VA

208/1

20V

3

5%

Z

1500 K

VA

208/1

20V

3A

2 %

Z

Page 23: A - Types of Overcurrents & Calculations

Ho

w t

o C

alc

ula

te T

ran

sfo

rme

r

Se

con

da

ry (a

ssu

min

g i

nfi

nit

e p

rim

ary

)

I sca

= (

Xfm

r F

LA)

x 1

00

/%

Z

(in

cre

ase

re

sult

by

10

% d

ue

to

UL

tole

ran

ce f

or

tra

nsf

orm

er

imp

ed

an

ces)

Page 24: A - Types of Overcurrents & Calculations

500 K

VA

480/2

77V

1

5 %

Z

500 K

VA

480/2

77V

1A

2 %

Z

13,2

22 A

33,0

55 A

1500 K

VA

480/2

77V

2

5%

Z

1500 K

VA

480/2

77V

2A

2 %

Z

39,6

66 A

99,1

65 A

Sh

ort

Cir

cu

it C

urr

en

t E

xam

ple

#1 A

nsw

ers

1500 K

VA

208/1

20V

3

5%

Z

1500 K

VA

208/1

20V

3A

2 %

Z

91,6

08 A

229,0

20 A

Page 25: A - Types of Overcurrents & Calculations

208/1

20V

4

4A

6

6A

480/2

77V

5

5A

480/2

77V

40,0

00 A

.40,0

00 A

.40,0

00 A

.

50 f

t # 1

50 f

t 250 k

cm

Sh

ort

Cir

cu

it C

urr

en

t E

xam

ple

#2

Page 26: A - Types of Overcurrents & Calculations

Ca

lcu

lati

ng

Sh

ort

Cir

cuit

Cu

rre

nts

Uti

lize

Po

int-

to-P

oin

t M

eth

od

Ste

ps

4,

5 &

6 f

or

Fau

lts

Page 27: A - Types of Overcurrents & Calculations

Sh

ort

Cir

cu

it C

urr

en

t E

xam

ple

#2 A

nsw

ers

208/1

20V

4

4A

40,0

00 A

.

50 f

t # 1

12,3

67 A

.

480/2

77V

5

5A

40,0

00 A

.

20,3

22 A

.

6

6A

480/2

77V

40,0

00 A

.

50 f

t 250 k

cm

28,8

18 A

.

Page 28: A - Types of Overcurrents & Calculations

Su

mm

ary

•O

ve

rcu

rre

nt

–o

ve

rlo

ad

s, b

olt

ed

fa

ult

s, &

arc

ing

fau

lts

•Fa

ult

cu

rre

nts

ca

n b

e d

ete

rmin

ed

th

rou

gh

ou

t

dis

trib

uti

on

syst

em

•S

ho

rt c

ircu

it c

urr

en

ts n

ee

de

d t

o a

sse

ss I

.R.,

Co

mp

on

en

t P

ro,

Co

ord

ina

tio

n,

Arc

Fla

sh

Ha

zard

s &

OC

PD

Se

lect

ion