ABB CIRCUIT SHIELD 87T User manual

ABB

IB

7.6

.

1.7

-

4

Issue

B

INSTRUCTIONS

Transformer

Differential

Relay

©

CIRCUIT

&

SHIELD

Catalog

Series

219

Standard

Case

TYPE

87

T

Catalog

Series

419

Drawout

Test

Case

TYPE

87

T

PIFFEBENT

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CAT

TWO

WINDING

RELAY

Courtesy of NationalSwitchgear.com

IB

7.6

.

1.7

-

4

Page

2

Transformer

Differential

Relay

TABLE

OF

CONTENTS

Introduction

Precautions

Placing

Relay

into

Service

App

1

ication

Data

Maintenance

and

Testing

.

..

Page

2

Page

2

Page

2

Page

5

Page

14

INTRODUCTION

These

instructions

contain

the

information

required

to

properly

install

,

operate

,

and

test

the

ABB

Circuit

-

Shield

Type

87

T

single

-

phase

Transformer

Differential

Relay

.

These

mounting

.

the

case

and

clearly

numbered

.

relays

are

housed

in

a

case

suitable

for

conventional

semi

-

flush

panel

All

connections

to

the

relay

are

made

at

terminals

located

on

the

rear

of

Relays

with

catalog

numbers

starting

with

419

are

similar

in

function

to

relays

of

the

earlier

219

series

,

but

offer

totally

drawout

construction

with

integral

test

faci

1

ities

.

and

blade

assembly

upon

removal

of

the

relay

from

its

case

,

are

of

partial

drawout

construction

,

with

the

input

transformers

remaining

in

the

case

upon

withdrawal

of

the

lower

electronic

circuit

board

.

Current

transformer

shorting

is

accomplished

by

a

direct

-

acting

spring

Units

of

the

219

series

PRECAUTIONS

The

following

precautions

should

be

taken

when

applying

these

relays

:

1

.

Incorrect

wiring

may

result

in

damage

,

diagram

before

energizing

,

correct

polarity

.

Be

sure

wiring

agrees

with

the

connection

Be

sure

that

the

dc

control

voltage

is

applied

in

the

2

.

Apply

only

the

rated

dc

control

voltage

marked

on

the

front

panel

.

3

.

Do

not

attempt

to

change

a

current

tap

setting

while

the

relay

is

in

service

.

This

wi

11

likely

cause

the

relay

to

trip

due

to

the

mismatch

created

.

(

Pulling

the

tap

does

not

open

the

ct

circuit

.

)

If

a

control

wiring

insulation

test

is

4

.

High

voltage

tests

are

not

recommended

,

required

,

withdraw

the

unit

from

its

case

before

applying

the

test

voltage

.

5

.

Follow

test

instructions

to

verify

that

the

relay

is

in

proper

working

order

.

CAUTION

:

since

troubleshooting

entails

working

with

energized

equipment

,

care

should

be

taken

to

avoid

personal

shock

.

Only

competant

technicians

familiar

with

good

safety

practices

should

service

these

devices

.

PLACING

THE

RELAY

INTO

SERVICE

1

.

RECEIVING

.

HANDLING

.

STORAGE

Upon

receipt

of

the

relay

(

when

not

included

as

part

of

a

switchboard

)

examine

for

shipping

damage

.

If

damage

or

loss

is

evident

,

file

a

claim

at

once

and

promptly

notify

Asea

Brown

Boveri

.

Use

normal

care

in

handling

to

avoid

mechanical

damage

.

Keep

the

relay

clean

and

dry

.

2

.

INSTALLATION

the

outline

dimensions

and

panel

drilling

are

given

in

Figure

1

.

Mounting

:

Courtesy of NationalSwitchgear.com

Transformer

Differential

Relay

IB

7.6

.

1.7

-

4

Page

3

Connections

:

Typical

connection

diagrams

for

two

and

three

Internal

connections

in

Figure

2

.

winding

transformers

are

shown

in

Figures

3

and

4

.

Special

care

should

be

taken

to

connect

the

dc

control

power

in

the

proper

polarity

.

These

relays

have

a

metal

front

panel

which

is

connected

through

printed

circuit

board

runs

to

a

rear

terminal

marked

"

G

"

.

In

all

applications

this

terminal

should

be

wired

to

ground

.

The

87

T

relay

includes

an

external

resistor

which

is

supplied

mounted

to

the

case

and

wired

between

terminals

9

and

operation

.

This

resistor

must

be

connected

for

proper

10

.

Note

:

Connections

are

critical

in

this

application

.

Most

problems

experienced

during

commissioning

are

due

to

incorrect

connections

:

interchanged

phases

,

or

ct

polari

-

ties

,

or

incorrect

make

-

up

of

the

delta

ct

connections

.

Using

the

transformer

nameplate

drawing

as

the

reference

,

a

phasing

-

check

should

be

made

to

be

sure

that

the

ct

secondary

connections

are

made

up

properly

for

the

particular

transformer

winding

configuration

.

In

case

of

difficulty

during

commissioning

,

a

plug

-

in

test

unit

is

available

on

loan

from

the

factory

as

an

aid

in

analyzing

connection

problems

;

however

,

significant

load

current

must

be

flowing

to

use

this

device

.

3

.

SETTINGS

Procedures

for

calculating

settings

for

a

particular

installation

are

given

in

the

Application

section

.

CURRENT

TAPS

:

The

ct

secondary

currents

seen

by

the

relay

are

usually

unequal

due

to

the

primary

to

secondary

ct

’

s

on

the

primary

and

secondary

sides

of

the

transformer

.

ratio

of

the

transformer

being

protected

and

the

choice

of

ratios

for

the

The

tap

settings

are

selected

to

equalize

the

input

currents

seen

by

the

differ

-

ential

measurement

unit

of

the

relay

.

The

tap

values

and

ratios

provided

are

shown

in

Table

1

.

Setting

calculations

will

determine

the

tap

selection

required

for

the

high

-

side

and

low

-

side

ct

inputs

.

It

is

important

to

identify

which

input

terminals

of

the

relay

are

connected

to

the

high

-

side

ct

’

s

and

to

the

low

side

ct

’

s

.

these

Then

when

making

settings

,

to

relate

the

front

panel

tap

to

the

input

current

terminals

it

affects

:

the

left

side

tap

for

input

terminals

1

-

2

,

the

right

side

tap

for

terminals

5

-

6

,

and

the

center

tap

(

on

3

winding

units

)

for

input

terminals

3

-

4

.

%

DIFFERENTIAL

:

The

ABB

Circuit

-

Shield

Type

87

T

relay

operates

when

its

input

currents

differ

in

magnitude

(

vectorial

1

y

)

.

The

operating

point

occurs

when

the

difference

,

expressed

as

a

percentage

of

the

smaller

,

exceeds

the

setting

.

The

percentage

differential

or

"

slope

"

setting

is

made

with

the

left

-

hand

front

panel

dial

.

illustrated

in

the

graphs

of

Figures

6

and

7

.

the

operate

current

relative

to

the

tap

value

,

This

continuously

adjustable

control

allows

settings

from

15

to

40

%

,

and

is

In

these

plots

the

vertical

axis

is

Mo

=

(

Ii

/

T

1

)

-

(

I

2

/

T

2

)

and

the

horizontal

axis

is

the

smaller

restraint

current

relative

to

tap

value

MR

=

I

2

/

T

2

The

Percent

Differential

is

D

=

MO

/

MR

where

i

nto

and

i

nto

11

is

the

1

arger

restraint

current

T

1

the

tap

value

,

12

is

the

smaller

restraint

current

T

2

the

tap

value

.

Note

:

MR

=

R

when

I

2

/

T

2

<

R

where

R

(

adjustable

)

Minimum

Restraint

Courtesy of NationalSwitchgear.com

Transformer

Differential

Relay

IB

7.6

.

1.7

-

4

Page

4

MINIMUM

RESTRAINT

(

Minimum

Operating

Current

)

:

The

minimum

operating

current

is

set

by

the

right

-

hand

dial

on

the

front

panel

,

which

is

continuously

adjustable

.

The

minimum

operating

current

value

is

determined

by

the

following

relationship

between

the

restraint

dial

setting

,

the

percent

differential

setting

,

and

the

current

tap

setting

:

(

D

/

(

100

+

D

)

x

R

x

T

Im

D

=

%

differential

setting

R

=

adj

min

restraint

setting

T

=

tap

setting

Im

=

minimum

operating

current

in

amperes

where

For

example

,

with

D

=

25

%

,

R

=

2.3

and

T

=

5

A

:

(

25

/

125

)

x

2.3

x

5

46

%

X

5

A

2.3

A

Im

effect

of

varying

the

Minimum

Restraint

adjustment

is

graphically

illustrated

in

The

Figure

7

and

tabulated

in

this

chart

:

Restraint

Dial

Setting

Minimum

Operate

Current

Percent

Differential

Setting

30

%

of

tap

1

.

05

40

%

1

.

5

43

%

65

%

2.3

21

%

25

%

1

.

05

30

%

1

.

5

45

%

2.3

1

.

05

14

%

15

%

1

.

5

20

%

30

%

2.3

The

setting

that

provides

a

30

-

40

%

minimum

operating

current

would

be

typical

for

most

applications

.

INSTANTANEOUS

:

The

instantaneous

element

operates

directly

on

magnitude

of

the

measured

current

difference

.

This

internal

adjustment

is

factory

set

at

10

x

tap

,

but

may

be

adjusted

from

8

to

20

x

as

may

be

required

by

the

application

.

This

element

must

be

set

by

test

.

Refer

to

the

section

on

Testing

for

the

calibration

procedure

.

HARMONIC

RESTRAINT

:

The

harmonic

restraint

unit

blocks

relay

operation

on

transformer

energization

by

detection

of

the

second

harmonic

content

of

the

inrush

current

.

This

element

is

factory

set

to

restrain

on

15

%

second

harmonic

,

which

is

appropriate

for

most

applications

.

An

internal

calibration

adjustment

provides

a

range

of

approximately

10

-

20

%

,

and

may

be

set

by

test

.

Refer

to

the

section

on

Testing

for

the

calibration

procedure

.

Courtesy of NationalSwitchgear.com

Transformer

Differential

Relay

IB

7.6

.

1.7

-

4

Page

5

APPLICATION

DATA

The

ABB

Circuit

-

Shield

Type

87

T

provides

high

-

speed

differential

protection

for

power

transformers

.

The

Type

87

T

is

a

single

-

phase

relay

.

A

set

of

three

relays

is

required

for

the

pro

-

tection

of

one

power

transformer

.

The

relay

is

offered

in

models

for

the

protection

of

two

winding

and

three

winding

transformers

.

The

relay

has

a

linear

percentage

differential

characteristic

in

order

to

accomodate

ct

ratio

mismatch

,

the

operation

of

on

-

load

tap

changers

,

and

ct

errors

at

high

fault

currents

external

to

the

protected

zone

.

Controls

are

provided

on

the

front

panel

to

adjust

the

slope

(

15

-

40

%

)

,

and

the

minimum

operate

current

.

A

second

-

harmonic

restraint

unit

prevents

relay

operation

on

transformer

magnetizing

inrush

current

.

content

,

but

may

be

adjusted

if

required

.

This

unit

is

factory

calibrated

to

restrain

on

15

%

second

harmonic

An

unrestrained

instantaneous

unit

which

operates

on

the

magnitude

of

the

difference

current

is

provided

to

back

up

the

percentage

differential

unit

.

The

instantaneous

unit

is

factory

set

at

10

x

tap

,

but

may

be

adjusted

(

8

-

20

x

)

in

the

field

.

The

relay

has

ratio

correcting

taps

,

therefore

,

external

auxiliary

ct

’

s

will

not

The

high

continuous

rating

of

each

tap

gives

the

user

more

The

low

burden

of

the

relay

(

see

table

3

)

provides

usually

be

required

,

flexibility

in

selecting

taps

,

improved

ct

performance

.

A

procedure

for

calculating

the

relay

settings

for

a

particular

application

follows

on

page

10

.

MOTE

t

DIMENSIONS

ARE

-

^

5

^

-

MM

8.36

213

6.56

6.66

1.19

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167

30.2

t

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TEST

CASE

STUD

NUMBERS

(

BACK

VIEW

)

3.19

81.0

6.62

168

Figure

1

:

Relay

Outline

and

Panel

Drilling

Courtesy of NationalSwitchgear.com

IB

7.6

.

1.7

-

4

Page

6

Transformer

Differential

Relay

Specifications

Input

Circuits

:

Ratio

Matching

Taps

:

Continuous

rating

:

1

Second

rating

:

Burden

:

see

Table

1

.

see

Table

2

.

200

Amperes

,

any

tap

.

see

Table

3

.

Percentage

Restraint

Element

:

Minimum

operate

current

:

adjustable

13

-

65

*

of

tap

setting

.

Slope

:

adjustable

15

-

40

*

.

Refer

to

Figures

6

and

7

.

Instantaneous

Element

:

Internally

adjustable

8

-

20

x

tap

.

Factory

set

at

10

x

tap

.

Harmonic

Restraint

Element

:

Restrain

on

2

nd

harmonic

.

Internally

adjustable

approximately

10

*

-

20

*

.

Factory

setting

-

15

*

.

Operating

Time

:

See

Figure

5

.

Output

Contact

Rating

:

@

125

vdc

@

250

vdc

closing

/

tripping

continuous

break

30

A

30

A

5

A

5

A

0.3

A

0.1

A

Operating

Temperature

Range

:

-

20

to

+

70

°

C

models

available

for

125

vdc

@

0.05

A

max

.

drain

110

vdc

@

0.05

A

max

.

drain

250

vdc

@

0.05

A

max

.

drain

220

vdc

@

0.05

A

max

.

drain

48

vdc

@

0.05

A

max

.

drain

32

vdc

and

24

vdc

:

Control

Voltage

:

(

allowable

range

100

-

140

vdc

)

(

allowable

range

88

-

125

vdc

)

(

allowable

range

200

-

280

vdc

)

(

allowable

range

176

-

250

vdc

)

(

allowable

range

38

-

58

vdc

)

see

note

2

below

,

and

page

22

.

TABLE

4

:

CATALOG

NUMBERS

-

COMMON

.

UNITS

Control

Voltage

Catalog

Numbers

Std

Case

Test

Case

System

Freq

.

Type

Windings

87

T

48

vdc

11

Ovdc

125

vdc

250

vdc

Two

60

Hz

.

219

T

2431

219

T

2401

219

T

2441

219

T

2451

419

T

2431

419

T

2401

419

T

2441

419

T

2451

48

vdc

11

Ovdc

125

vdc

220

vdc

50

Hz

.

219

F

2431

219

F

2401

219

F

2441

219

F

2421

419

F

2431

419

F

2401

419

F

2441

419

F

2421

48

vdc

11

Ovdc

125

vdc

87

T

Three

60

Hz

.

219

T

3431

219

T

3401

219

T

3441

419

T

3431

419

T

3401

419

T

3441

50

Hz

.

48

vdc

11

Ovdc

125

vdc

219

F

3431

219

F

3401

219

F

3441

419

F

3431

419

F

3401

419

F

3441

Select

419

series

units

for

new

applications

due

to

the

improved

testing

features

.

Notes

:

1

.

2

.

Contact

factory

for

availability

of

units

for

control

voltage

ratings

not

listed

.

Type

96

dc

/

dc

inverter

and

48

vdc

rated

87

T

:

see

page

22

.

For

24

or

32

vdc

control

use

Courtesy of NationalSwitchgear.com

Transformer

Differential

Relay

IB

7.6

.

1.7

-

4

Page

7

Type

87

T

Tap

Ratios

TABLE

1

3.2

4.0

2.9

3.6

4.5

5.0

8.7

2.9

1.00

1.10

1.24

1.38

1

.

72

3.00

1

.

55

3.2

1

.

00

1

.

25

1.13

1

.

41

1.56

2.72

3.6

1

.

00

1.11

1.39

2.42

1

.

25

4.0

1.00

1.13 1.25

2.18

4.5

1.00

1.11

1.93

5.0

1

.

00

1.74

8.7

1.00

Continuous

Current

Ratings

TABLE

2

TABLE

3

Burden

Data

Tap

Setting

Continuous

Rating

Tap

Setting

Ohms

P

.

F

.

2.9

A

3.2

A

3.6

A

4.0

A

4.5

A

5.0

A

8.7

A

1

0

A

2.9

A

3.2

A

3.6

A

4.0

A

4.5

A

5.0

A

8.7

A

0.05

0.04

1.0

10

A

1.0

10

A

1.0

1

OA

1

.

0

1

OA

1.0

1

OA

1

.

0

17

A

1.0

Figure

2

:

Internal

Connection

Diagrams

16

D

419

B

Transformer

Differential

Relay

Drawout

Test

Case

Type

87

T

Type

87

T

Transformer

Differential

Relay

Standard

Case

16

D

219

B

++

1

06

O

5

o

3

02

ol

fccJ

fee

?

1

5

a

I

T

G

JT

°

G

r

°

ii

i

$

1

E

$

15

OlH

Oja

$

12

$

11

10

?

q

ID

9

*

Notes

:

1

.

Omit

coil

3

-

4

for

two

winding

relays

.

2

.

The

tap

block

associated

with

coil

1

-

2

is

on

left

hand

side

,

when

viewed

from

the

front

of

the

relay

.

3

.

The

tap

block

for

coil

5

-

6

is

on

the

right

hand

side

.

4

.

If

provided

,

the

tap

block

for

coil

3

-

4

is

in

the

center

.

5

.

The

external

resistor

shown

connected

between

terminals

9

and

10

is

supplied

mounted

on

the

relay

and

must

be

in

place

for

proper

operation

.

6

.

The

diode

shown

between

terminals

15

and

16

is

used

for

testing

only

.

See

test

procedures

.

Courtesy of NationalSwitchgear.com

Transformer

Differential

Relay

IB

7.6

.

1.7

-

4

Page

8

Connections

shown

for

Phase

Rotation

a

-

b

-

c

Connections

shown

for

Phase

Rotation

a

-

b

-

c

Delta

-

Wye

Transformer

Delta

-

Delta

Transformer

b

a

c

b

a

c

52

-

1

52

-

1

H

2

r

;

ii

m

r

?

n

rt

-

A

„

,

Hi

DLJL

|

»

y

LhJL

nnr

!

£

2

JT

|

<

i

6

-

C

£

J

fsjt

:

!

<

5

!

£

!

"

5

j

®

I

|

oo

L

.

_

J

®

L

)

i

*

1

CM

CO

*

3

£

I

?

5

£

?

5

£

co

|

_

J

co

I

J

oo

L

_

t

5

J

52

-

2

52

-

2

*

+

<

>

87

T

-

3

1

*

87

T

-

2

"

S

4

=

rM

87

T

-

1

8

7

8

7

12

11

10

9

\

6

ovo

12

11

10

9

h

6

o

^

o

12

11

10

9

<

Hki

ovo

D

.

C

.

Control

Power

ft

5

f

r

~

r

.

86

X

52

-

2

86

52

-

1

a

^

86

52

-

1

52

-

2

TC

+

Typical

Connections

for

Differential

Protection

Two

-

Winding

Three

-

Phase

Power

Transformer

Figure

3

:

Note

:

using

the

transformer

nameplate

drawing

as

a

reference

,

a

"

phasing

-

check

"

should

be

made

to

insure

that

the

ct

secondary

connections

are

made

up

properly

for

the

particular

transformer

winding

configuration

.

Courtesy of NationalSwitchgear.com

I

2

3

2

3

H

3

52

-

1

52

-

3

93

!

92

94

3

I

92

94

96

'

6

2

94

961

o

>

Irirta

CSJ

IO

,

I

o

K

H

I

H

I

N

-

I

3

K

I

CO

1

±

N

3

!

00

®

I

®

I

3

45

*

1

9.5

J

3

&

3

o

“

b

~

b

Y

2

®

"

5

(

D

YI

3

r

+

Y

3

93

n

_

p

..

p

ZO

®

0

)

v

:

52

-

2

XI

X

3

+

4

C

IW

=

1

'

87

T

-

2

87

T

-

3

87

T

-

1

[

M

H

8

7

12

1110

9

o

4

W

ovo

12

11

10

9

0

-

4

KJ

OVO

12

11

10

9

y

6

o

+

*

o

D

.

C

.

ff

Sr

r

Control

Power

“

X

xlx

“

i

52

-

3

86

52

-

1

aa

a

^

86

52

-

1

52

-

3

52

-

2

M

2

3

TC

CO

-

J

O

)

“

O

*

0

)

CD

•

®

-

J

Figure

4

:

Typical

Connections

for

Differential

Protection

-

Three

-

Winding

Three

-

Phase

Transformer

to

Courtesy of NationalSwitchgear.com

Transformer

Differential

Relay

IB

7.6

.

1.7

-

4

Page

10

CALCULATION

OF

SETTINGS

:

A

procedure

for

calculating

the

relay

settings

follows

,

end

of

the

procedure

.

An

example

is

given

at

the

1

.

Determine

the

maximum

load

currents

of

the

transformer

,

on

the

high

-

side

IH

and

the

1

ow

-

side

II

.

2

.

Determine

the

maximum

through

-

fault

currents

,

IHF

and

ILF

.

3

.

Choose

the

ct

ratio

in

accordance

with

step

1

to

give

approximately

5

A

secondary

current

at

the

maximum

load

current

;

and

,

also

so

that

the

maximum

external

fault

current

is

less

than

100

amperes

secondary

.

others

,

check

that

they

meet

the

requirements

above

.

applications

,

through

-

fault

current

is

limited

by

the

transformer

impedance

.

)

If

the

ct

ratios

have

been

specified

by

(

Note

that

for

2

winding

4

.

Calculate

the

load

currents

on

the

ct

secondary

side

,

IHS

and

ILS

.

remembering

to

multiply

by

for

ct

5

.

Calculate

the

relay

currents

,

IHR

and

ILR

secondaries

connected

in

delta

.

6

.

Calculate

the

ratio

of

the

relay

currents

taking

the

1

arger

/

smal

1

er

A

.

7

.

Referring

to

Table

1

,

choose

the

taps

so

that

the

tap

ratio

(

T

)

is

closest

to

the

relay

current

ratio

(

A

)

.

Properly

assign

the

larger

and

smaller

value

tap

selections

to

the

high

-

side

and

low

-

side

relay

currents

culated

in

step

7

.

in

the

same

manner

as

(

A

)

was

cal

-

if

more

than

one

selection

of

taps

is

available

for

the

tap

ratio

desired

usual

practice

to

choose

tap

values

closest

to

the

normal

relay

currents

IHR

and

ILR

.

I

t

i

s

8

.

Check

that

the

relay

currents

are

within

the

continuous

rating

of

the

relay

refer

to

Table

2

.

|

(

T

-

A

)

/

(

sma

11

er

of

T

or

A

)

|

10

.

Select

the

slope

(

percentage

differential

)

setting

.

Both

the

mismatch

and

transformer

’

s

load

tap

changer

range

must

be

considered

.

S

=

M

+

LTC

.

security

,

the

25

%

setting

should

be

used

for

calculated

slopes

of

15

%

and

below

,

the

40

%

setting

for

calculated

slopes

of

15

-

30

%

.

M

=

Mismatch

9

.

Calculate

the

mismatch

:

the

For

and

11

.

Select

the

Instantaneous

setting

.

This

internal

adjustment

is

factory

set

at

10

x

tap

value

.

The

recommended

setting

is

approximately

10

times

the

power

transformers

self

-

cooled

load

current

rating

:

(

a

)

Calculate

the

relay

current

at

the

transformers

self

-

cooled

rating

.

(

b

)

Divide

by

the

high

-

side

relay

tap

value

.

(

c

)

Multiply

by

1.2

for

margin

.

Set

the

internal

potentiometer

R

119

using

the

calibration

procedures

given

in

the

section

on

Testing

.

The

maximum

burden

presented

to

a

ct

is

given

by

the

12

.

CT

performance

check

:

following

equations

:

2.3

RL

+

RB

+

RA

for

wye

-

connected

ct

’

s

.

3.4

RL

+

3

RB

+

3

RA

for

delta

-

connected

ct

’

s

.

(

this

includes

a

correction

factor

for

the

increased

resistance

of

the

leads

due

to

self

-

heating

effects

)

R

RL

is

the

one

-

way

lead

resistance

from

ct

to

the

relay

.

RB

is

the

relay

burden

.

RA

is

the

burden

of

other

devices

in

series

with

the

relay

.

where

(

See

Table

3

)

CT

performance

should

be

satisfactory

if

the

maximum

external

fault

current

flowing

through

R

does

not

require

a

voltage

higher

than

one

-

half

the

ANSI

Class

C

accuracy

rating

of

the

ct

.

Therefore

Vc

>

2

x

RIFS

must

be

satisfied

for

both

the

high

-

side

and

low

-

side

ct

’

s

.

(

Note

:

if

a

multi

-

ratio

ct

is

being

used

,

and

if

the

full

secondary

winding

is

not

being

used

,

then

adjust

the

"

C

"

rating

by

the

factor

NS

/

NT

to

account

for

the

lower

effective

"

C

“

rating

.

(

Ns

=

secondary

turns

used

;

NT

-

total

secondary

turns

.

)

13

.

Minimum

operating

current

:

a

setting

giving

25

-

40

%

minimum

operating

current

would

be

usual

practice

.

See

chart

on

page

4

.

Courtesy of NationalSwitchgear.com

ABB CIRCUIT SHIELD 87T User manual

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