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Minimum

Values

for

Insulation

Resistance

Rotating

Machinery

Report

+43,

entitled

“Recommended

Practices

for

Test-

ing

Insulation

Resistance

of

Rotating

Machinery”

published

by

the

American

Institute

of

Electrical

Engineers,

deals

with

the

problem

of

making

and

interpreting

insulation

resistance

measurements

for

rotating

machinery.

It

reviews

the

factors

which

affect

or

change

insulation

resistance

characteristics,

outlines

and

recommends

uniform

methods

for

making

tests,

and

presents

formulas

for

the

calculation

of

approximate

min-

imum

insulation

resistance

values

for

various

types

of

a-c

and

d-c

rotating

machinery.

It

is

strongly

recommended

that

those

who

operate

and

main-

tain

rotating

machinery

with

ratings

of

1000

kva

and

above

in

the

a-c

class

and

100

kw

and

above

in

the

d-c

class,

obtain

copies

of

this

publication

for

guidance.

They

may

be

obtained

direct

from

American

Institute

of

Electrical

Engineers,

33

West

39th

Street,

New

York

City

or

through

the

James

G.

Biddle

Co.

In

the

case

of

small

a-c

machinery

rated

999

kva

and

less,

this

report

states

that

since

these

small

units

are

not

likely

to

be

designed

for

use

on

high

voltages,

and

since

it is

seldom

that

users

of

these

machines

wish

more

than

a

simple

test

to

deter-

mine

the

insulation

condition,

the

following

simplified

formula

is

recommended.

It

is

in

effect,

the

old

rule

“One

megohm

per

1000

volts”

with

a

minimum

of

one

megohm:

R,

డ్󰾭

Kv

ol

]

where

Κι

=

Standard

insulation

resistance

of

class

A

or

B

stator

armature

windings

in

megohms

at

wind-

ing

temperatures

up

to

75°

C,

obtained

by

ap-

plying

500.

volts

d-c

to

the

entire

winding

for

one

minute.

Kv

=

Rated

machine

voltage

in

kilovolts

In

the

case

of

small

d-c

machines

rated

99

kv

and

less

the

report

states

that

since

these

small

units

are

not

likely

to

be

designed

for

use

on

high

voltages,

and

since

the

users

of

these

machines

seldom

wish

more

than

a

simple

test

to

determine

the

condition

of

the

insulation,

the

standard

insulation

resistance

for

these

armature

windings

should

be

of

the

order

of

one

meg-

ohm

at

winding

temperatures

up

to

75°

C,

obtained

by

apply-

ing

500

volts

d-c

to

the

winding

for

one

minute.

Bushings

In

the

case

of

outdoor

oil

circuit

breaker

bushings,

experi-

ence

has

shown

that

any

bushing,

with

its

assembled

associated

144]