Biddle MEGGER - Page 41

62 pages

Save Page as PDF

To Next Page

To Next Page

To Previous Page

To Previous Page

Loading...

ues

to

be

stable

at

2500

volts

there

will

be

no

change

in

the

insulation

resistance

value

obtained,

which

is

shown

by

the

dotted

extension

of

the

horizontal

line

in

the

figure.

When

nonlinear

conditions

appear

at

a

higher

voltage

the

voltage-

resistance

curve

reveals

this

very

clearly

by

a

lower

resistance

value,

indicated

by

the

downward

curve

in

the

figure.

The

figure,

therefore,

reveals

the

simplicity

of

determining

the

change

in

insulation

stability

by

using

three

fixed

voltages

which

are

easily

reproducible

when

making

3-voltage

tests

on

a

routine

basis.

We

wish

to

emphasize

that

the

curve

in

Figure

43

indicates

the

resistance

change

due

to

leakage

current

only,

and

not

the

absorption

current

which

may

appear

for

a

period

of

time

with

each

change

in

voltage.

It

may

be

necessary

to

wait

an

appreciable

time

after

each

voltage

change

for

the

absorption

current

to

disappear

before

taking

a

reading.

To

understand

better

the

technique

of

making

insulation

resistance

tests

at

two

or

more

voltages

the

following

steps

are

suggested,

using

an

industrial

or

traction

motor

classed

in

the

300

to

1000

volt

range

as

an

example:

1)

Make

a

one

minute

Megger

instrument

test

at

500

volts

to

serve

as

a

basis

of

comparison

for

subsequent

steps.

2)

After

a

careful

cleaning

operation

make

a

second

500

volt

Megger

instrument

test

to

determine

the

effective-

ness

of

this

cleaning.

3)

If

the

one

minute

insulation

resistance

value

is

sub-

normal,

or

if

the

60

second/30

second

insulation

resis-

tance

ratio

is

no

greater

than

one

at

this

point

then

a

drying

operation

may

be

desirable

before

using

a

higher

test

voltage.

However,

making

another

test

at

1000

volts

and

comparing

these

readings

with

those

from

the

500

volt

test

will

help

in

determining

the

need

for

drying.

If

the

1000

volt

test

value

is

appreciably

less

than

that

at

500

volts

then

a

drying

operation

should

be

per-

formed.

On

the

other

hand,

if

the

1000

volt

and

500

volt

test

values

are

approximately

the

same

it

is

reason-

able

to

assume

that

the

decision

to

perform

a

drying

operation

can

be

deferred

until

after

the

next

step.

4)

Make

a

Megger

instrument

test

at

2500

volts.

If

there

is

no

appreciable

difference

in

the

500

and

2500

volt

test

[39]