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Vill

TESTS

USING

MULTI-VOLTAGE

MEGGER

INSULATION

TESTERS

Maintenance

practice

trends

indicate

the

value

of

testing

insulation

with

d-c

voltages

at

levels

somewhat

higher

than

the

peak

value

of

the

rated

a-c

voltage

of

the

equipment

being

tested.

Such

d-c

tests

have

in

some

cases

been

shown

to

reveal

nondestructively

incipient

weaknesses

in

insulation

which

could

not

otherwise

be

found

except

possibly

by

corona

detection

at

nondestructive

a-c

test

voltage

levels.

The

technique

involves

the

application

of

two

or

more

d-c

voltages,

and

critically

observing

any

reduction

of

insulation

resistance

at

the

higher

voltage.

Any

marked

or

unusual

re-

duction

in

insulation

resistance

for

a

prescribed

increase

in

applied

voltage

is

an

indication

of

an

incipient

weakness.

It

is

important

to

mention

that

the

merits

of

this

technique

arise

from

more

recent

investigations

which

indicate

that

rather

high

d-c

voltage

can

be

used

to

detect

weaknesses

with-

out

damaging

the

insulation.

The

maximum

value

of

voltage

which

should

be

used

will

depend

largely

on

the

cleanliness

and

dryness

of

the

insulation

to

be

tested.

In

making

tests

on

insulation

at

such

6-6

voltages

the

ohm-

meter

method

has

at

least

two

advantages.

First,

prescribed

fixed

voltages

are

switched

into

use,

and

one

instrument

meas-

urement

made

with

the

direct

reading

ohmmeter.

This

is

a

more

simple

and

reproducible

method

than

one

in

which

many

choices

of

voltage

are

available.

Another

important

ad-

vantage

of

the

ohmmeter

can

best

be

explained

by

referring

to

Figure

43.

In

this

figure

the

change

which

may

occur

in

leakage

current

after

the

absorption

current

has

disappeared

is

shown

plotted

in

terms

of

insulation

resistance

against

three

different

voltages.

It

is

noted

that

there

is

no

change

in

resistance

shown

in

the

figure

between

500

and

1000

volts,

indicating

no

change

in

the

insulation

as

a

result

of

applying

these

two

voltages.

This

is

an

assumption,

but

is

a

condition

which

is

not

uncommon

in

practice.

If

the

insulation

contin-

[38]