Biddle MEGGER - Page 55

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across

the

outer

surface

of

the

instrument

between

the

Earth

and

‘Line

terminals.

The

leakage

current

is

simply

shunted

away

from

the

Line

terminal

and

returns

to

the

current

supply

without

passing

through

the

deflecting

coil

of

the

ohmmeter.

POSITIONS

6

DISCHARGE

5

1000¥

DEFLECTING

σοι.

FIGURE

47.

Basic

diagram

of

Cat.

Νο.

21159

“Major

Megger

Insulation

Tester.

Figure

47

shows,

in

principle,

how

the

Guard

System

is

used

inside

the

instrument.

Most

“Megger”

Insulation

Testing

Instruments

have

the

Guard

System,

except

the

Midget

type,

which

is

of

low

range

and

has

a

molded

plastic

case

of

good

insulating

quality.

Voltage

Characteristics

When

using

a

“Megger”

Insulation

Tester

the

voltage

actually

applied

to

the

apparatus

under

test

is

a

function

of

the

resistance

under

test

in

series

with

the

instrument

ballast

resistance

R’

in

Fig.

45.

Voltage

regulation

of

the

generator

or

other

source

of

supply

may

be

considered

negligible.

The

voltage

characteristic

is

therefore

expressed

by

the

formula—

R

ταῦ

Where

V;=the

terminal

or

applied

voltage

R=Insulation

Resistance

of

the

apparatus

under

test

R’=Instrument

ballast

resistance

Vi=instrument

voltage,

i.e.,

the

voltage

generated

or

applied

to

the

instrument

[53]