Biddle MEGGER - Page 52

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PRINCIPLE

OF

OPERATION

OF

“MEGGER”

INSULATION

TESTING

INSTRUMENTS

The

“Megger”

ohmmeter

is

a

permanent

magnet,

cross-coil

in-

dicating

instrument

whose

accuracy

is

independent

of

the

exact

voltage

supplied

for

the

test.

No

compensation

or

adjustment

is

necessary-

The

ohmmeter

consists

essentially

of

two

coils—A

and

B

m

Figure

45,

mounted

on

the

same

moving

system

with

pointer

attached

and

free

to

rotate

in

a

permanent

magnet

field.

The

system

is

pivoted

in

spring-supported

jewel

bearings.

EARTH

LINEO

+

HAND

|

GENERATOR

OUMMETER

GUARD

FIGURE

45.

Basic

dicgram

of

electrical

connections

for

the

“Megger”

Insulation

Tester.

There

are

no

control

springs

such

as

are

used

in

ammeters

and

voltmeters.

Current

is

led

to

the

coils

by

flexible

conduct-

ing

ligaments

which

offer

the

least

possible

restraint,

so

that

when

the

instrument

is

level

and

no

current

is

supplied,

the

pointer

should

float

towards

Infinity.

Deflecting

and

Control

Coils

The

deflecting

coil

A

is

connected

in

series

with

a

fixed

ballast

resistance

R’

and

the

resistance

under

test,

the

latter

being

connected

to

the

Earth

and

Line

terminals.

[501