Biddle MEGGER - Page 60

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found

to

be

correct,

the

standard

is

switched

back

to

the

1

megohm

position

and

a

shunting

resistance

is

connected

be-

tween

the

Guard

and

Line

terminals.

The

value

of

shunting

resistance

is

determined

from

the

A

coil

circuit

resistance

of

approximately

400,000

ohms,

thus

the

necessary

shunt

must

have

at

least

1/9

of

this

value

to

give

a

multiplying

factor

of

10.

It

is

not

necessary

to

measure

this

value

by

auxiliary

means,

since

after

connecting

it

between

Line

and

Guard,

it

is

only

necessary

to

adjust

it

until

the

pointer

on

the

Instrument

again

indicates

exactly

10

megohms.

At

this

point

the

scale

factor

is

10,

so

the

shunting

resistance

must

be

exactly

1/9

(approximately

45,000

ohms)

of

the

A

coil

circuit

resistance.

Now

the

scale

graduations

from

10

to

100

megohms

can

be

checked

by

switching

the

standard

resistance

through

its

steps,

1

to

10

megohms.

When

the

100

megohm

graduation

has

been

checked,

the

standard

resistance

is

again

returned

to

the

1

megohm

posi-

tion

and

the

shunt

resistance

reduced

until

the

pointer

again

indicates

100

megohms.

At

this

point

the

shunt

is

1/99

(ap-

proximately

4,000

ohms)

of

the

resistance

in

the

A

coil

circuit,

giving

a

scale

factor

of

100.

With

the

shunt

adjusted,

the

scale

graduations

from

100

to

1,000

megohms

may

be

checked

by

again

switching

the

standard

resistance

through

its

steps.

When

the

1,000

megohm

graduation

has

been

checked,

the

standard

and

shunt

resistances

are

again

adjusted.

Now

the

shunt

resistance

is

adjusted

to

1/999

of

the

A

coil

circuit

re-

sistance,

giving

a

scale

factor

of

1,000.

The

2,000

megohm

graduation

is

checked

by

switching

the

standard

resistance

to

its

2

megohm

step.

Thus,

all

the

scale

graduation

above

1

megohm,

except

15

and

150

megohms,

are

checked.

These

two

values

are

not

checked

by

this

method

since

there

is

no

1.5

megohm

step

on

this

particular

standard.

An

alternate

procedure,

that

may

only

be

applied

to

the

hand

operated

Meg

type

Instruments

and

certain

other

Meg-

ger

Instruments,

involves

using

only

one

standard

resistance,

preferably

1

megohm

and

a

shunt

resistance.

However,

the

shunt

resistance

must

be

re-adjusted

for

each

scale

graduation

to

be

checked.

First,

the

A

coil

circuit

resistance

must

be

accurately

measured

from

Line

to

Guard

with

a

Wheatstone

bridge

and

recorded.

Then

each

shunt

resistance

value

5

must

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