Biddle MEGGER - Page 59

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]

tage

rating

may

be

taken

as

(

s

VANG

క

$

x

R,,

where

V,

is

the

rated

voltage

of

the

instrument

and

R,

is

the

standard

resistance

under

test.

When

R,

is

made

up

of

units

in

series,

Re=R;+R-2e

;

δον;

VNE

ΕΤ,

etc.,

the

watts

in

any

one

unit

is:

R

©)

x

R,,

where

R,

Ng

is

the

resistance

of

that

particular

unit.

As

shown

on

pages

53

and

54,

V,

will

vary

with

the

load

and

wattage

capacity

of

the

resistance

standard

may

be

lessened

as

V,

is

reduced.

This

method

MUST

be

used

on

rectifier

operated

and

dual

operated

Meg

Instruments

and

certain

Megger

Instruments.

as

will

be

explained

later.

Shunting

Method:

The

second

method

involves

the

use

of

only

a

limited

number

of

resistance

standards,

supplemented

by

the

shunt

principle.

The

choice

of

this

method

is

based

on

commercially

available

standards

rated

from

1

to

10

megohmis

in

10

steps,

1

watt

per

step.

A

rheostat,

covering

a

range

up

to

approximately

50,000

ohms

is

also

required.

The

shunting

method

can

be

best

understood

by

noting

the

‘typical

circuit

in

Fig.

45.

Here,

the

resistance

for

a

divide-by-10

shunt

must

be

exactly

1/9

of

the

total

A

or

deflecting

coil

circuit

resistance,

(consisting

of

R’

and

the

resistance

of

the

A

coil

in

series).

A

rheostat

of

1/9

of

the

total

resistance

(R’

and

A)

is

connected

between

the

LINE

and

GUARD

terminals.

This

makes

the

total

resistance

between

Line

and

Guard

exactly

1/10

the

value

for

direct

reading.

Therefore,

only

1/10

of

the

load

current

flows

through

the

A

coil

circuit,

causing

the

pointer

to

indicate

10

times

the

actual

resistance

connected

to

the

Earth

and

Line

terminals.

This

same

shunt

principle

may

be

used

to

check

the

entire

resistance

range

above

1

megohm

of

Megger

Insulation

Test-

‘ers

with

only

a

limited

προς

of

resistance

standards

avail-

able.

A

single

range

Meg

type

of

Insulation

Tester,

Cat.

|

#7680,

rated

at

1,000

V

and

2,000

megohms,

will

be

given

|as

an

example

and

checked

by

using

a

1

to

10

megohm

stand-

ard

cited

above.

The

standard

resistance,

having

10

steps

of

l

to

10

megohms,

is

first

used

to

check

the

corresponding

scale

|

graduations

on

the

Instrument

directly

by

connecting

to

the

Earth

and

Line

terminals,

and

then

switching

through

the

కం

of

the

standard.

After

the

10

68010

graduation

is

|

[57]

|