Biddle MEGGER - Page 61

62 pages

Save Page as PDF

To Next Page

To Next Page

To Previous Page

To Previous Page

Loading...

|

|

|

|

|

A+R’

be

calculated

from

the

formula

5

=

S

where

A

+

R’

is

the

measured

A

coil

circuit

resistance

and

f

is

the

scale

factor

(the

scale

graduation

to

be

checked

divided

by

the

value

of

the

standard

resistance).

As

an

example,

the

15

megohm

i

graduation,

one

of

the

two

missed

by

the

previous

example,

|

|

|

could

be

checked

by

this

method.

First,

the

resistance

is

meas-

ured

accurately

from

Line

to

Guard

and

recorded.

Then,

sub-

stituting

into

the

formula

AER

μοι

Coe

τι

ποτ

IE

using

a

megohm

standard.

The

shunt

is

then

set

for

1/14

of

the

A

coil

circuit

resistance.

When

the

shunt

is

connected

between

Line

and

Guard,

with

a

1

megohm

standard

between

Earth

and

Line,

the

Instrument

should

indicate

15

megohms.

On

the

Meg

type

Instrument

having

a

divide-by-10

switch,

this

switch

may

be

used

instead

of

an

external

shunt

to

extend

the

range

of

the

available

standard

resistances.

However,

the

'

resistance

values

between

Line

and

Guard

must

first

be

checked

to

insure

that

the

resistance

in

the

divide-by-l

range

is

exactly

10

times

the

resistance

in

the

divide-by-10

range.

If

the

available

standards

do

not

cover

the

entire

range

of

the

Instrument

by

using

the

divide-by-10

range,

the

switch

may

be

set

to

divide-by-1

and

external

shunts

used

instead.

|

Earlier

it

was

mentioned

that

the

shunt

method

was

ap-

plicable

only

to

certain

models.

The

shunt

method

may

be

used

on

all

hand

operated

models

of

the

Meg

type.

However,

it

cannot

be

used

on

rectifier

operated

types

because

the

bal-

last

resistance

is

inserted

in

the

earth

circuit

instead

of

the

line

circuit

to

protect

the

rectifier

against

external

short

cir-

cuits

between

the

Earth

and

Guard

terminals.

On

hand

oper-

ated

models

any

such

short

circuit

would

immediately

become

apparent

because

of

difficulty

in

cranking.

Furthermore,

it

is

impractical

to

shunt

the

A

coil

alone

since

tne

shunt

setting

would

never

be

correct

due

to

the

change

in

resistance

of

the

A

coil

caused

by

the

heating

effect

of

the

current.

This

same

resistance

change

is

present

in

the

hand

operated

types,

but

the

percentage

change

is

made

negligible

by

masking

this

small

change

by

the

relatively

large

ballast

resistor

in

series

[59:1

a