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APPENDIX

Instructions

for

Checking

the

Calibration

of

Megger

Insulation

Testers—AIll

Types

In

checking

the

calibration

of

Megger

Insulation

Testers

two

methods

are

suggested.

One

method

involves

the

use

of

direct

measured

resistance

standards

for

the

full

range.

These

are

usually

difficult

to

obtain

except

at

great

expense.

The

second

method

makes

use

of

low

resistance

standards,

adapted

to

the

full

range

by

means

of

shunting.

This

permits

use

of

commercially

available

standards.

As

an

aid

to

understanding

the

application

of

the

two

methods,

refer

to

Figure

47,

in

which

a

Guard

terminal

and

the

guard

system

are

shown.

As

explained,

all

types

of

Meg-

ger

Insulation

Testing

Instruments

have

guard

systems,

ex-

cept

the

Midget

type.

However,

the

Guard

terminal

is

not

used

on

low

range

types

(below

1,000

megohms,

500

V),

even

though

internally

the

elements

of

the

guard

system

are

pres-

ent.

On

Megger

Instruments

not

having

Guard

terminals,

and

manufactured

prior

to

approximately

1948,

the

external

guard

connection

may

be

made

to

the

brass

disk

in

back

of

the

hard-rubber

disk

on

which

the

Line

terminal

is

mounted.

This

brass

disk

is

connected

directly

to

the

internal

guard

system.

The

guard

disk

of

the

Meg

Instrument

is

set

in

a

deep

groove

in

the

insulating

washer

supporting

the

Line

terminal.

The

connection

to

the

internal

guard

system

can,

therefore,

be

made

by

winding

a

fine

wire

tightly

in

the

groove.

If

the

Guard

connection

is

not

externally

accessible

the

direct

substitution

method

must

be

used.

Direct

Substitution

Method:

When

using

the

direct

substitution

method,

it

is

only

neces-

sary

to

connect

a

standard

resistance

unit

of

suitable

multiple

values

between

the

Earth

and

Line

terminals

of

the

Instru-

ment.

The

calibration

is

checked

by

operating

the

generator

at

rated

speed

and

reading

the

scale.

The

reading

corresponds

to

the

value

of

standard

resistance

inserted

between

Line

and

Earth.

There

are

several

precautions

to

be

observed

in

using

this

method.

For

higher

range

measurements,

1000

megohms

and

up,

the

resistance

should

be

adequately

guarded

against

leakage,

as

explained

in

Chapter

VII.

Also,

the

wattage

rat-

ing

of

the

resistance

used

should

not

be

exceeded.

The

wat-

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