Difference in Models

TM 5-6625-2691-13&P

1 - 7 . Difference in Models

auxiliary contacts used in the control circuit which

are not present on the Model A427.

This manual describes the Military Standard

Model A427 and Model 427B electrical test sets.

The Model A427 and Model A427B test sets are

e. Voltage Sensing Module. The Voltage Sensing

identical in function, capacity and operation. How-

Module on the two models is physically different,

ever, there are numerous internal changes which

and is not physically interchangeable. Model A427

makes most of the internal major assemblies non-

uses an internal control relay within the module.

Model A427B is all solid state. Also Model A427B

interchangeable. The following is a tabulated listing

of salient differences between the two models:

i n c o r p o r a t e s a time delay feature in the wind

a. Unit Enclosure. Figure 1-6 and figure 1-6.1

switch control circuit which allows the test set to

operate for about 5 seconds with the wind switch

are envelop line drawings of Model A427 and Model

open.

A427B which delineate the external package details.

f. Rectifier Assembly. The Model A427 test set

As illustrated the major difference is that skid run-

uses a selenium rectifier stack to generate the 28

ners were added to the bottom of Model A427B.

VDC control power required for fan operation. On

This raises the base of the unit approximately 6

Model A427B three separate silicon rectifier

inches and facilitates fork lift truck handling of the

bridges are used. Also the Model A427B requires

test set. Also, swing-down handles were added to

two 25 watt dropping resistors not used on Model

these skid runners to better facilitate hand lifting

of the test set by four men. The hand lift handles

A427. These dropping resistors are mounted with

were removed from the top of the housing as pro-

the silicon rectifiers on a single plate with a ter-

vided in Model A427. The housing assembly of the

minal strip,

two units is interchangeable.

g. Control Transformer. The Control Transform-

b. Control Panel. The control panel of Model

ers on the two models are physically different; and

A427 contains three (3) glass cartridge fuses for

not interchangeable. On Model A427B, a change in

p r o t e c t i o n of the unit control circuit. On Model

the control circuit reduced the VA capacity require-

A427B these fuses were replaced with three (3)

ment of these transformers.

thermal lag automatic circuit breakers. All other

h. Power Absorbers. The construction of the in-

controls and the method of operation of the two

dividual load elements within each Power Absorber

models are identical.

is different on the two model test sets. The electri-

c. Terminal Bracket Mounting. On Model A427

cal capacity and function of corresponding elements

the INPUT TERMINAL BOARD contains four (4)

of the two models is the same. The physical size

terminals which are .312 inch diameter studs with

and shape of the two Power Absorber assemblies is

insulating screw-on type clamping nuts for hand

also the same. However, differences in the method

tightening without tools. On Model A427B these

of electrical termination of the elements within the

Power Absorber are different. For this reason, the

studs are similar in form except they are .375 inch

P o w e r Absorbers of Model A427 are not inter-

diameter, and two studs are provided for connec-

changeable with Model A427B Power Absorbers.

tion of lead LO or neutral. The ground stud on

i. Fan Motor. The Fan Motors of Model A427

both models is identical. The input voltage select-

and Model A427B are identical, and interchange-

ing VOLTAGE RECEPTICAL is the same on both

able.

models. The operating procedures for connection

j. Wind Switch. The Wind Switch used on Model

of supply and selection of operating voltage is the

A427B is different than the switch used on Model

same for both models.

A427. The wind switches are not physically inter-

d. Contactor. T h e I N P U T P O W E R C O N T A C -

changeable.

TOR (K1) is physically different on the two

k. Variable Transformer. The Variable Trans-

models; and is not physically interchangeable. The

former of Model A427 and Model A427B are iden-

energizing coil electrical characteristics are differ-

ent in the two models. Also Model A427B has two

tical, and interchangeable.

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