ATO-
250™ Operating Procedures
1
1.0 Introduction
1.10 Applicability
This manual applies to the Series II of the
model ATO-250™, identified by the part
number VIC-25000-S2 manufactured by
Vanguard Instruments Company. A
separate manual applies to the Series I, the
first design configuration of the Model
ATO-250
.
2.0 Supersede Notice
This Operator’s manual is the basic issue for
the Series II model ATO-250
and does not
supersede any previously published manual.
3.0 General Description
The ATO-250™ is a digital micro-ohm
meter designed for precisely measuring non-
inductive resistances ranging from 1 micro-
ohm to 300 milliohms. Applications include
resistance measurement of circuit-breaker
contacts, bushing joints or any other low
resistant devices. The test unit is field-
portable, rugged and can be easily operated
by first time users. The ATO-250 is built
with a 16-key pushbutton pad for entering
test parameters and control functions.
Control-option menus, measured resistance
values and other related identifying data are
outputted onto a LCD screen. The ATO-
250
has a built-in thermal printer, which
prints test data onto a 2.5 inch wide
thermally sensitive paper.
Thus, the measured resistance data can
either be displayed or be printed from the
thermal printer. The measured resistance
data can also be stored in the FLASH
EEPROM, where up to 63 records of 96
readings each can be retained. Operators can
recall the stored resistance measurements
and related data at a later time for review
and printing.
A Windows based software package is
supplied with each ATO-250 that allows the
user to retrieve test records stored in the
unit’s memory. Operation requires little
more than connecting the test leads to an
unknown resistance, selecting the test
parameters, and test current (10 to 250
amperes).
3.1 Functional Description
The fundamental operation of the ATO-
250
is based upon the electrical
relationship described by Ohm’s law:
R=V/I
. Where
I
is a known test current and
V
is the voltage across the unknown
resistance, such as breaker contacts. Since
the test current through the device under test
is known and the voltage across it is read by
the microprocessor, the resistance of the
device is calculated using Ohm’s law.
The ATO-250
test voltage is supplied by
an unfiltered DC (direct current) power
supply. The DC test current ranging from 10
to 250 Amperes is set by a “CURRENT
CONTROL” knob on the front panel.
The ATO-250
can make precise micro-
ohm measurements possible without
compensating for the error induced by the
test leads’ resistance, by running the
voltmeter test leads separately from the
current bearing test leads. Therefore, by
measuring the voltages at the terminals of
the resistance being measured eliminates the
error from the I•R voltage drop in the test
current cables.
Resistance readings are then displayed onto
a backlit 4-line by 20-character LCD.
NOTE
The ATO-250 test voltage is a full-
wave rectified DC, which is not a pure
DC voltage. At 25 Amperes or more of
test current, a circuit breaker bushing
CT may detect an AC (alternate
current) ripple through the breaker’s
contact. Precaution should be taken to
eliminate the CT sensing or any
accidental tripping of the Substation
Bus Differential Relay.
