Vanguard ATO-600 - Manual (Page 6)

ATO-400 / ATO-600 Operating Procedures
6
1.0 INTRODUCTION
1.1 Applicability
This manual applies to the Model ATO-400™ and Model ATO-600™ (hereafter, ATO), made
by Vanguard Instruments Company, Inc.
1.2 General Description
The ATO-400/600 are third generation micro-ohmmeters made by Vanguard Instruments
Company. The ATO-400/600 feature high accuracy microprocessor-controlled measurement of
very low resistances, ranging from 1 micro-ohm to 300 milli-ohms. The ATO is field-portable,
rugged, and is easily operated by first-time users having a minimum of training. It features a 16-
key push button keypad for entering test parameters and control functions and a 4-line by 20-
character LCD alpha/numeric readout for displaying control-option menus, measured resistance
values, and related data. The operation requires little more than connecting test leads to an
unknown resistance and selecting the desired functions and its options. Operators select the test
current (10 to 400 amperes for the ATO-400 or 10 to 600 amperes for the ATO-600) and test
time (5 to 60 seconds). Measured resistance data is displayed on the ATO’s LCD. The
measured resistance data can also be stored (up to 63 records of 96 readings each) in FLASH
EEPROM memory. Operators can recall the stored resistance measurements and related data at a
later time for review or down loading to the PC. Down loading test records is accomplished by
using the ATO’s built-in RS-232C or USB ports.
1.3 Functional Description
The ATO’s operation is based on the electrical relationship described by Ohm’s law: R=V/I,
where I is a known current and V is the dc voltage measured across the unknown resistance
(typically, a circuit breaker’s contacts). Since the current (user selected) through the unknown
resistance is known and the voltage across the unknown resistance is measured by a precision
voltmeter, the resistance displayed on the LCD is calculated using Ohm’s law. The ATO test
voltage is supplied by an unfiltered DC power supply. A regulated constant-current source
outputs the preset current selected by the user.
The DC test current is selectable from 10 to 400 amperes for the ATO-400 and from 10 to 600
amperes for ATO-600. Since the ATO-400 and ATO-600 use unfiltered DC current supplies,
there is the possibility of inductively tripping a circuit breaker bus differential relay. Each
ATO is supplied with a set of current cables and voltage sensing cables. Voltage sensing test
cables are separate from the current-carrying test cables to the resistive load. Thus, voltages are
measured at the terminals of the resistance being tested, eliminating any I•R voltage drop error in
the current cables. These ATO features make very precise micro-ohm measurements possible
without having to compensate for current-lead resistance errors.
1.4 Furnished Test Accessories
The ATO is supplied with two 15-foot (#1/0 AWG) test cables with heavy-duty alligator clamps
and two 15-foot voltage-sensing cables with alligator clamps. A ground cable and power cord is
also included with each ATO.