ATO-400P / ATO-600P Operating Procedures
6
1.0 INTRODUCTION
1.1 Applicability
This manual applies to the Model ATO-400P™ and Model ATO-600P™ (hereafter, ATO),
made by Vanguard Instruments Company, Inc.
1.2 General Description
The ATO-400P/600P are third generation micro-ohmmeters made by Vanguard Instruments
Company. The ATO-400P/600P 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 pad 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 ATO has a built-in thermal printer, which prints test data
on 2.5-inch-wide thermal sensitive paper. 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 ATO-400P or 10 to 600 amperes for ATO-600P)
and test time (5 to 60 seconds). Measured resistance data is displayed and can be printed on the
thermal printer. 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, printing 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 read-out can be 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-400P and from 10 to 600
amperes for ATO-600P.
Since the ATO-400P and ATO-600P 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 leads are separate from the current-bearing test leads 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-resistances 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
included with each ATO.
