Vanguard Auto-Ohm - Manual (Page 5)

Auto-Ohm Operating Procedures
1
1.0 INTRODUCTION
1.1 Applicability
This manual applies to Series II of the
Model Auto-Ohm™ (hereafter, Auto-Ohm),
identified by Part Number VIC-60000-S2
made by Vanguard Instruments Company.
A separate manual applies to Series I (the
first design configuration) of the Auto-Ohm.
1.2 Supersedure Notice
This Operator’s manual is the basic issue for
Series II Auto-Ohm and does not supersede
any previously published manual.
1.3 General Description
The Auto-Ohm micro-ohmmeter is made by
Vanguard Instruments Company and
features microprocessor-control for
measuring very low resistances ranging
from 1 micro-ohm to 300 milliohms with
high accuracy. The Auto-Ohm is field-
portable, rugged, and is easily operated by
first-time users having little or no training; it
features a one-knob control and an LCD
alpha/numeric display of the resistance
measured. The one-knob control operation
is logical and simple: Turning the knob
scrolls through a menu of possible options
(which display in sequence) and pressing
the knob activates the selected function. As
its name implies, the Auto-Ohm operation is
automatic, requiring little more from the
user than connecting it to an unknown
resistance and selecting what it is to do and
how the test results are to be presented. The
Auto-Ohm stores the last 3 resistance
measurements, which can be displayed
during test.
1.4 Functional Description
The Auto-Ohm’s operation is based on the
electrical relationships 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 read by a precision voltmeter,
the resistance read-out is a direct function of
the voltage being measured. The voltage is
read directly as resistance (translated and
displayed on a digital readout). The dc test
current is selectable in 2-amp steps, from 10
to 100 amperes. Test current is gradually
ramped up and down (automatic function),
which virtually eliminates magnetically
induced transients through the circuit-
breaker current transformers. The risk of
inductively tripping a circuit breaker (bus
differential relay) is virtually non-existent.
The Auto-Ohm performs all of these
functions in one simple process. Voltmeter
test leads run separately 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. The
Auto-Ohm test voltage is supplied by a DC
power supply. A precisely regulated
constant-current source ensures measured
voltage is a function of the resistance alone,
and not affected by current-cable resistance
losses. These Auto-Ohm features make very
precise micro-ohm measurements possible
without having to calculate compensations
for current lead resistances errors.
1.5 Furnished Test Accessories
The Auto-Ohm is supplied with one 35-foot
test cable with quick disconnect test plugs at
the unit end and heavy-duty alligator clamps
at the test-load end. Heavy-duty welding-
type C-clamps are available as optional
accessories (these C-clamps allow test lead
connections to a wide variety of bushing
sizes, bus bars, and conductors that require
low-resistance test-lead contacts).