Vanguard Herculito - Manual (Page 5)

Herculito Operating Procedures
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1.0 INTRODUCTION
1.1 Applicability
This manual applies to the Model Herculito, made by Vanguard Instruments Company.
1.2 General Description
The Herculito features microprocessor-controlled low resistance meter. The Herculito was
designed to measure very low resistances ranging from 1 micro-ohm to 300 milli-ohms with high
accuracy. The Herculito is light weight, rugged, and is easily operated by first-time users having
a minimum of training. It features one-knob control and an LCD alpha/numeric display. 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. The Heculito operation is automatic, only requiring the user to connect it to an
unknown resistance and selecting the desired functions and its options. The Herculito stores the
last 3 resistance measurements, which can be displayed after a test.
1.3 Functional Description
The Herculito’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 flowing through the
unknown resistance is known and the voltage across the unknown resistance is read by a
precision voltmeter, the resistance read-out can be calculated using Ohm’s law.
The Herculito’s 5 Vdc test voltage is supplied by a true DC power supply. The test current is
regulated by the test load and the test cables. Test current varies slightly from 80 to 100 Ampere
depending on the resistance of the load.
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. This feature makes possible very precise micro-ohm
measurements without having to calculate compensations for current lead resistance errors.
Note
Since the test current rise and fall time is not controlled by a current regulator circuit, precaution
should be taken to eliminate any CT sensing that might inadvertently trip Substation Bus
Differential Relays.