voltmeters installed in switchboards and control
ELECTRICAL INDICATING
INSTRUMENTS
consoles (fig. 5-24) all have a fixed resistance
value. Portable voltmeters, used as test equip-
Electrical indicating instruments (meters) are
ment, usually have a variable resistance. These
used to display information that is measured by
resistances are calibrated to the different ranges
some type of electrical sensor. Meters on the
that the meters will display. The normal range for
control console, although they display units such
the switchboard and electric plant meters is 0 to
as pressure or temperature, are in fact dc
600 volts.
voltmeters. The signal being sensed is conditioned
by a signal conditioner. It is then converted to
AMMETERS
0- to 10-volt dc, proportional to the parameters
Ammeters (fig. 5-25) are used to indicate the
being sensed. Electrical values, such as wattage
current passing through a conductor. Different
and current, are measured and displayed at ship's
types of ammeters are used for either ac or dc.
service switchboards. The electrical values are then
However, any ac ammeter will indicate on dc with
conditioned and displayed on the electric plant
a lower degree of accuracy.
console. The meters on the electrical plant
The construction of installed ammeters is such
console are also the 0- to 10-volt dc type of meters.
that they are not able to handle the current that
The meters we will discuss in this chapter are
passes through the conductor being measured.
found on the ship's service switchboards.
Ammeters are required to be connected in series
Normally, shipboard repair is not done on switch-
with the circuit to be measured. Since the meters
board meters. If you suspect the switchboard
cannot handle the high switchboard current, the
meters are out of calibration or broken,
switchboard ammeters operate through current
you should have them sent to a repair facility.
transformers. This arrangement isolates the
You can find more information on the theory
instruments from line potential. The current
of operation of these meters in the Navy
transformer produces in its secondary a definite
Electricity and Electronics Training Series
fraction of the primary current. This arrangement
(NEETS), Module 3, Introduction to Circuit
makes it possible to measure large amounts of
Protection, Control, and Measurement, NAV-
current with a small ammeter.
EDTRA 172-03-00-79.
CAUTION
Both dc and ac voltmeters determine voltage
THE SECONDARY OF A CURRENT
the same way. They both measure the current that
TRANSFORMER CONTAINS A DAN-
the voltage is able to force through a high
GEROUS VOLTAGE. NEVER WORK
resistance. Voltage measuring is always connected
AROUND OR ON CURRENT TRANS-
i n parallel with (across) a circuit. The
F O R M E R S WITHOUT TAKING
PROPER SAFETY PRECAUTIONS.
Figure 5-24.--An ac voltmeter.
Figure 5-25.--An ac ammeter.
5-19
