GEH-15398 Types AB and DB Ammeters and Voltmeters
dimensions of the external resistors are given in Fig.
10, 11, and 12.
D-C AMMETERS
D-e ammeters designed for current measurement
above 60 amperes necessitate the use of shunts which
are to be connected to the ammeter by means of the
leads provided. In the event that leads are supplied
by the purchaser, their resistance should agree with
the value stamped on the instrument nameplate. Any
change in the resistance of these leads will introduce
an error in the reading. General Electric shunts of
the same ampere rating and millivolt drop are inter
changeable.
Warning:
All ammeter. shunts should be connected
in the grounded side of the line when possible. When
this is not possible, or the circuit cannot be grounded,
the metal parts of manually operated contact devices
or auxiliary pointer knobs which project through the
cover are at line potential. When the cover is remov
ed, also remember that the instrument windings, as
well as the scale and pointer, are at line potential.
Use caution to avoid contacting this potential.
As the resistance of the shunt leads used in certain
applications cannot be predetermined at the factory,
it has been found desirable to furnish some d-e am
meters with an internal rheostat to adjust for non
standard lead resistances.
The nameplate on the instrument has an INSTRU
MENT
+
LEAD RESIS. value stamped upon it.
This value is to be that of the total circuit, which is
considered to include the resistance of the instrument
and its leads with the shunt disconnected. This total
circuit resistance must be adjusted correctly in order
to attain the specified accuracy of the instrument.
To adjust the rheostat for a given lead resistance.
proceed as follows:
If a resistance bridge is available, connect it across
the shunt end of the leads (shunt disconnected) and
the instrument across the opposite end. Set the re
sistance bridge for the INSTRUMENT
�
LEAD
RESIS. value stamped on the instrument nameplate.
Loosen the lock nut (located under the nameplate).
and turn the rheostat shaft (which projects through
the nameplate) until a zero deflection is secured on
the bridge galvanometer. After the correct setting
has been obtained, retighten the lock nut.
If a resistance bridge is not available. adjustment
may be made by connecting a standard millivoltmeter
across the shunt end of the leads (shunt disconnected)
and the instrument across the opposite end. Apply
a low adjustable d-e voltage across the shunt end
of the leads and adjust this voltage until the indication
on the standard millivoltmeter agrees with the full-
4
scale millivolts (marked FULL SCALE MV) on the
instrument nameplate. Loosen the lock nut (located
under the nameplate), and turn the rheostat shaft
(which projects through the nameplate) until the
ammeter reads full scale. Retighten the lock nut.
Note:
The maximum lead resistance is about 1 ohm
for a 100-millivolt instrument and about 0.26 ohm for
a 50-millivolt instrument.
D-C VOLTMETERS
D-e voltmeters designed for use on circuits above
750 volts and d-e ground detectors for voltages up to
and including 750 volts are furnished with external
resistors and are connected directly to the circuit to
be measured.
INSTALLATION
MOUNTING
The appropriate dimension drawings shown in this
book should be followed when drilling the switch
board. All drilling and all wiring on the switchboard
should be completed before mounting the instruments.
Supporting studs are provided for mounting on the
switchboard. Mount the instruments in a level posi
tion.
These instruments are practically uneffected by
stray fields, but it is advisable to keep transformers
and wires carrying heavy current as far as possible
from all indicating instruments.
When there is no current flowing through the in
strument and it is properly leveled, the pointer should
indicate zero. Any deviation from zero should be
corrected by means of the zero adjustor.
TERMINAL CONNECTIONS
The threads on the terminal studs and nuts should
be clean and bright. The contact surfaces of nuts.
busbars. and cable terminals must be thoroughly clean
to insure good contact. If a nut turns hard, it should
be run over a tap of the proper dimensions; otherwise.
the threads on the stud may be ruined or the stud
turned in the instrument, thereby breaking the con
nection. After the instrument has been connected.
tighten the nuts sufficiently to insure good contact.
Do not overtighten.
The nuts and studs used in these instruments are
U.
S. standard.
Connect the instrument as shown in the appropriate
diagram. The numbers in these connection diagrams
correspond to the numbers stamped on the instrument
terminals.
On instruments manufactured prior to 1951, the
terminal studs are stamped with letters instead of
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