Quantcast Procedures for meters

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Figure 12-1. Method of connecting a phantom load for a field test, if on a single-phase, two-wire watthour meter
for solid-state units may be different from the fol-
lowing general data applying to electromechanical
units. Some adjustments of fixed instruments can
be made without removal from their panel assembly
mounting. Other adjustments, however, require
that the instrument be taken to a shop or labora-
tory. All instruments should be adjusted for null
(zero) reading. In most cases, this reading can be
obtained with the instrument disconnected. For
watt and varmeters, the null reading should be
checked with only the current coil energized from a
test source. The full scale adjustment should be
obtained by slowly increasing the load to be mea-
sured to the full scale value. For example, a voltme-
ter is adjusted by varying the series resistance con-
nected to the test power supply. Watch the pointer
as the instrument is slowly loaded to its full scale
value to check whether there is any friction in the
movement. When a watt or varmeter is being ad-
Figure 12-2. Method of connecting a phantom load for
field test, if on a three-phase, three-wire watthour meter
justed, an ammeter should be used in series with
the current coil of the meter and the current should
work is done on it. Careful planning of the work will
be limited to the rating of the coil to prevent over-
reduce the chances of an accident from energized
heating. After the instrument has been calibrated
and adjusted for full scale reading, the load should
a. Preparation. Before making adjustments on
be gradually decreased and the null position
checked again.
instruments or meters that require removal of the
cover, it is important that the outside of the instru-
c. Procedures for meters. Meter adjustments are
ment be thoroughly cleaned to prevent the entrance
made for full load, light load, lag, and creep. Most
of any dirt, dust, or magnetic material. Also, the
polyphase meters have an additional adjustment to
working space around the instrument or meter
obtain balance between elements. The meter manu-
should be cleared to permit setting up test devices
facturers' instruction book should be consulted for
required for calibration and adjustment. Test de-
instructions before making any adjustments. Gen-
vices should not be placed directly under the equip-
eral instructions given here may differ from manu-
ment to be adjusted.
facturers' instructions for solid-state meters. The
b. Procedure for instruments. Adjustment proce-
lag and balance adjustments are usually made in a
shop before installation. The adjustment of a meter
dures will vary according to the type of instrument.
is done by loading it with a phantom (artificial) load
The manufacturer's instrument book should be con-
and comparing its performance with that of a cali-
sulted before making any adjustment. Adjustments


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