3300 ACM Installation and Operation Manual Power Measurement Ltd.
2-4 Installation
2.5.2 PHASE CURRENT INPUTS
The 3300 ACM uses CTs to sense the current in each phase
of the power feed. The selection of the CTs is important
because it directly affects accuracy.
Current Input Options
The 3300 ACM offers a choice of phase current input
options to match the type of CTs being used. The basic
model 3300 ACM is compatible with CTs with 5 Amp full
scale secondaries. The 1AMP option provides compatibility
with 1 Amp CT secondaries.
CAUTION
Refer to the label on the base module of
the 3300 ACM to determine the equipped
current input option(s). Applying current
levels incompatible with the current input
configuration will permanently damage
the device.
CT Ratings
The CT secondary should have a burden capacity greater
than 3 VA.
The CT primary rating is normally selected to be equal to the
current rating of the power feed protection device. However,
if the peak anticipated load is much less than the rated
system capacity then improved accuracy and resolution can
be obtained by selecting a lower rated CT. In this case the
CT size should be the maximum expected peak current
+25%, rounded up to the nearest standard CT size.
Other factors may affect CT accuracy. The length of the CT
cabling should be minimized because long cabling will
contribute to inaccuracy. Also, the CT burden rating must
exceed the combined burden of the 3300 ACM plus cabling
plus any other connected devices (burden is the amount of
load being fed by the CT, measured in Volt-Amps). The
3300 ACM burden rating is given in Appendix C.
Overall accuracy is dependent on the combined accuracies of
the 3300 ACM, the CTs, and the PTs (if used). Instrument
accuracy Class 1 or better is recommended.
2.5.3 PT & CT CONNECTION
Figures 2.5.4a to 2.5.6 illustrate all required phase voltage
and phase current connections for various circuit configura-
tions to ensure correct installation. Phasing and polarity of
the AC current and voltage inputs and their relationship is
critical to the correct operation of the unit.
All phase voltage sense leads should be protected by
breakers or fuses at their source. In cases where PTs are
required, if the power rating of the PTs is over 25 Watts the
secondaries should be fused.
DANGER
PT secondary circuits are capable of
generating lethal voltages and
currents with their primary circuit
energized. Standard safety precau-
tions should be followed while
performing any installation or service
on the device (e.g. removing PT fuses,
etc.)
CTs should be connected to the device via a shorting block or
test block to facilitate the safe connection and disconnection
of the CTs.
DANGER
CT secondary circuits are capable of
generating lethal voltages and
currents when open circuited with
their primary circuit energized.
Standard safety precautions should
be followed while performing any
installation or service on the device
(e.g. shorting CT secondaries, etc.)
Questions regarding proper working procedures should be
referred to qualified personnel.
Courtesy of NationalSwitchgear.com
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