5-4
IM CW120-E
5.3
Wiring the Circuit under Test with
External VT/CT
WARNING
• When using an external current transformer (CT), be careful not to allow the secondary stage
of the CT to become open-circuited while the primary stage is being electrified. Otherwise, a
high-voltage may develop on the secondary stage, causing extreme danger.
• A measuring current flows through the bold lines shown in the figure below. For these lines,
use wires having an adequate margin of current-carrying capacity.
If the maximum voltage or current level being measured exceeds the
maximum measurement range of the CW120/121, use an external voltage
transformer (VT) or current transformer (CT). This strategy enables the
voltage or current levels beyond the maximum range to be measured.
When to use a VT and how?
If the maximum voltage of the circuit exceeds 450 V, connect an external VT
and connect the secondary stage of the VT to the voltage input terminals.
When to use a CT and how?
If the maximum current of the circuit exceeds the following value, connect an
external CT and clamp the secondary stage wire of the CT with a current
probe.
96033 probe:
50 A (rated at 5/10/20/50 A)
96030 probe:
200 A (rated at 20/50/100/200 A)
96031 probe:
500 A (rated at 50/100/200/500 A)
96032 probe:
1000 A (rated at 200/500/1000 A)
Example for single-phase two wires (1
∅
2W)
1
N
V1
Voltage input
Current input
N
CH1
V
v
L
VT
CT
Source
Load
Scaling
When using a VT and/or CT, you can set the VT ratio and/or CT ratio to
display the readings of the primary circuit voltages and currents. (This is
called the scaling function.)
SEE ALSO
For details of how to set the CT and VT ratios, see Sections 7.2.13 and 7.2.14, “Setting VT Ratio” and
“Setting CT Ratio.”
