PQube 3 Installation Manual
Manual#: 851-000135
Version 3.4 – 08/11/2022 Page 31 of 47
Installing CTs at the secondary of measuring current transformers (1 A or 5 A
secondary)
If you will be monitoring the 1 A or 5 A secondary of existing metering CTs, you will need to use the CTI-
1A or the CTI-5A module. They have 1 A and 5 A nominal inputs, respectively.
The CTI module inputs are installed in series with your 1 A or 5 A secondary circuit. The terminal block on
your CTI module is connected to the 0.333 V current input channels on your PQube 3. Each CTI module
includes 4 current channels, so you can use up to 2 CTI modules per PQube 3.
WARNING: When installing CTs with 1 A or 5 A secondaries, take extra precautions to ensure
that an open circuit does not develop on the secondary wires. Shorting blocks are typically
used to avoid the possibility of an open circuit during installation. CTs must be installed only
by qualified personnel for electrical installations.
1 A or 5 A vs. 0.333V secondary, what’s the difference?
Most current transformers are designed to have 1 A or 5 A of current flowing
through the secondary circuit while full rated current is flowing through the
primary circuit.
While installing CTs with 1 A or 5 A secondary, it is imperative that an open
circuit does not develop in the secondary. If an open circuit develops while
current is flowing through the primary of the CT, a very hazardous open circuit
voltage (OCV) will develop across the opening. In this condition, typical OCV
values can range from hundreds to thousands of volts.
For this reason, POWERSIDE offers CTs exclusively with 0.333 V secondary. Our
CTs includes a built-in burden resistor in the secondary circuit so that the
current always has a path to flow through. The resistor value is calibrated and
tuned to achieve a 0.333 V drop across the resistor at full rated current. This
0.333 V signal can then be measured using the 2 wires coming out of the CT.
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