Figure. 5.2.1. - 30. Current measurement terminology
PRI: The primary current, i.e. the current which flows in the primary circuit and through the primary side
of the current transformer.
SEC: The secondary current, i.e. the current which the current transformer transforms according to its
ratios. This current is measured by the protection relay.
NOM: The nominal primary current of the protected object.
For the measurements to be correct the user needs to ensure that the measurement signals are
connected to the correct inputs, that the current direction is connected to the correct polarity, and that
the scaling is set according to the nominal values of the current transformer.
The relay calculates the scaling factors based on the set values of the CT primary, the CT secondary
and the nominal current. The relay measures the secondary current, the current output from the
current transformer installed into application's primary circuit. The rated primary and secondary
currents of the CT need to be set for the relay to "know" the primary and per-unit values. With motors
and other speci c electrical apparatus protections, the motor's nominal current should be set for the
values to be in per unit with regards to the apparatus nominal instead of the CT nominal. This is not
always mandatory as some relays still require manual calculations for the correct settings; however,
setting the relay's nominal current makes motor protection much easier and more straightforward. In
modern protection devices this scaling calculation is done internally after the current transformer's
primary current, secondary current and motor nominal current are set.
AQ-E215
Instruction manual
Version: 2.01
33
© Arcteq Relays Ltd
