REG 316*4 1MRB520049-Uen / Rev. C ABB Switzerland Ltd
7-9
7.2.3. Checking the c.t. connections
The c.t’s must be connected in strict accordance with diagram
supplied with the equipment.
The following checks must be carried out to check the c.t’s and
c.t. circuits:
· polarity check
· primary injection test
· plot the excitation curve
· c.t. circuit grounding.
The polarity check (as close as possible to the protection equip-
ment) not only checks the current input circuit as a whole, it also
checks the phase-angle of the c.t.
Primary injection checks for a ratio error and the wiring to the
protection equipment. Each phase-to-neutral and phase-to-
phase circuit should be injected. In each case, the phase cur-
rents and the neutral current should be measured.
The relative polarities of the c.t’s and their ratios can also be
checked using load current.
Plotting the excitation curve verifies that the protection is con-
nected to a protection core and not a metering core.
Each electrically independent current circuit may only be earthed
in one place, in order to avoid balancing currents created by po-
tential differences.
Core-balance c.t’s
If the residual current is obtained from a core-balance c.t., the
ground for the cable screen must first be taken back through the
core-balance c.t. before connecting it to ground. The purpose of
this is to ensure that any spurious E/F current flowing along the
screen of the cable cancels itself and is not measured falsely as
an E/F on the relay’s own feeder.
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