Basler Electric BE1-700 - Manual (Page 280)

CT Polarity (BE1-700C)
CT polarity is critical to the proper operation of the BE1-700C. Fundamental information on CT polarity
and protective relays is provided below.
By ANSI convention, current transformer polarity will face away from the protected winding of a
transformer, motor, generator, or reactor, and away from the contacts in a circuit breaker. Therefore,
primary current flow towards the winding or contacts (direction of protected zone) will result in a
secondary current out X1 in phase with the primary (see Figures 12-8 and 12-9).
On occasion, however, protection engineers will run into situations where CT polarity is reversed for a
specific application. That is, non-polarity of the CT secondary will be in phase with the primary current
flow (Figure 12-10). For example, a transformer differential CT from a breaker with a different polarity
convention, such as low voltage switchgear or a bus differential CT taken from the low side of a
transformer.
Orientation of CT polarity relative to primary current flow establishes the secondary CT terminal that
should be connected to polarity of the protective relay.
Figure 12-8. Standard CT Polarity
Figure 12-9. Current Transformer Action
Figure 12-10. Example of Reversed CT Polarity
12-10 BE1-700 Installation 9376700990 Rev M