12-16
Installation
BE1-951
Figure 12-17. Standard CT Polarity
Figure 12-18. Current Transformer Action
Figure 12-19. Example of Reversed CT
Polarity
Terminal Blocks
There are two sizes of terminal blocks used on the H1 case design, the current circuit block and the
input-output blocks. The current circuit terminal block is the larger of the two types and uses 8/32 Phillips
head screws with lock washers. The input-output block is the smaller of the two types and uses 6/32 slot or
Phillips head screws with no washer. The S1 case design uses slotted 10-32 washerless "pan-head" screws
for current and potential inputs. The input-output blocks are the same size as those incorporated in the H1
design, and use 6/32 slot or Phillips head screws.
NOTE
Except as noted, connections to the relay should be made with a minimum wire size of
#14 AWG.
CT Polarity
CT polarity is critical to the proper operation of the BE1-951. Sidebar 12-1 provides fundamental information
on CT polarity and protective relays.
Sidebar 12-1 Current Circuit Polarity
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 12-17 and 12-18).
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 12-19). 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.
