(2) Application. Refer to paragraph 5-3b(2) for
c. Current transformers. A Current Transformer
(CT) is an instrument transformer having low
losses whose purpose is to provide a fixed primary
HIGH AIR PRESSURE
LOW AIR PRESSURE
to secondary step down current ratio. The primary
to secondary current ratio is in inverse proportion to
the primary to secondary turns ratio. The secondary
winding thus has multiple turns. The CT is usually
either a toroid (doughnut) winding with primary
conductor wire passing through the "hole" or a unit
section of bus bar (primary), around which is wound
the secondary, inserted into the bus run. The CT
ratio is selected to result in a five ampere secondary
current when primary rated current is flowing.
(1) Ratings. Current transformers are usually
rated at 5 amperes in the secondary circuit.
(2) Application. Refer to paragraph 5-3c(2) ap-
Figure 5-10. Air blast arc interrupter, diagram.
brought out to the metering and control panel. The
d. Control circuits. Switchgear control circuits for
medium voltage are functionally similar to those
PT isolates the high voltage primary from the me-
tering and control panel and from personnel. The
used for low voltage systems. The control circuits
step down ratio produces about 120 VAC across the
are similarly classified as either AC or DC.
secondary when rated voltage is applied to the pri-
(1) AC control circuits. Refer to the description
mary. This permits the use of standard low voltage
provided in paragraph 5-3e( 1).
meters (120 VAC full scale) for all high voltage cir-
(2) DC control circuits. Refer to the description
cuit metering and control.
provided in paragraph 5-3e( 3).
(1) Ratings. Potential transformers are usually
e. Service practices. Service practices for medium
rated at 120 volts in the secondary circuit.
voltage switchgear consist of a complete mainte-
Figure 5-11. Cross sectional view of vacuum arc interrupter: