A CT, current transformer, produces a secondary current proportional to the primary, usually 1 A or 5 A at rated current, for meters and protection relays. Metering CTs are specified for accuracy at normal load, while protection CTs must reproduce fault currents many times rated without saturating, described by classes such as 5P20 or PX. The secondary must never be open-circuited with primary current flowing, because the voltage across the open terminals can reach thousands of volts.
Short the CT secondaries before you pull that meter out, there's 400 amps on the primary.
Never open a CT is drilled into every apprentice; the shorting links in the test block are there for exactly that reason.
Also heard as
- current transformer
Related terms
VT
Voltage transformer: an instrument transformer that steps high voltage down to a standard level for relays and meters.
Burden
Load connected to the secondary of an instrument transformer, stated in VA or ohms.
Differential protection
Scheme that compares current entering and leaving a protected zone and trips when the two disagree.
X/R ratio
Ratio of reactance to resistance in the fault path, which sets how large and slow the DC offset in fault current is.