A conventional current transformer (CT) helps instruments and protective relays work with a representation of alternating current flowing in another circuit. The questions below explain basic terms; equipment selection and work on CT circuits require qualified personnel and the applicable equipment documentation.
What does a current transformer do?
A CT produces a secondary current related to the current in its primary circuit. Meters and protective relays use that secondary signal. The transformer also provides electrical isolation between the primary circuit and the connected instruments, but that isolation does not make the secondary circuit safe to handle.
What are the main parts of a CT?
A CT has a magnetic core and a secondary winding. Its primary may be a winding, a bar that forms part of the device, or a conductor passing through an opening in the core. Insulation and the surrounding housing depend on the design and intended application.
What is the CT ratio?
The ratio describes the relationship between rated primary and secondary currents. It helps a meter or relay interpret the secondary signal in terms of primary current. Actual accuracy depends on the CT's specified operating conditions and connected load.
What is the secondary circuit?
The secondary circuit includes the CT's secondary winding and the connected wiring and devices, such as a meter or relay. Its design and condition matter to both measurement and safety. A qualified person should assess the complete circuit using the manufacturer's information and site procedures.
What does CT burden mean?
Burden describes the load connected to the secondary circuit, commonly expressed as apparent power under specified conditions. The rated burden is one of the conditions used to define a CT's accuracy. Connected instruments and wiring must be considered together when assessing the application.
Are metering and protection CTs interchangeable?
Not automatically. Metering and protection have different performance needs, so the CT and connected equipment must be selected for the intended duty. Check the relevant specifications and the requirements of the meter or protective relay.
Why is an open CT secondary hazardous?
If primary current is flowing while the secondary circuit is open, a dangerous voltage can develop at the secondary terminals and the CT can overheat. Do not disconnect or alter a CT circuit as a routine troubleshooting step. A qualified person must control the hazard using the equipment manufacturer's instructions and the site's procedures.
What information matters when choosing a CT?
Relevant information includes the rated ratio, insulation level, frequency, accuracy class, rated burden and the intended metering or protection duty. The equipment's short-time current capability and installation conditions also need review. Selection belongs to a qualified designer using the applicable requirements and manufacturer documentation.
Source
Based on the NIMI Electrician (Power Distribution), second-year theory, printed pages 99–101 and 156–157. Consult the actual CT manufacturer's instructions for the equipment in use.