Current transformer (CT) symbol
Also called CT
A current transformer steps the line current down to a small secondary current (1 A or 5 A) for meters and protective relays. Its ratio, such as 600/5, is written next to it.
What the current transformer (CT) symbol means
A current transformer steps the line current down to a small secondary current (1 A or 5 A) for meters and protective relays. Its ratio, such as 600/5, is written next to it.
ANSI and IEC symbols
A small loop beside the conductor, with a polarity dot and the ratio written next to it.
ANSI SVGWhere you see it
On every breaker that has protection or metering, often several CTs per breaker for different relays.
What is written next to it
On a one-line, the current transformer (CT) carries its tag and its ratings: ratio (for example 600/5 A), accuracy class (5P20 for protection, 0.5 for metering) and burden in VA.
What gets checked at commissioning
Before the current transformer (CT) is energised, the test records usually cover: ratio, polarity, insulation resistance, winding resistance and the excitation (knee point) curve. Each check is recorded against the tag, so the system can be handed over with its evidence. See commissioning software.
ANSI device numbers
Protective functions you will often find with a current transformer (CT):
- 50/51 overcurrent
- 87 differential
See the full list of ANSI device numbers with their IEC equivalents.
Related symbols
Frequently asked questions
What does the current transformer (CT) symbol look like?
On ANSI drawings: A small loop beside the conductor, with a polarity dot and the ratio written next to it. In IEC 60617: A circle threaded on the conductor. The coil form is S00851.
Is the current transformer (CT) symbol the same in IEC and ANSI?
No. A circle threaded on the conductor. The coil form is S00851. The IEC 60617 number is S00850.
Where is a current transformer (CT) used?
On every breaker that has protection or metering, often several CTs per breaker for different relays.
Which ANSI device numbers go with a current transformer (CT)?
50/51 overcurrent, 87 differential.
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