# Interposing current transformer

Canonical URL: https://eloragrid.ai/symbols/interposing-current-transformer
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: ICT
Drawing use: Schematic, SLD
Reference families: IEC, Manufacturer

Auxiliary CT inserted in a CT secondary circuit to correct ratio, phase shift or provide isolation for a relay.

## In plain English

### What does Interposing current transformer mean?

An interposing CT sits between a main CT and a relay or meter. It corrects ratio mismatch, provides phase-shift compensation for transformer differential schemes, or isolates circuits, and it was standard in electromechanical differential protection before relays could compensate internally.

### A simple example

The interposing CT adds burden to the main CT and can introduce its own errors at high fault current. The symbol shows its presence, but its ratio, connection (star or delta) and taps must be shown beside it because they decide whether the differential scheme balances.

### What the drawings help you do

Read the ratio and connection group of the interposing CT. Check the added burden on the main CT. An interposing CT is an auxiliary in the secondary circuit; a main CT carries the primary circuit current.

### This particular reference

Auxiliary CT inserted in a CT secondary circuit to correct ratio, phase shift or provide isolation for a relay. An interposing CT is an auxiliary in the secondary circuit; a main CT carries the primary circuit current.

### Terms

- Winding: A coil of conductor inside an electrical machine or transformer.
- Coil: A wound conductor that can produce a magnetic field when energised.
- Phase: One of the related AC waveforms in a power system.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [Current transformer symbol variants](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-40AAEF4A-AC3F-4B3D-90A5-A9E0707C1241.htm)
- [Transformer vector-group interpretation in protection application](https://techdoc.relays.protection-control.abb/r/SSC600-and-SSC600-SW-Technical-Manual/1.5/en-US/TR2PTDF-Operation-principle?contentId=_tVeRElk4P5wHpFgF2sYvw)


## What it represents

An interposing CT sits between a main CT and a relay or meter. It corrects ratio mismatch, provides phase-shift compensation for transformer differential schemes, or isolates circuits, and it was standard in electromechanical differential protection before relays could compensate internally.
The interposing CT adds burden to the main CT and can introduce its own errors at high fault current. The symbol shows its presence, but its ratio, connection (star or delta) and taps must be shown beside it because they decide whether the differential scheme balances.

## How to read it

- Read the ratio and connection group of the interposing CT.
- Check the added burden on the main CT.

## Information to show or reference

- State ratio and taps
- Show connection group
- Reference relay balance calculation

## Important distinction

An interposing CT is an auxiliary in the secondary circuit; a main CT carries the primary circuit current.

## Facts

- Purpose: Ratio matching, phase correction or isolation
- Location: CT secondary circuit
- Preview: Independent educational concept; consult the applicable standard and project legend for the exact graphic

## Public references

- [Current transformer symbol variants](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-40AAEF4A-AC3F-4B3D-90A5-A9E0707C1241.htm) — Autodesk. Public AutoCAD Electrical current transformer family used for context; this variant is independently drawn.
- [Transformer vector-group interpretation in protection application](https://techdoc.relays.protection-control.abb/r/SSC600-and-SSC600-SW-Technical-Manual/1.5/en-US/TR2PTDF-Operation-principle?contentId=_tVeRElk4P5wHpFgF2sYvw) — ABB. Public explanation of vector-group compensation in transformer protection.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.