# Interposing CT ratio-matching interface

Canonical URL: https://eloragrid.ai/symbols/interposing-ct-ratio-matching-interface
Publisher: Elora Grid
Updated: 2026-11-03
Lookup label: ICRI
Drawing use: Schematic
Reference families: Manufacturer

Representation: Functional block
Installation focus: substations
Input: Source CT circuit
Output: Matched relay current circuit

Shows the matching purpose of an interposing CT between assigned measurement circuits.

## In plain English

### What this block does

Shows the matching purpose of an interposing CT between assigned measurement circuits.

### A simple example

In a substation protection panel, separate CT cores can supply metering and protection circuits with different duties. A test boundary must preserve the secondary arrangement required by the selected CT and test device. For this reference, the drawing identifies “Source CT circuit” at the input and “Matched relay current circuit” at the output. The interposing CT adds its own burden and error.

### What the drawings help you do

Start at the named input: Source CT circuit. Identify its source device or document and the state represented on the drawing. Follow the named output: Matched relay current circuit. Cross-reference its destination and separate the command from confirmation that the equipment responded.

### Terms

- Input: Source CT circuit
- Output: Matched relay current circuit
- Application reference: An explanation of a named drawing function or circuit relationship; the project documents define the equipment and its connections.

Concept diagram: Source CT circuit → ICRI → Matched relay current circuit
A reading guide. The entry's original illustration and input/output facts describe this application.

- [Indoor current and voltage transformers](https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers)


## What it represents

Shows the matching purpose of an interposing CT between assigned measurement circuits. A current-transformer secondary arrangement carries a measuring current from a defined primary location to metering or protection inputs. Core identity, polarity and the connected circuit determine the measurement's meaning.
In a substation protection panel, separate CT cores can supply metering and protection circuits with different duties. A test boundary must preserve the secondary arrangement required by the selected CT and test device. For this reference, the drawing identifies “Source CT circuit” at the input and “Matched relay current circuit” at the output. The interposing CT adds its own burden and error.
Trace every secondary connection and the assigned earthing point against the actual connection diagram. These illustrations explain circuit roles and never prescribe a live CT switching or disconnection procedure. The functional block is an independent explanation of this named application. Equipment-specific ratings, terminal connections and operating conditions must be established in the project documentation.

## How to read it

- Start at the named input: Source CT circuit. Identify its source device or document and the state represented on the drawing.
- Follow the named output: Matched relay current circuit. Cross-reference its destination and separate the command from confirmation that the equipment responded.
- The interposing CT adds its own burden and error.
- Trace every secondary connection and the assigned earthing point against the actual connection diagram. These illustrations explain circuit roles and never prescribe a live CT switching or disconnection procedure.

## Information to show or reference

- Label the source and connection reference for source ct circuit.
- Identify the destination and defined state of matched relay current circuit.
- Record the equipment or control-document revision supporting this functional block.
- Check this application limit in the project design: The interposing CT adds its own burden and error.

## Important distinction

The interposing CT adds its own burden and error. This reference describes the named application; it does not establish a new IEC graphical symbol, an ANSI device-function number or an exact manufacturer connection diagram.

## Facts

- Representation: Functional block
- Installation focus: Substations
- Input / boundary: Source CT circuit
- Output / connection: Matched relay current circuit
- Lookup label: ICRI is a library lookup label, not a standard device designation.

## Public references

- [Indoor current and voltage transformers](https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers) — ABB. Public technical context for current-transformer secondary arrangements. Current and voltage measurement transformers and their secondary interfaces. The application text and educational graphic are independently authored; the publisher does not certify this representation.

## 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.