# Control-transformer fused-primary interface

Canonical URL: https://eloragrid.ai/symbols/control-transformer-fused-primary-interface
Publisher: Elora Grid
Updated: 2026-11-03
Lookup label: CFI
Drawing use: Schematic
Reference families: Manufacturer

Representation: Functional block
Installation focus: switchgear
Input: Auxiliary AC source
Output: Control transformer primary

Shows the protected primary connection of the selected control-power transformer.

## In plain English

### What this block does

Shows the protected primary connection of the selected control-power transformer.

### A simple example

In a switchgear lineup, separate protected branches may feed trip coils, closing coils and protection electronics. Their alarm contacts help the station controller identify which operating capability is unavailable. For this reference, the drawing identifies “Auxiliary AC source” at the input and “Control transformer primary” at the output. Fuse selection follows transformer and protection data.

### What the drawings help you do

Start at the named input: Auxiliary AC source. Identify its source device or document and the state represented on the drawing. Follow the named output: Control transformer primary. Cross-reference its destination and separate the command from confirmation that the equipment responded.

### Terms

- Input: Auxiliary AC source
- Output: Control transformer primary
- Application reference: An explanation of a named drawing function or circuit relationship; the project documents define the equipment and its connections.

Concept diagram: Auxiliary AC source → CFI → Control transformer primary
A reading guide. The entry's original illustration and input/output facts describe this application.

- [SEL-RPM redundant power module](https://selinc.com/products/RPM/)


## What it represents

Shows the protected primary connection of the selected control-power transformer. Control power supplies the command, protection and indication equipment associated with a switchgear assembly. The drawing must distinguish the source, each distribution branch and the functions lost if that branch fails.
In a switchgear lineup, separate protected branches may feed trip coils, closing coils and protection electronics. Their alarm contacts help the station controller identify which operating capability is unavailable. For this reference, the drawing identifies “Auxiliary AC source” at the input and “Control transformer primary” at the output. Fuse selection follows transformer and protection data.
Follow the actual positive, negative or neutral returns and the protective device on each branch. A healthy voltage indication does not establish that every downstream path can deliver its required operating current. 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: Auxiliary AC source. Identify its source device or document and the state represented on the drawing.
- Follow the named output: Control transformer primary. Cross-reference its destination and separate the command from confirmation that the equipment responded.
- Fuse selection follows transformer and protection data.
- Follow the actual positive, negative or neutral returns and the protective device on each branch. A healthy voltage indication does not establish that every downstream path can deliver its required operating current.

## Information to show or reference

- Label the source and connection reference for auxiliary ac source.
- Identify the destination and defined state of control transformer primary.
- Record the equipment or control-document revision supporting this functional block.
- Check this application limit in the project design: Fuse selection follows transformer and protection data.

## Important distinction

Fuse selection follows transformer and protection data. 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: Switchgear
- Input / boundary: Auxiliary AC source
- Output / connection: Control transformer primary
- Lookup label: CFI is a library lookup label, not a standard device designation.

## Public references

- [SEL-RPM redundant power module](https://selinc.com/products/RPM/) — SEL. Public technical context for switchgear control-power supervision. Independent AC and DC sources for station auxiliary power. 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.