# Control-power automatic source selection

Canonical URL: https://eloragrid.ai/symbols/control-power-automatic-source-selection
Publisher: Elora Grid
Updated: 2026-11-03
Lookup label: CASS
Drawing use: Schematic
Reference families: Manufacturer

Representation: Functional block
Installation focus: switchgear
Input: Auxiliary-source health
Output: Selected control-power source

Selects the documented auxiliary source according to its availability inputs.

## In plain English

### What this block does

Selects the documented auxiliary source according to its availability inputs.

### 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-source health” at the input and “Selected control-power source” at the output. Selection logic must not unintentionally parallel sources.

### What the drawings help you do

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

### Terms

- Input: Auxiliary-source health
- Output: Selected control-power source
- Application reference: An explanation of a named drawing function or circuit relationship; the project documents define the equipment and its connections.

Concept diagram: Auxiliary-source health → CASS → Selected control-power source
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

Selects the documented auxiliary source according to its availability inputs. 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-source health” at the input and “Selected control-power source” at the output. Selection logic must not unintentionally parallel sources.
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-source health. Identify its source device or document and the state represented on the drawing.
- Follow the named output: Selected control-power source. Cross-reference its destination and separate the command from confirmation that the equipment responded.
- Selection logic must not unintentionally parallel sources.
- 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-source health.
- Identify the destination and defined state of selected control-power source.
- Record the equipment or control-document revision supporting this functional block.
- Check this application limit in the project design: Selection logic must not unintentionally parallel sources.

## Important distinction

Selection logic must not unintentionally parallel sources. 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-source health
- Output / connection: Selected control-power source
- Lookup label: CASS 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.