# Source-availability selection interface

Canonical URL: https://eloragrid.ai/symbols/source-availability-selection-interface
Publisher: Elora Grid
Updated: 2026-11-03
Lookup label: SSI
Drawing use: Schematic
Reference families: Manufacturer

Representation: Functional block
Installation focus: switchgear
Input: Incoming source health
Output: Available-source selection

Compares the declared health of available incoming sources for transfer logic.

## In plain English

### What this block does

Compares the declared health of available incoming sources for transfer logic.

### A simple example

In a switchgear lineup with two incomers and a bus tie, the single-line arrangement helps the engineer identify normal supply paths and which loads change source during a transfer. For this reference, the drawing identifies “Incoming source health” at the input and “Available-source selection” at the output. Source voltage alone may not establish supply suitability.

### What the drawings help you do

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

### Terms

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

Concept diagram: Incoming source health → SSI → Available-source selection
A reading guide. The entry's original illustration and input/output facts describe this application.

- [SEL-451 protection, automation and bay control](https://selinc.com/products/451/)


## What it represents

Compares the declared health of available incoming sources for transfer logic. A source and bus arrangement shows how incoming supplies and outgoing circuits can be connected. Breaker and tie positions determine which sections are energized and whether sources can become electrically paralleled.
In a switchgear lineup with two incomers and a bus tie, the single-line arrangement helps the engineer identify normal supply paths and which loads change source during a transfer. For this reference, the drawing identifies “Incoming source health” at the input and “Available-source selection” at the output. Source voltage alone may not establish supply suitability.
Keep normal positions, transfer limits, source independence and protection-zone boundaries visible. An arrangement drawing explains connectivity; the permitted switching sequence needs a separate approved scheme. 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: Incoming source health. Identify its source device or document and the state represented on the drawing.
- Follow the named output: Available-source selection. Cross-reference its destination and separate the command from confirmation that the equipment responded.
- Source voltage alone may not establish supply suitability.
- Keep normal positions, transfer limits, source independence and protection-zone boundaries visible. An arrangement drawing explains connectivity; the permitted switching sequence needs a separate approved scheme.

## Information to show or reference

- Label the source and connection reference for incoming source health.
- Identify the destination and defined state of available-source selection.
- Record the equipment or control-document revision supporting this functional block.
- Check this application limit in the project design: Source voltage alone may not establish supply suitability.

## Important distinction

Source voltage alone may not establish supply suitability. 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: Incoming source health
- Output / connection: Available-source selection
- Lookup label: SSI is a library lookup label, not a standard device designation.

## Public references

- [SEL-451 protection, automation and bay control](https://selinc.com/products/451/) — SEL. Public technical context for switchboard source and bus interfaces. Bus arrangements, breaker control and bay-level coordination. 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.