# Breaker-and-a-half middle-bay interface

Canonical URL: https://eloragrid.ai/symbols/breaker-and-a-half-middle-bay-interface
Publisher: Elora Grid
Updated: 2026-11-03
Lookup label: BMI
Drawing use: Schematic
Reference families: Manufacturer

Representation: Functional block
Installation focus: switchyards
Input: Two diameter circuits
Output: Middle-breaker interface

Identifies the shared middle-breaker connections within the assigned diameter.

## In plain English

### What this block does

Identifies the shared middle-breaker connections within the assigned diameter.

### A simple example

In a switchyard single-line drawing, the bay arrangement helps the reader follow the circuit from its bus connection to the external line, transformer or compensation equipment and identify the measurement boundaries along the path. For this reference, the drawing identifies “Two diameter circuits” at the input and “Middle-breaker interface” at the output. Shared breaker failure affects both associated circuits.

### What the drawings help you do

Start at the named input: Two diameter circuits. Identify its source device or document and the state represented on the drawing. Follow the named output: Middle-breaker interface. Cross-reference its destination and separate the command from confirmation that the equipment responded.

### Terms

- Input: Two diameter circuits
- Output: Middle-breaker interface
- Application reference: An explanation of a named drawing function or circuit relationship; the project documents define the equipment and its connections.

Concept diagram: Two diameter circuits → BMI → Middle-breaker interface
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

Identifies the shared middle-breaker connections within the assigned diameter. A bay circuit arrangement groups the switching, measuring and protection interfaces associated with a specific network circuit. A line bay, transformer bay and reactor bay share some devices but connect different kinds of primary equipment.
In a switchyard single-line drawing, the bay arrangement helps the reader follow the circuit from its bus connection to the external line, transformer or compensation equipment and identify the measurement boundaries along the path. For this reference, the drawing identifies “Two diameter circuits” at the input and “Middle-breaker interface” at the output. Shared breaker failure affects both associated circuits.
Preserve the actual order of primary connections, isolation points and CT locations. These conceptual arrangements do not establish equipment clearances, terminal layouts or an approved switching sequence. 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: Two diameter circuits. Identify its source device or document and the state represented on the drawing.
- Follow the named output: Middle-breaker interface. Cross-reference its destination and separate the command from confirmation that the equipment responded.
- Shared breaker failure affects both associated circuits.
- Preserve the actual order of primary connections, isolation points and CT locations. These conceptual arrangements do not establish equipment clearances, terminal layouts or an approved switching sequence.

## Information to show or reference

- Label the source and connection reference for two diameter circuits.
- Identify the destination and defined state of middle-breaker interface.
- Record the equipment or control-document revision supporting this functional block.
- Check this application limit in the project design: Shared breaker failure affects both associated circuits.

## Important distinction

Shared breaker failure affects both associated circuits. 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: Switchyards
- Input / boundary: Two diameter circuits
- Output / connection: Middle-breaker interface
- Lookup label: BMI 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 switchyard bay circuit arrangements. Bay arrangements and coordinated breaker controls. 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.