# Bus differential check-zone interface

Canonical URL: https://eloragrid.ai/symbols/bus-differential-check-zone-interface
Publisher: Elora Grid
Updated: 2026-11-03
Lookup label: BDCI
Drawing use: Schematic
Reference families: Manufacturer

Representation: Functional block
Installation focus: substations
Input: Check-zone current inputs
Output: Check-zone permission

Provides the separately defined overall check-zone condition used by a bus protection scheme.

## In plain English

### What this block does

Provides the separately defined overall check-zone condition used by a bus protection scheme.

### A simple example

In a substation feeder bay, local current and voltage measurements may combine with a remote-end signal before the protection issues the assigned trip request. Backup actions use their own defined initiations and supervision. For this reference, the drawing identifies “Check-zone current inputs” at the input and “Check-zone permission” at the output. Check zones and discriminating zones have different roles.

### What the drawings help you do

Start at the named input: Check-zone current inputs. Identify its source device or document and the state represented on the drawing. Follow the named output: Check-zone permission. Cross-reference its destination and separate the command from confirmation that the equipment responded.

### Terms

- Input: Check-zone current inputs
- Output: Check-zone permission
- Application reference: An explanation of a named drawing function or circuit relationship; the project documents define the equipment and its connections.

Concept diagram: Check-zone current inputs → BDCI → Check-zone permission
A reading guide. The entry's original illustration and input/output facts describe this application.

- [SEL-411L line differential protection overview](https://selinc.com/products/411l/)


## What it represents

Provides the separately defined overall check-zone condition used by a bus protection scheme. A protection scheme combines measurements, decision elements and communication or trip outputs to protect a defined part of the network. A function number names a job; the scheme interface shows how that job connects to other equipment.
In a substation feeder bay, local current and voltage measurements may combine with a remote-end signal before the protection issues the assigned trip request. Backup actions use their own defined initiations and supervision. For this reference, the drawing identifies “Check-zone current inputs” at the input and “Check-zone permission” at the output. Check zones and discriminating zones have different roles.
Keep each protected zone, initiating quantity, blocking signal and trip destination distinct. Settings, communication performance and coordination require the approved protection study and selected relay documentation. 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: Check-zone current inputs. Identify its source device or document and the state represented on the drawing.
- Follow the named output: Check-zone permission. Cross-reference its destination and separate the command from confirmation that the equipment responded.
- Check zones and discriminating zones have different roles.
- Keep each protected zone, initiating quantity, blocking signal and trip destination distinct. Settings, communication performance and coordination require the approved protection study and selected relay documentation.

## Information to show or reference

- Label the source and connection reference for check-zone current inputs.
- Identify the destination and defined state of check-zone permission.
- Record the equipment or control-document revision supporting this functional block.
- Check this application limit in the project design: Check zones and discriminating zones have different roles.

## Important distinction

Check zones and discriminating zones have different roles. 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: Check-zone current inputs
- Output / connection: Check-zone permission
- Lookup label: BDCI is a library lookup label, not a standard device designation.

## Public references

- [SEL-411L line differential protection overview](https://selinc.com/products/411l/) — Schweitzer Engineering Laboratories. Public technical context for substation protection scheme interfaces. Line protection, breaker failure, communications and protection 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.