# Line-voltage synchronising input interface

Canonical URL: https://eloragrid.ai/symbols/line-voltage-synchronising-input-interface
Publisher: Elora Grid
Updated: 2026-11-03
Lookup label: LSII
Drawing use: Schematic
Reference families: Manufacturer

Representation: Functional block
Installation focus: substations
Input: Line-side VT signal
Output: Synchronising voltage input

Identifies the line-side voltage supplied to the assigned synchronising measurement.

## In plain English

### What this block does

Identifies the line-side voltage supplied to the assigned synchronising measurement.

### A simple example

In a substation bay, one voltage circuit can feed protection while a separately identified winding supplies metering. A voltage-selection circuit must make clear which bus or line measurement is being used. For this reference, the drawing identifies “Line-side VT signal” at the input and “Synchronising voltage input” at the output. Both sides require identified references.

### What the drawings help you do

Start at the named input: Line-side VT signal. Identify its source device or document and the state represented on the drawing. Follow the named output: Synchronising voltage input. Cross-reference its destination and separate the command from confirmation that the equipment responded.

### Terms

- Input: Line-side VT signal
- Output: Synchronising voltage input
- Application reference: An explanation of a named drawing function or circuit relationship; the project documents define the equipment and its connections.

Concept diagram: Line-side VT signal → LSII → Synchronising voltage input
A reading guide. The entry's original illustration and input/output facts describe this application.

- [Indoor current and voltage transformers](https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers)


## What it represents

Identifies the line-side voltage supplied to the assigned synchronising measurement. A voltage-transformer secondary arrangement connects a defined primary voltage measurement to protection, metering or control inputs. Winding identity, fuse protection and the voltage reference must remain visible across the drawing.
In a substation bay, one voltage circuit can feed protection while a separately identified winding supplies metering. A voltage-selection circuit must make clear which bus or line measurement is being used. For this reference, the drawing identifies “Line-side VT signal” at the input and “Synchronising voltage input” at the output. Both sides require identified references.
Show the primary measurement point, secondary connection, protective devices and relay reference. A missing measurement can resemble a real system disturbance unless the voltage-circuit health is supervised appropriately. 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: Line-side VT signal. Identify its source device or document and the state represented on the drawing.
- Follow the named output: Synchronising voltage input. Cross-reference its destination and separate the command from confirmation that the equipment responded.
- Both sides require identified references.
- Show the primary measurement point, secondary connection, protective devices and relay reference. A missing measurement can resemble a real system disturbance unless the voltage-circuit health is supervised appropriately.

## Information to show or reference

- Label the source and connection reference for line-side vt signal.
- Identify the destination and defined state of synchronising voltage input.
- Record the equipment or control-document revision supporting this functional block.
- Check this application limit in the project design: Both sides require identified references.

## Important distinction

Both sides require identified references. 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: Line-side VT signal
- Output / connection: Synchronising voltage input
- Lookup label: LSII is a library lookup label, not a standard device designation.

## Public references

- [Indoor current and voltage transformers](https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers) — ABB. Public technical context for voltage-transformer secondary arrangements. Current and voltage measurement transformers and their secondary interfaces. 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.