# Synchronising lamps

Canonical URL: https://eloragrid.ai/symbols/synchronising-lamps
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: SL
Drawing use: Schematic
Reference families: IEC, Manufacturer

Set of lamps connected between incoming and running phases that brighten and dim with the phase difference during synchronising.

## In plain English

### What does Synchronising lamps mean?

Synchronising lamps are connected between corresponding phases of the running system and the incoming machine. As the phase difference changes the lamps brighten and dim, and in the dark-lamp method the breaker is closed when they are dark, showing the voltages are in phase.

### A simple example

Lamps cannot show small residual voltage differences reliably, since a lamp can look dark with a significant voltage across it. They are therefore a backup or visual aid, and the drawing should show a synchroscope or check-synchronising relay for the actual permission to close.

### What the drawings help you do

Identify which phases each lamp is connected across. Look for a check-synchronising relay in the closing circuit. Synchronising lamps give a visual indication; a synchroscope shows phase angle and slip direction directly.

### This particular reference

Set of lamps connected between incoming and running phases that brighten and dim with the phase difference during synchronising. Synchronising lamps give a visual indication; a synchroscope shows phase angle and slip direction directly.

### Terms

- Coil: A wound conductor that can produce a magnetic field when energised.
- Phase: One of the related AC waveforms in a power system.
- Rating: A value stating the conditions for which equipment is specified.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/)
- [Generator switchgear and paralleling controls](https://www.cummins.com/en-na/generators/products/switchgear)


## What it represents

Synchronising lamps are connected between corresponding phases of the running system and the incoming machine. As the phase difference changes the lamps brighten and dim, and in the dark-lamp method the breaker is closed when they are dark, showing the voltages are in phase.
Lamps cannot show small residual voltage differences reliably, since a lamp can look dark with a significant voltage across it. They are therefore a backup or visual aid, and the drawing should show a synchroscope or check-synchronising relay for the actual permission to close.

## How to read it

- Identify which phases each lamp is connected across.
- Look for a check-synchronising relay in the closing circuit.

## Information to show or reference

- Show lamp connections per phase
- State lamp voltage rating
- Reference check-synchronising function

## Important distinction

Synchronising lamps give a visual indication; a synchroscope shows phase angle and slip direction directly.

## Facts

- Connection: Across incoming and running phases
- Common method: Dark-lamp closing
- Preview: Independent educational concept; consult the applicable standard and project legend for the exact graphic

## Public references

- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/) — IEC. IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced.
- [Generator switchgear and paralleling controls](https://www.cummins.com/en-na/generators/products/switchgear) — Cummins. Public generator paralleling controls context.

## 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.