# Double frequency meter

Canonical URL: https://eloragrid.ai/symbols/double-frequency-meter
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: Hz Hz
Drawing use: SLD, Schematic
Reference families: IEC, Manufacturer

Instrument with two frequency elements showing running and incoming frequency together on a synchronising panel.

## In plain English

### What does Double frequency meter mean?

A double frequency meter shows the frequency of the running system and the incoming machine on one instrument. During manual synchronising the operator adjusts the incoming governor until the two readings match closely, then uses the synchroscope for final phase alignment.

### A simple example

The meter shows frequency only and gives no information on phase angle. Closing with matched frequencies but a large phase difference can still cause a severe disturbance, so it must always be read with the synchroscope or check-synchronising relay.

### What the drawings help you do

Identify running and incoming inputs. Read together with the synchroscope. A double frequency meter compares frequency; a synchroscope shows the phase difference and its rate of change.

### This particular reference

Instrument with two frequency elements showing running and incoming frequency together on a synchronising panel. A double frequency meter compares frequency; a synchroscope shows the phase difference and its rate of change.

### Terms

- Normally open / normally closed: The contact state in the normal condition defined for the drawing.
- Coil: A wound conductor that can produce a magnetic field when energised.
- Phase: One of the related AC waveforms in a power system.

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

A double frequency meter shows the frequency of the running system and the incoming machine on one instrument. During manual synchronising the operator adjusts the incoming governor until the two readings match closely, then uses the synchroscope for final phase alignment.
The meter shows frequency only and gives no information on phase angle. Closing with matched frequencies but a large phase difference can still cause a severe disturbance, so it must always be read with the synchroscope or check-synchronising relay.

## How to read it

- Identify running and incoming inputs.
- Read together with the synchroscope.

## Information to show or reference

- Label both inputs
- Show VT sources
- Reference synchronising check

## Important distinction

A double frequency meter compares frequency; a synchroscope shows the phase difference and its rate of change.

## Facts

- Inputs: Running and incoming voltage
- Displayed quantity: Two frequencies
- 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 context for generator paralleling controls where synchronising instruments are used.

## 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.