# Offline standby UPS

Canonical URL: https://eloragrid.ai/symbols/offline-standby-ups
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: UPS VFD
Drawing use: SLD, Schematic
Reference families: IEC, IEEE / ANSI

A UPS that passes raw mains straight to the load and starts its inverter only when mains is lost.

## In plain English

### What does Offline standby UPS mean?

An offline or standby UPS connects the load directly to mains through a transfer relay. A separate charger keeps the battery topped up, and the inverter starts only when mains fails, at which point the relay transfers the load. Transfer typically takes several milliseconds to around ten milliseconds.

### A simple example

Drawings show the mains path through a changeover contact to the output, with the charger, battery and inverter forming a parallel branch that connects to the other contact position. These units are mainly small desktop or control panel units.

### What the drawings help you do

The load sees mains disturbances and the transfer gap, and the inverter waveform may be stepped rather than sinusoidal. The symbol does not show waveform quality, transfer time or whether surge filtering is included.

### This particular reference

A UPS that passes raw mains straight to the load and starts its inverter only when mains is lost. An offline UPS gives no correction until it transfers; an online UPS conditions the output continuously.

### 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.
- Signal: Electrical information passed between devices, rather than necessarily the main power supply.

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.

- [DC UPS, AC UPS and capacitor buffer modules](https://www.phoenixcontact.com/en-gb/products/uninterruptible-power-supplies-ups)
- [DIN-rail power supply, buffer, redundancy and UPS families](https://www.emea.lambda.tdk.com/uk/products/din-rail)


## What it represents

An offline or standby UPS connects the load directly to mains through a transfer relay. A separate charger keeps the battery topped up, and the inverter starts only when mains fails, at which point the relay transfers the load. Transfer typically takes several milliseconds to around ten milliseconds.
Drawings show the mains path through a changeover contact to the output, with the charger, battery and inverter forming a parallel branch that connects to the other contact position. These units are mainly small desktop or control panel units.
The load sees mains disturbances and the transfer gap, and the inverter waveform may be stepped rather than sinusoidal. The symbol does not show waveform quality, transfer time or whether surge filtering is included.

## How to read it

- The normal path has no conversion stage.
- Check output waveform type for sensitive loads.

## Information to show or reference

- State VA rating and transfer time.
- Show transfer relay, charger and inverter.
- Reference output waveform.

## Important distinction

An offline UPS gives no correction until it transfers; an online UPS conditions the output continuously.

## Facts

- Normal load path: Direct mains
- Inverter: Starts on mains failure
- Typical size: Small single-phase units
- Preview: Independent educational illustration; consult the applicable standard and project legend for the exact graphic

## Public references

- [DC UPS, AC UPS and capacitor buffer modules](https://www.phoenixcontact.com/en-gb/products/uninterruptible-power-supplies-ups) — Phoenix Contact. Public UPS function context.
- [DIN-rail power supply, buffer, redundancy and UPS families](https://www.emea.lambda.tdk.com/uk/products/din-rail) — TDK-Lambda. Public DIN-rail UPS family 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.