Uninterruptible Power Supplies Topologies - Standby UPS

An Uninterruptible Power Supply is a power protection device with a battery supply that provides runtime when the mains fails or fluctuates wildly and some degree of power quality control when a mains or other alternating current (AC) supply is present.

Uninterruptible Power Supplies - Standby (off-line)

Standby - the inverter only switches on (with a several millisecond break in supply) when the mains fails or fluctuates wildly outside the input voltage window of the EMI filter or if fitted Automatic Voltage Stabiliser (AVS). The normal output waveform on battery is a square wave. The UPS offers an budget conscious alternative to a line interactive UPS for the protection of computer and telecoms peripherals.

ENTRY level power protection from 350VA to 1kVA (single phase only) - Voltage and frequency dependent (VFD) according to BS EN 62040-3:2001.

The most basic form of Uninterruptible Power Supply (UPS) system, where the load is supplied by the incoming mains supply without stabilisation or regulation whilst the mains supply remains within a specified input window.

The Uninterruptible Power Supply tracks the mains supply in terms of voltage and frequency variations. Once the incoming mains supply exceeds the specified window, the Uninterruptible Power Supply switches on the inverter to continue to supply power to the load. There is an inverter switch on time of a few milliseconds and this will be generally longer than that encountered with a line interactive Uninterruptible Power Supply.

How The Standby (off-line) Uninterruptible Power Supply Tackles Power Problems:

Output Waveform

When the mains supply is present it is filtered by the UPS without any stabilisation or regulation then passed directly through to the load. The output waveform from the inverter is generally a square-wave output. The square-wave output may not be suitable for all types of loads.

Sags and surges

The load is supplied by the incoming mains supply which is filtered by the UPS whilst the inverter remains switched off (provided that the input window is not exceeded). The battery charger continues to convert the mains AC into DC to charge the battery. If the sag or surge exceeds the input window then the inverter is switched on to maintain the required output voltage tolerance.

Transients, spikes and electrical noise

The output to the load is filtered within the UPS. These filters attenuate (reduce) the incoming transients, spikes or electrical noise. These disturbances will not be completely prevented from passing through to the load and any large voltage spikes or transients that may exceed the input voltage window will cause the UPS to switch to inverter power.


When the incoming mains supply exceeds the specified window, the UPS switches on the inverter to continue to supply power to the load.

Short duration mains failures

During short duration mains supply failures the UPS will continue to supply power to the load using the inverter, however constant short duration supply failures will lead to unnecessary discharging of the battery.

Long duration mains failure - blackouts

The load is supplied by the inverter which is switched on the moment the incoming mains supply fails. No additional battery packs can be added for additional autonomy, therefore the only option is to oversize the UPS to achieve a longer runtime as most stand by UPS are only provided with a short autonomy of approximately 5-10 minutes.


The battery charger will generally recharge the batteries to 80% within 8 hours to provide sufficient battery autonomy ready for the next supply failure. The UPS is not capable of supporting any external battery packs.