Standby or Offline UPS is one of the three main types of the static UPS.It is the simplest and least expensive uninterruptible power supply system. Actually, some don’t even consider it as a UPS, because of the presence of a momentary break during transfer or switching. Due to that, an offline UPS is sometimes described as a standby power supply (SPS). Nonetheless, the design employed by the standby UPS is the one commonly available at retail stores for protection of desktop and other small computers.

Standby or Offline UPS Block Diagram

Common power problems

The primary role of any UPS is to provide short-term power when the input power source fails. However, most UPS units are also capable in varying degrees of correcting common utility power problems:

• Voltage spike or sustained overvoltage
• Momentary or sustained reduction in input voltage
• Voltage sag
• Noise, defined as a high frequency transient or oscillation, usually injected into the line by nearby equipment
• Instability of the mains frequency
• Harmonic distortion, defined as a departure from the ideal sinusoidal waveform expected on the line

Offline/Standby UPS

When mains supply is present, the UPS output is supplied via a built-in EMI/RFI filter which provides the load with protection from spikes and transients by clamping peak voltage to pre-defined levels. While the mains supply fails or fluctuates outside of the UPS’s operating window, a relay connects the load to the inverter output (resulting in a 4-8ms transfer time). In normal operation, with mains supply present, both output voltage and frequency will track the input voltage and frequency respectively.