The two main types of UPS operation (known as the UPS system’s topology) used in Network/Server UPS Systems are line-interactive and on-line. At the most basic level, line-interactive UPS systems are less expensive than on-line UPS systems (approximately 20 to 40% less, depending on the model and manufacturer), but they also provide less protection than on-line UPS systems. It’s helpful to explore the differences between line-interactive and on-line models to understand the trade-offs involved. Note: If you need a UPS system larger than 5,000 VA (4,000 watts), an on-line UPS is your most likely choice. Let’s consider four key UPS features and how line-interactive and on-line technology deliver each feature to connected equipment.
Line-interactive UPS systems use automatic voltage regulation (AVR) to correct abnormal voltages without switching to battery. (Regulating voltage by switching to battery drains your backup power and can cause batteries to wear out prematurely.) The UPS detects when voltage crosses a preset low or high threshold value and uses transformers to boost or lower the voltage by a set amount to return it to the acceptable range. On-line UPS systems use a more precise method of voltage regulation: they continuously convert incoming AC power to DC power and then convert the DC power to ideal AC output power. This continuous double-conversion operation isolates connected equipment from problems on the AC line, including blackouts, brownouts, overvoltages, surges, line noise, harmonic distortion, electrical impulses and frequency variations. In “line” mode (i.e. when not operating from battery), line-interactive UPS systems typically regulate output within ±8-15% of the nominal voltage (e.g. 120, 208, 230 or 240 volts). On-line UPS systems typically regulate voltage within ±2-3%.
All Network/Server UPS Systems include surge suppression and line noise filtering components to shield your equipment from damage caused by lightning, surges and electromagnetic (EMI/RFI) line noise. On-line UPS systems offer superior protection because the double-conversion operation isolates equipment from problems on the AC line.
Pure Sine Wave Output
When operating from battery power, a line-interactive UPS system generates the waveform of its AC output. An on-line UPS does this continuously. All on-line and many line-interactive UPS systems have pure sine wave output. Pure sine wave output provides maximum stability and superior compatibility with sensitive equipment. Pure sine wave power is required by some equipment power supplies and prevents others from overheating, malfunctioning or failing prematurely.
Transfer Time to Battery
During an outage, line-interactive UPS systems typically transfer from line power to battery-derived power within two to four milliseconds, which is more than fast enough to keep all but a small percentage of the most power-sensitive equipment operating without interruption. On-line UPS systems do not have a transfer time because the inverter is already supplying the connected equipment load when an outage occurs.