On-line (or Continuous Operation)

Top-of-the-range technology for the centralized protection of servers

Operating diagram

On-line technology has changed a great deal in the last few years, resulting in a dramatic cut in the size of these products. Contrary to what many people believe, an On-line UPS does not supply output current from its battery at all times. The current is completely regenerated by its transformation from AC to DC and then back to AC, thus guaranteeing a constant level of quality, regardless of disturbance on the mains power source. The batteries, themselves, are only used in the event of an outage. Claims that an On-line UPS wears out its batteries more quickly than a Line Interactive UPS are therefore a gross misrepresentation.

High Frequency

Instead of chopping the current at the same frequency as the mains power supply (50 Hz), more and more On-line UPSs use high-frequency chopping: Pulsar EX UPSs, for instance, operate at 85 kHz. This technique means that the size of the transformer used inside the UPS can be significantly reduced. The difference is remarkable at all power ratings: for a 4 or 5 kVA device, a low frequency transformer weighs about 60 kg. The same technology, but operating at a higher frequency, only weighs 4 kg, which reduces the footprint and weight of the UPS. This technology can only be implemented for ratings in excess of 500 VA on On-line UPSs.

Filtering capacities

By carrying out continuous double conversion, an On-line UPS significantly attenuates electrical disturbances, which might otherwise damage a computer, forming a natural barrier. This possibility is not available on Standby UPSs, which explains why an On-line device will always deliver better filtering capacity than Line Interactive or Off-line UPSs.

Automatic bypass

Theoretically, this system guarantees the supply of electricity to the load in the event of a fault on the UPS. In fact, although this function really is implemented on the largest high power UPSs, it is not fully implemented on small devices on account of the cost of the technology involved. Only a battery-charger fault will result in load transfer to the bypass without interruption in the event of a problem. Consequently, the principal use of the bypass is to absorb startup inrush currents. When a UPS uses its bypass, the load is no longer protected, which may result in other problems, particularly if there is no galvanic isolation (see following pages). For all these reasons, Merlin Gerin has chosen not to implement an automatic bypass on Pulsar PSX On-line UPSs, which plug into an ordinary wall socket. The Pulsar CSX range, which connects via a terminal block, does implement an automatic bypass.

Manual bypass

The other bypass function is intended for maintenance work: it is used to power down the UPS without interrupting the power to the load. It is thus possible to work on a faulty UPS in complete safety. This function is of no use on a UPS which is replaced by a standard exchange unit when it requires repairs as it is necessary to stop the load in order to replace the UPS. The Pulsar CSX range, for which servicing is carried out on-site, on the various sub-assemblies, is fitted with a manual bypass. Problems related to long battery-backup timesTelecommunications applications, for instance, require very long battery-backup times, from 6 to 8 hours. Only On-line UPSs can guarantee proper operation under these conditions. This operating mode places continuous demands on all UPS components, as is always the case on an On-line UPS. On the other hand, a Line Interactive is not designed to operate for so long in battery mode, which may result in overheating problems and premature wear of the UPS.