Line Interactive UPS
On-Line UPS
UPS designs come in three different types - Off-line, Line Interactive, and On-line
Off-line and Line Interactive work in fundamentally the same way. Power is supplied to the UPS, some 'power cleaning' is applied as it passes through the UPS and in the event of a power failure, a switch automatically operates to draw power from the battery bank. When power is restored then the switch automatically returns to its normal position.
The main differences between off-line and line interactive are that with line interactive, the 'power cleaning' is superior, and the switching in most cases is much faster.
For example, a typical pc will not notice a gap in power of less than 20 milliseconds. A line interactive switch takes less than 5 milliseconds to operate.
On-line design is very different. Here, the power coming into the UPS is converted from AC to DC, and runs in parallel with the battery bank. On the output side, the power is re-converted back to AC. There is no switching, the battery simply starts to supply power as soon as the mains power disappears. Also, the 'power cleaning' comes from the double conversion process, and is more effective than the other two types.
Line Interactive and off-line UPS's are generally cheaper than on-line units, and are generally more suited to smaller applications where economy is the priority.
UPS Sizing concepts
It is always important to oversize the UPS you choose. The reason for this is that any UPS you buy will be expected to last at least 3 years, and larger units should be expected to last for anything up to 15 years.
It is impossible to accurately predict future requirements, but in general loading is likely to increase, therefore additional spare capacity is always a benefit.
Backup times for UPS's vary from about 5 minutes to several hours. The main questions to ask before deciding the UPS backup time are:
In the event of a long power outage do we need to:
1. - simply shut down the servers and wait until power is restored. In which case a basic 5-10 minute backup time will be perfectly adequate
2. - have a longer backup time in order to shut down a large database, or manually shut down certain applications. In this case something like a 30 minute backup time will probably be required
3. - keep key personnel, the telephone system or communications to to remote sites operational. In this case a longer backup time will be required. Please bear in mind that air-conditioning units are not normally supported by the UPS, and the server room may become overheated. This can often be solved by simply opening doors and windows. If this is likely to be inadequate, then the air conditioning can be powered from the UPS, or by a generator.