Specifications

8 007-4603-001
2: Site Preparation Concepts
It is very important that you consider the inrush current of the system when you select a
UPS or SPS. Unlike power-company lines, these power-conditioning devices may not be
able to supply the current that is required during power-on, even if they are sized
appropriately for nominal current loads. For more information, refer to the following
“Power-line Treatment” section.
It is possible for the inrush current drawn by a device to cause a slight drop in the line
voltage. Although it is very brief, this drop can, in unusual situations, be enough to cause
problems in other devices on the same line.
Inrush current is a characteristic of the power supplies in a system. The inrush current
values apply whether the system is heavily or lightly loaded. Therefore, although a
lightly loaded system may draw less power while it is running, it may still draw a very
large inrush current.
SGI Origin 3900 servers typically have low inrush characteristics.
Power-line Treatment
Power-line treatment may be required if the site has unstable power that results in
problems such as fluctuating voltage, transients, surges and spikes, and noise. Common
causes of unreliable power are old wiring; load-switching equipment, such as welding
and plating devices; and variable-speed motors or motors that start and stop frequently.
A variety of devices are available to improve the quality of a power line, including:
• Line conditioners
• Line regulators
• Isolation transformers
• UPSs
Total Harmonic Distortion
Table 6-2 lists total harmonic distortion (THD). Total harmonic distortion is a measure
of the extent to which a waveform is distorted by harmonic content. This rating indicates
how much the power supply in the system affects the quality of power delivered to other
systems that are supplied by the same transformer.
Note: While the term total harmonic distortion can be applied to either voltage or
current, all of the numbers listed in this guide apply to current.