Specifications

Electrical Requirements
007-4603-001 7
Grounding Requirements
• Ensure that the ground has sufficiently low impedance in order to limit the voltage
to ground and to facilitate the operation of protective devices in the electrical
circuit.
• Ensure that all grounds entering the room are interconnected somewhere within the
building to provide a common ground potential. This includes any separate power
sources, lighting, convenience outlets, and other grounded objects such as building
steel, plumbing, and ductwork. Refer to the IEEE Emerald Book: IEEE
Recommended Practices for Powering and Grounding Electronic Equipment and
the National Electric Code (NEC) for power, grounding, and life safety issues.
Warning: Any difference in ground potential greater than 250 millivolts
between two racks that are connected with NUMAlink or Xtown cables can
cause severe equipment damage.
• To maintain your entire SGI Origin 3900 server at the same electrical potential, all
multiple-rack systems must be bolted together.
Power Factor
Power factor is a number between 0 and 1 that represents the ratio of the total power in
watts to the total volt-amperage input. A system with a power factor of one (sometimes
called “unity”) is making full use of the energy that it draws. A system with a power
factor of 0.75 is effectively using only three-quarters of the energy that it draws.
SGI Origin 3900 servers are power-factor corrected and thus have a power factor very
close to 1. Some peripherals do not have this correction built in.
Caution: Ensure that you consider the power factor of the system when you select an
uninterruptible power supply (UPS).
Inrush Current
Inrush current is the peak current that flows into a power supply as AC power is applied.
The inrush current is usually much higher than the nominal current. This temporary
increase is due to the charging of the input filter capacitors in the power supply and is
limited only by the input impedance of the power supply and the wiring that supplies
power to the system.
The inrush current often far exceeds the rating of the electrical outlet to which the system
is connected. If the system is connected directly to “wall power” (that is, it is not on a
UPS or a standby power system [SPS] ), this is typically not a problem. The peak inrush
current lasts for only a part of one AC cycle (less than 1/60 of a second). This is not long
enough to damage wiring and, in most cases, will not trip a circuit breaker (depending on
the delay curves of the circuit breaker).