High-Line Or Low-Line Input Voltage; Inrush Current - Compaq 226824-001 - ProLiant - ML750 Basics Manual

Under steady-state operating conditions and for reliable operation, the full-load current should not
exceed 80 percent of the rated current.
Once AC power reaches a server rack, a power supply converts AC power to DC (for example,
208VAC to 12 VDC to power a server blade) and when necessary, a converter provides DC-to-DC
voltage conversion for internal operations.
In traditional data centers, power goes from the transformers to a number of sub-panels that contain
circuit breakers. A wire from each circuit breaker provides power to each outlet where the equipment
is connected. Once installed in a rack, each non-blade server requires single-phase power from the
PDU. The PDU typically provides circuit-breaker protection for components plugged into its outlets.

High-line or low-line input voltage

Selection of the proper input voltage is a very important issue in the Americas and other regions
around the world that follow North American commercial wiring practices. Choosing between low-
line operation and high-line operation affects the following areas: power supply output capacity,
power conversion efficiency, power supply thermal operation, and power supply reliability.
HP defines low line as 100 to 120VAC and high line as 200 to 240VAC. In North America,
commercial power is delivered to the server at 120 V or 208 V. All HP servers now have auto-sensing
input circuitry that automatically adjusts to the applied input voltage. The only exceptions are those
devices that are defined as high-line operation only.
Some ProLiant servers are equipped with power supplies that have greater capacity when connected
to high-line input power. Power supplies operate more efficiently when operating at high line. ProLiant
and BladeSystem servers all operate with efficiencies of 85 percent or greater when connected to a
high-line voltage source. Operating at low line causes the power supply to operate at a lower
efficiency and to draw more current for the same power output.
Power supply thermal operation also depends on the choice of input voltage. Many input
components, such as diodes, actually run hotter when operating with low-line input power. This is
caused by the almost double input current. The formula for heat generated in a component is I 2 x R,
where I is the input current and R is the resistance of the component. Therefore, if the input current is
doubled, the heat generated in any given component is going to be four times higher. This heat must
then be cooled by the data center air-conditioning system, which increases costs. HP provides power
calculators to help determine the power and cooling requirements for HP ProLiant servers. These
calculators are revised often and the latest version should be downloaded from
http://h30099.www3.hp.com/configurator/powercalcs.asp or
www.hp.com/go/bladesystem/powercalculator.

Inrush current

For several milliseconds during start-up, electronic devices containing solid-state power supplies,
transformers, and capacitors experience an initial current that can be several times greater than their
operating current. This phenomenon is called inrush current. High inrush current surge during start-up
can affect weak electrical systems by unnecessarily tripping circuit breakers.
Typical inrush events that occur in the real world last far less than 2 ms. Inrush current of individual
servers is not problematic; however, inrush current is additive. Multi-server implementations
experiencing simultaneous inrush current can easily surpass 50 A for a data center, even if it is for a
very short period of time.
Given the large number of servers in a data center, power supplies must contain inrush current surge
protection to limit the current drawn during startup. If the power supply does not use current surge
protection, relays and fuses must be rated higher than any possible surge current. Server power
supplies currently offered by HP actively limit inrush current. Circuitry in the front end of the power
supply limits the amount of current drawn by the power supply during the initial period when AC
power is applied. It is always prudent to open circuit breakers and power off loads in an affected
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