HP DL360 Power basics for IT professionals - Page 23

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 lowline 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 I2 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 23