HP DL360 Critical factors in intra-rack power distribution planning for high-d - Page 3

Key factors of power distribution in high-density racks Power distribution units (PDUs) are highly recommended for high-density systems that place heavy loading demands on the facility alternating current (AC) power bus. PDUs allow IT infrastructure designers to: • Use segmented circuits to improve serviceability and enable sequential startup • Achieve balanced loading between dual power buses • Support redundant operation of equipment with redundant power supplies A variety of PDU types are available to meet different needs. Key considerations for choosing an appropriate PDU include: • Level and phase of power to be distributed • Connector type • Estimated power consumption of active components in the rack • Physical space available for PDU mounting • Special requirements or features such as circuit monitoring Power level and phase PDUs are designed to distribute a specific amount of current at either a low-line (100 - 127 VAC) or high-line (200 - 240 VAC) voltage range, depending on which level is supplied to the rack. Most HP server equipment features auto-sensing input circuitry that automatically detects and adjusts to the input voltage range. High-line voltage is the favored choice for electrical equipment requiring high power consumption. As determined by Ohm's law [voltage (E) x current (I) = power (W)], higher voltage allows power supplies to produce the necessary power with less AC current. For example, a power supply that is required to produce 400 watts of power for a server will need to draw 3.3 amperes if operated with 120 VAC. The same power supply operating at 208VAC only requires 1.9 amperes to produce 400 watts-over a 30 percent reduction of AC current. High-line operation is generally more economical because it allows power supplies to create less heat (thereby reducing the cooling requirements). High-line voltage also can enhance reliability by extending equipment meantimebetween-failure (MBF) periods. Note that some power supplies require high-line voltage to operate at maximum potential. High-line voltage (208 VAC in North America, 200 VAC in Japan, 220 VAC elsewhere) is becoming commonly available in data centers. High-line voltage can be distributed as single or three-phase power. With single-phase power, voltage reaches its maximum and minimum 120 times a second (at 60 Hz). Three-phase power uses three cycles 120 degrees out of phase to provide a more constant voltage to the load (Figure 1). The efficiency of three-phase power allows the use of smaller circuit sizes (smaller-gauge conductors) to distribute the same amount of AC power. Figure 1. Comparison of single- and three-Phase AC Single-Phase AC Three-Phase AC 3