Direct Current (DC) Power Systems

DC power systems that serve critical loads are common in two forms:

• The primary power source for access provider and carrier equipment.
• As an alternative to AC power in computer rooms because of energy efficiency, design simplification and ease of paralleling alternative energy sources DC power distribution systems function within the data center in the same way as the AC systems providing power to a variety of loads. However, DC-based systems can offer additional features that can be attractive to data center designers and operators. It is also possible to mix DC and AC systems in hybrid configurations, for example, providing DC to critical loads and AC to mechanical loads.

DC power system operating voltages are affected by several factors such as:

• Battery technology (e.g., lead-acid varieties, nickel-cadmium, nickel-metal-hydride, lithium-ion varieties, sodium varieties, flow varieties)
• Rectifier output regulation in maintaining constant voltage under dynamic loads
• Voltage drops in DC power conductors—cable sizes and lengths
• Operating voltage limits of various connected loads
• Derating for environmental conditions such as altitude
• All DC equipment shall be properly and clearly marked according to the applicable electrical or building codes and as required by the appropriate listing agency. DC equipment includes, but is not limited to:
• Cords
• Cables
• Raceways
• Bus ways
• Power connectors
• Junction boxes
• Batteries
• Generators
• Flywheels
• Circuit breakers
• PDUs
• Rectifiers
• DC-UPS
  

The design of all DC-powered ITE shall likewise comply with applicable sections of required electrical and building codes and shall be rated, listed, and marked for DC operation at the anticipated voltage range.

Electrical safety clearance rules are applicable to both AC and DC circuits. For DC circuits, the clearance requirements shall generally be the same as those for AC circuits having the same nominal voltage to ground.

Direct current systems should meet the same requirements for availability, reliability, maintenance, and safety as AC power systems.

DC-based critical power systems are an emerging and potentially viable option for mission-critical environments, it is advisable at the present time to work closely with the designers, equipment providers, and electrical contractors who have experience in the direct current applications.

Designers, operators and others involved in DC systems should refer to the appropriate standards such as:

• Over current protection devices (OCPD) and disconnect devices for DC power systems will need further investigation for higher voltage DC systems. AC-rated devices cannot automatically be used on a DC power system at the same rated voltage and current.
• The established 2:1 protection coordination scheme for DC fuses is not readily applicable since data centers typically utilize circuit breaker technology.
• Transients for other than 48 VDC systems are not well described.
• Battery disconnect or other DC disconnect usage at voltages above 50 VDC is not well established.
• Disconnects may not be required, depending upon local requirements. 2-pole or even 3-pole disconnects may be required, perhaps at multiple points within a system. Interrupting capacity and withstand ratings may need to be selectively coordinated.
• Conductor sizing regulations for DC are not well established; sizing per AC code requirements may
  be inappropriate for DC conductors.
• If the rectifier technology is switched mode power supply (SMPS), it may require use of electrical noise control via power line filters (possibly connected to the return or DC equipment ground conductor) and might create additional audible noise from cooling fans.
• Rectifier operation modes at the higher voltages, such as 380 VDC, may need verification for AC
  input power factor correction, load sharing, paralleling, voltage sensing, and current limitation.
• The DC power distribution topology is historically bus or cabling. Further, there may also be a need for an underfloor topology. Distribution (such as rigid bus bar) withstand under extreme fault current conditions (such as a direct short across the batteries [if batteries are used]) will probably vary considerably from that for telecommunications 48 VDC system.
• Voltage drop is a concern, especially if DC is run for distances longer than a few meters. Guidelines for calculating voltage drop can be found in IEEE 946.
  
• For metallic battery racks, determine the local authority requirements for sizing the bonding conductor. 13.3 mm2 (6 AWG), which is typically specified for telecommunications applications below 50 VDC, may not be acceptable for higher voltages (e.g., 380 VDC).
• Consult with the local authority regarding limitations for the location of centralized DC power systems in the computer room.
• Determine the grounding methods required by the ITE. The 380 VDC system might be operated
  as a positive grounded system (similar to the 48 VDC telecommunications system) as a negative grounded system (similar to the 24 VDC telecommunications system) or as an ungrounded system (similar to higher voltage UPS systems).