RESEARCH REVIEW

Nvidia is pushing large AI training facilities toward adopting 800V DC power architectures in a drive to further densify racks. Using 800V DC for power distribution frees up valuable space in IT racks currently taken up by AC-DC power supplies, which instead can be used for additional compute. This is integral to Nvidia's roadmap toward the megawatt-scale rack (see Vendors gearing up for 800V DC adoption).

Adding 800V DC to the data hall will bring new power distribution equipment and electrical code requirements, including those relating to safety. DC systems have characteristics distinct from AC, which can be unfamiliar to many electrical engineers operating and maintaining data center power systems. Industry practices around 800V DC have yet to mature.

Chief among the concerns is arc flash, due to the elevated voltage levels and the high currents that 800V DC power distribution equipment will carry. A recent study from Schneider Electric provides a starting point. It concludes that 800V DC systems can be deployed with an acceptable level of arc flash risk, but there is a caveat: the risks depend strongly on the 800V DC power architecture chosen, so detailed analysis is required.

Existing arc flash analysis commonly uses the National Fire Protection Association (NFPA) 70E, a US standard that is used worldwide, but it was designed primarily for AC systems. To demonstrate equivalent safety, the application of NFPA 70E needs fresh thinking, Schneider notes.

The report, DC arc flash analysis: a practical study on 800V DC AI data centers, looks at two 800V DC architectures: one that uses dedicated power racks next to the IT racks (also known as sidecars), and another that uses a centralized 800V DC power system for facility-wide distribution. It concludes that both architectures can be used safely, as long as operators perform arc flash analysis that covers their specific DC systems and configurations.

The study prepared a specific design for each architecture and used NFPA 70E to analyze its arc flash risk. In each case, Schneider identified energy sources, estimated their preliminary current and used transient simulation tools (MATLAB/Simulink) and power system analysis tools (such as an electrical transient analyzer program, or ETAP) to model detailed fault-current behavior.

The basics of arc flash analysis

Arc flash analysis is required for all electrical equipment operating above 50V AC or 120V DC unless it is deenergized before work is performed on it. An analysis determines the available short-circuit current and, therefore, the incident energy. This is then used to classify the arc flash risk into five distinct categories, on a scale of 0 to 4, where 0 denotes no arc flash risk.

A fault becomes a dangerous arc flash incident if the system delivers more than the threshold energy density before the clearing time — the installation-specific delay before upstream protection devices such as fuses and circuit breakers can interrupt the circuit.

NFPA 70E defines the threshold for Category 1 arc flash incident as one where the incident energy density exceeds 1.2 cal/cm2 (around 5 J/cm2) at the arc flash boundary. Arc flash incidents are similar to a lightning strike: the air ionizes, its resistance falls dramatically, and energy flows rapidly and destructively. Workers not wearing personal protection equipment (PPE) appropriate for the arc flash energy can suffer skin burns.

Many installations use standard tables based on equipment type, voltage, available fault current and clearing time to determine arc flash risks and establish rules for PPE use. As existing tables do not cover 800V DC installations, new electrical system design analysis is required, using incident energy to calculate thermal energy exposure at a given working distance.

Rack-level: 800V DC sidecars

The sidecar architecture will enable the support of 800V DC IT racks without major changes to facility-level power systems (see Figure 1). A sidecar manages conversion from standard AC (400/480V), provides an isolation point, can have redundancy built in or achieve redundancy through multiple units for larger 800V DC installations. Most sidecars are also expected to have an increased amount of energy storage (capacitor banks or batteries) to stabilize power on the AC side.

Figure 1 Simplified diagram of a rack-level 800V DC architecture

In its study, Schneider analyzed faults occurring at the 800V DC input connection to the power supply units (PSUs) in the IT rack. The AC/DC converters in the sidecar isolate the 800V DC system from upstream sources, so capacitance is the dominant energy source in this architecture. The study included capacitance energy found both in the sidecar converters and in the input capacitors of the DC-DC PSUs in the IT rack, which step down 800V DC to 50V DC.

The arc flash simulation found that the arc fault waveform would rise to a rapid peak and decay in less than half a millisecond. The arc flash was driven by stored energy in the sidecar's capacitance, rather than by the sustained current from the AC feed, as would be the case with an AC system arc flash — a fundamental difference. The analysis found that the energy was well below the safety threshold for PPE use.

The paper cautions that other design choices could affect the arc flash risk. A system that uses larger-capacity power sidecars, as several manufacturers are planning for the future, is unlikely to change the arc flash risk categorization, as the energy would remain well below the threshold. However, combining several sidecars into a bigger group to support a larger, multi-megawatt 800V DC system may bring the energy closer to the threshold.

Schneider predicts that a real-world test would very likely show an even lower arc flash risk, as analysis based on NFPA 70E is generally conservative compared with test data and more detailed modeling. Also, Schneider's conservative analysis intentionally omitted distributed induction (e.g., from cables), which can limit the initial discharge current.

Centralized 800V DC distribution

A centralized 800V DC power architecture for facility-wide distribution delivers more of the benefits from higher voltage, maintainability and IT floor density (see Figure 2). However, it also expands the use of 800V DC equipment, including distribution switchboards, busways and UPS systems using energy storage devices. Facility-level topologies are more complex than rack-based topologies to analyze for arc flash risks and provide more possible locations for faults. Further, they will likely have higher incident arc flash energy than rack-based topologies.

Figure 2 Simplified diagram of a facility-level 800V DC architecture

Schneider's facility-level case study considers a data center using a centralized medium-voltage AC-to-800V DC converter, coupled with a battery energy storage system (BESS) connected to the 800V DC bus. Both the AC source and BESS use converters that limit fault current magnitude and duration to around 150% of nominal current for 100 ms.

Fault currents can be influenced by the AC/DC converter, the BESS, the rack input capacitors, PSU internal capacitance, and the intervening busways. The design uses IT rack PSUs that prevent fault-current propagation from IT racks to upstream systems.

Schneider analyzed possible fault currents in the DC switchboard and in a rack, using conservative maximum power analysis and fault-current waveform modeling. The switchboard fault delivered energy faster than the rack fault. The conservative power analysis predicted that the switchboard fault would cross the 1.2 cal/cm2 threshold in less than 5 ms, while the rack fault would take around 10 ms. A more accurate ETAP analysis pushed both times out to around 60 ms.

The study suggests that both fault types can be kept below the incident energy limits at which PPE would be required, using common protection devices such as appropriately fast-clearing breakers. Schneider assumed that molded case circuit breakers (MCCBs) could clear faults in 3 ms, while air circuit breakers (ACBs) could clear them in 30 ms.

Outlook

Arc flash analysis results will clearly vary significantly depending on the system architecture, but the analysis presented in Schneider's study suggests that 800V DC can be implemented safely at both rack and facility level, with appropriate precautions. Arc flash risks can either remain below the threshold defined by NFPA 70E for dangerous arc flash incidents or be mitigated through the use of fast-clearing circuit breakers.

The study further suggests that data center operators can base 800V DC arc flash analysis on the existing NFPA 70E framework, but it needs to be applied with an awareness of the detailed architecture and the time-dependent nature of faults in converter-fed DC systems.

As 800V DC systems are only now beginning to emerge, it will take time for design and operational practices to mature. Early dialogue between all data center project participants is therefore essential to ensure that arc flash is considered from the planning and design stages onward, as mitigation will depend largely on power system architecture choices and equipment selection.