Case study — DC Arc flash and safety considerations in a 400VDC UPS architecture power system equipped with VRLA battery

Today´s 400VDC UPS power systems that are coming to market for data centers and DC micro-grids are often looked at by potential users as relying on a seemingly high and dangerous DC voltage, with respect to familiar telecom -48VDC power systems, or even familiar data center AC UPS systems operating with 208VAC distribution. After all, ICT loads sitting on a 400VDC common distribution bus is unfamiliar. And the user, not electrician, has to make the end connections (400VDC) to the equipment (servers), typically via a plug to the power strip. In today´s 400VDC Data center UPS power systems, papers have already been written explaining basic system safety considerations and techniques (i.e., high resistance mid-point grounding, with line to ground potentials lowered to +/-190VDC, and personnel/equipment fault currents faults are kept under 5 milli-amps). However, there´s been limited understanding and not much mainstream discussion about DC arc fault energies that can be delivered by energy storage devices such as VRLA batteries (the most common UPS back-up method) on a typical 400VDC bus. It is generally known that batteries can provide extremely high short circuit fault currents, but this information and the relationship to DC arc flash energy calculations (and resultant PPE hazard risk categories) remains a mystery for many potential end users. This is interesting because in today´s traditional AC UPS systems, the nominal 480VDC battery exists, but is isolated from the user due to the inverter. Thus, the same issue of arc flash still exists up to the inverter, but is unfamiliar to the end customer. In our 400VDC UPS, the DC bus is extended all the way to the end equipment and user. After all, arc fault (a.k.a. “arc flash” or “arc blast”) concerns have only become mainstream in the AC world in the past several years in ICT installations, and not yet generally discussed in 400VDC systems. Today, arc flash will mostly be familiar wit- those doing maintenance on >208VAC systems. Typically in -48VDC telecom systems, relatively little concern is shown to arc fault energies from the battery plant, since the low voltage (<;60V) is seen as benign from a shock hazard perspective. Arc flash safety studies are seldom or never done on these systems, and a review of the current research on DC faults shows work still needing to be done. Furthermore, only recently (2012) have national safety specs even addressed high (>100V) DC working environments, and that often from a prescriptive perspective, vs. a theoretical calculation perspective, as exists today in industry accepted AC type arc fault analysis. This paper shall therefore provide an illuminating arc fault study on a hypothetical 400VDC UPS power system. The following topics will be covered: · Description of a typical 120kW system with VRLA UPS batteries. · DC arc flash calculations on this typical system, including analysis of hazards at multiple locations (battery cabinet, power system bus, etc.). · General discussion of incident energy and PPE requirements for workers.