Shock & Arc Flash Analysis of DC Electrical Systems

Most concepts of electrical safety revolve around AC electrical systems.  While AC electrical systems are the most prevalent in the workplace, DC electrical systems are being used in more applications and equipment, e.g. photovoltaic (PV) systems, within UPS equipment, control cabinets, industrial machines and telecommunication systems.

In the most recent edition (2012) of the Standard for Electrical Safety in the Workplace, NFPA 70E, information to protect qualified employees from the shock and arc flash hazards associated with working on exposed live (energized) circuits in DC systems is starting to be addressed.  As noted in Table 130.4(C)(b), the limited, restricted, and prohibited approach boundaries for workers working near or on exposed live (energized) electrical conductors in direct current (DC) electrical systems are provided [1]. 

For DC electrical systems operating at less than 100Vdc, the limited, restricted, and approach boundaries are not specified.  For DC electrical systems operating at 100 Vdc to 300 Vdc, the limited approach boundary is 42 inches, while the restricted and prohibited approach boundary is to simply avoid contact.  For DC electrical systems operating at 301 Vdc to 1,000 Vdc, the limited approach boundary is 42 inches, the restricted approach boundary is 12 inches, and the prohibited approach boundary is 1 inch.

While shock hazard boundaries are now identified, information associated with arc flash boundaries, incident energy associated with DC electrical systems is not specified differently than AC electrical systems.  Additionally, the common electrical simulation tools do not have the capabilities to calculate the arc flash boundaries and incident energy for DC electrical systems.

The literature on arc flash hazards associated with DC electrical systems is limited [2, 3, 4].  While limited, this research data combined with an additional understanding of how DC electrical systems are rectified from AC electrical systems will add to the understanding of the shock and arc flash hazards for a DC electrical system [5].  When combining literature, interpolation of overcurrent protective devices, an understanding of the DC electrical system parameters, and equipment, an engineer can create a mathematical model that will adequately represent the arc flash boundaries and incident energy for a DC electrical system.

Once the shock and arc flash hazard analysis for electrical equipment connected to the DC electrical system has been evaluated, the equipment is required to be labeled in accordance with NFPA 70E and ANSI Z535.4.

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References:

1.     National Fire Protection Association (NFPA), Standard for Electrical Safety in the Workplace, NFPA 70E – 2012, Quincy, MA USA.

2.     Ammerman, R.F., Gammon, T, Sen, P.K., and Nelson, J.P. (2010), DC-Arc Models and Incident-Energy Calculations.  IEEE Transactions on Industry Applications, Volume 46, Number 5.

3.     Doan, D.R. (2010).  Arc Flash Calculations for Exposures to DC Systems.  IEEE Transactions on Industry Applications, Volume 46, Number 6.

4.     Fontaine, M.D. and Walsh, P (2012).  DC Arc Flash Calculations – Arc-in-open-air & Arc-in-a-box – Using a Simplified Approach (Multiplication Factor Method).  IEEE Paper No. ESW2012-25.

5.     RailCorp (2010).  Rectified Transformer & Rectifier Characteristics.  Document EP 03 00 00 01 TI, Version 3.0, Issued May 2010.