New Guide Helps Engineers Navigate DC Protection Specification

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Direct current (DC) distribution systems was the main technology in electricity’s early days, but after some years alternating current (AC) was used in most applications because production of electricity is mainly alternative (turbo generators) and voltage adaptation is simple (transformers). However, DC has maintained its importance in a number of specific applications – especially those involving battery-based storage equipment and uninterruptible power supplies. Designing protection into DC distribution systems can be complicated, though, and require special attention from the consulting engineers working on such projects.

DC technology is compatible with simple battery storage solutions, and it has benefited from the development of power supplies with electronic converters and batteries. DC systems can be found performing a number of tasks in remote and industrial settings, including:

  • Powering telecommunications infrastructure
  • Providing the electrical supply for industrial programmable logic controllers (PLCs)
  • Supplying auxiliary uninterruptible DC power, via a connected battery

Many options, little guidance

However, DC distribution poses challenges to system designers. Most significantly, the core difference between direct and alternating current makes protection more difficult in DC designs. While AC cycles to a zero-current state every half-cycle, DC remains at a constant current, which makes arcing a much bigger hazard in any attempt to break the current. You can see this difference in action in a YouTube video produced by Electric Experiments Roobert33.

Additionally, the broad range of DC voltages and their related protection requirements can be confusing. While AC distribution systems generally fall into either 230V or 400V categories, DC distribution might be specified in any of eight different voltages, ranging from 24V up to 400V. Protection and controls selection varies widely with system design characteristics, and determining the right devices for any given project can be a complicated process. International Electrotechnical Commission (IEC) Standard 60364-1, “Electrical Installations for Buildings,” provides a number of options for system designers, but it’s not always a helpful resource in narrowing down those options to a single ideal choice.

Voltage isn’t the only variable playing into the specification process in DC distribution – designers also must consider how their system will be earthed. Additionally, DC devices are all derived from their AC counterparts, which means sometimes 2-pole products aren’t available and 3-pole versions must be used instead. And DC panels aren’t standardized in the way AC panels are, so standard accessories also aren’t available.

New resource for specifiers

To help specifiers navigate the broad catalog of requirements and related product offerings, Schneider Electric has developed the “Distribution guide direct current: Choosing and implementing protective devices for voltage below 380 V DC.” The guide covers DC systems supplied by rectifier, and/or battery, isolated or connected to earth. It also provides information on insulation monitoring systems for DC applications. You can download the guide here. Also, be sure to visit our Consulting Engineer Portal, where you can find more information on DC distribution design and get in touch with professional engineers who can help answer your design questions.

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