A Safety Case for the Use of Bipolar ±60V DC for Microgrids

Low voltage dc microgrids are an increasingly viable option for low-volume distribution of electrical power in developing countries. 48V systems have proven popular, owing largely to familiarity from the automotive sector, but there remains to date no universal consensus on optimal system voltage. In this work we propose the use of ±60V bipolar dc distribution and explore the relative advantages and disadvantages of such an approach from the standpoint of electrical safety. Despite significant technical advantages over existing ac and unipolar 48V dc microgrids, two safety aspects must be considered: the detection and extinguishing of series arcs and control of touch safety voltages. We show that an arc suppression technique using load-side voltage monitoring can reliably detect and extinguish series arcs. This functionality has been incorporated into a practical system providing protection against series, line-line and line-earth arc faults. A comprehensive touch voltage assessment with reference to IEC 60479 confirms that the proposed system voltage does not present risk of ventricular fibrillation based on values of body impedance corresponding to the 5th percentile of the population.