Cost efficient Cable Distribution Grids currently and to a greater extend in the future, must meet the following Smart-Grid requirements: 1. High performance and reliability (Self-Healing) 2. Integration and control of Distributed Generation (Islanding) and e-car charging. 3. Reduction of power losses (Congestion Management resulting in "Green Smart Grid") The only point in the network where these requirements can be addressed is the Ring Main Unit. It is therefore evident that a "Smart-Grid" must rely on a "Smart RMU". Already several major Distribution Utilities and RMU manufacturers are in the process of developing the "Smart RMU" by applying telecoms and sensors, as well as motorizing or replacing the LBFM Switches and the HRC fuses with vacuum breakers. It is now widely accepted that in the ideal maintenance-free "Smart RMU" the LBFM Switches and the HRC Fuses should be completely eliminated and replaced with three vacuum breakers. The "Smart Three-Breaker RMU" as shown in Figure 1, is clearly the ideal component of the smart-grid secondary substation. The only disadvantage being its high cost as a result of utilising three vacuum breakers. In this paper we will present the "Smarter Single Breaker RMU" which has all the advantages of the Smart Three-Breaker RMU as well as being considerably more costefficient. Furthermore, it offers the possibility of modernizing old generation RMUs in a cost-effective way. Figure 1. Single Line Diagrams of a Conventional RMU with 3-LBFM Switches c/w HRC Fuses and the Smart Three-Breaker RMU.

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