The fused ring main unit-options for choice

The 11 kV street corner substation forms a vital link in the chain of power supply stretching from the generating station via the transmission and distribution networks all the way down to the domestic table lamp and toaster. The protection of such substations has always posed some unique problems; prospective fault levels (up to 350 MVA or even more) are far higher than is the case on the LV network downstream, yet the equipment itself has often to be sited close to the general public where any malfunction could result in a serious incident. Conversely, spurious operation of the protection equipment could result in outages of considerable extent. In this paper, the author explores the stated arguments for changes away from existing protection policy and argues the case for the retention of HRC fuse protection with the use, where appropriate, of the now widely available full range type of fuse to give an even greater degree of system protection than would be possible by any other means