Failure cases of COS and cause of these were investigated. The fractured porcelain body had a large part in distribution system of KEPCO recently. Quality of the material of porcelain and cement was examined and manufacturing processes were checked. There was no notable difference in microstructure of the porcelains compared with a sound one but the failures was concentrated on which sulfur based compound used to stick steel on the porcelain body instead of the cement. The mechanical strength of sulfur compound varies depending on melting temperature. The compound started to melt around $135^{circ}C$ and fired with blue flame above $160^{circ}C$ because it contains sulfur. The mechanical strength of the solidified compound is in inverse proportion to the melting temperature. To guarantee the characteristics of the compound the melting temperature should be controlled.