In this study, an analysis of the used RCD's condition is conducted to assess the effect on environment of installation area, terms of use, corrosion, contamination etc. According to KS C 4613, the used RCDs were tested to get date which is the RCD's life time. And then, the date is analysed using Minitab which is a statistical program for reliability analysis. As a result, The RCD's MTTF for the all samples is 12.7?.23 years. and the MTTF of RCDs which are used indoor area is 12.9?.34 years, the MTTF of RCDs which are used outdoor area is 11.9?.30 years. Futhermore, the MTTF of RCDs which are used in dry area is a 13.1?.26 years, the MTTF of RCDs which are used in dust area is 13.0?.57 years, the MTTF of RCDs which are used in moisture area is 8.4?.77 years.