Supervisory Control and Data Acquisition (SCADA) Systems are used to control and monitor physical processes in real time. It is specifical-ly applied to monitor and control activities from remote end covering a wide geographical area. Examples of SCADA application are found in control and supervision of devices behavior in transmission of electricity, transportation of gas and oil pipelines, water distribution, traffic lights and other systems used as bases of modern society. Transmission Company of Nigeria (TCN) SCADA System, being a closed system seems to be free from external threats, except perhaps the insider intrusion. In Nigeria, electricity industry deregulation has commenced and Independent Power Producers (IPPs) now generate electricity into TCN Grid. There are also bulk electricity traders buying electricity for sale through TCN network. Data from these new entrants into the industry would be remotely acquired through integration with TCN SCADA System. Integration with the existing SCADA System of TCN would likely be through public communication Network and internet, etc. Migrating from a fairly secure closed SCADA System to an open SCADA System comes with its security challenges. This paper examines the distinguishing characteristics of closed (isolated) and open (exposed) SCADA System using the Transmission Company of Nigeria as a case study.