Recently, most electric power substations have adopted production control systems, such as SCADA systems, which communicate with field devices and remotely control processes from a computer screen. However, these systems together with protection measures and additional control actions (using protocol IEC61850) seem not to be enough to free substations of security attacks (e.g. virus, intruders, forgery or unauthorized data manipulation). This paper analyzes the main features of an electric power substation together with the aspects that might be significantly affected by cyber-attacks. The paper also presents the implementation of a specific security system (i.e. firewall-wise system) intended to protect a target distribution network.