Any form of deliberate physical or cyber activity that attempts to undermine the control mechanisms that maintain the key goals of reliability, efficiency and safety of a physical system can be considered to be an attack on the system. Indeed, an attack can be as subtle as a configuration change that prevents the optimal operation of a power system.This chapter describes an approach that enhances the security of the interlocking function in a power distribution substation by using the power flow behavior of the physical system during switching events as direct power feedback. The approach detects potential over-the-network data modification attacks on the interlocking function using out-of-bounds sensor measurements. The direct power feedback adds an extra layer of security and redundancy to existing power substation interlocking function protection mechanisms.