This paper presents a novel approach for topological observability analysis using heuristic rule based expert system. The observability problem is split in P-S observability and Q-V observability by P-δ/Q-V decouple characteristic of power systems. A heuristic rule based expert system is developed for finding the existence of an observable spanning tree for P-δ measurement graph and Q-V measurement graph. This expert system finds the existence of an observable spanning tree in measurement graph on the basis of heuristic rules, directly without making a spanning tree. Inference in this approach is done by the process of chaining through rules until a conclusion about the observability or unobservability is reached. The proposed heuristic rule based expert system has been tested on the standard IEEE 5 bus and 14 bus test systems and an 87 bus real power system, which is a part of Northern grid network of India. Results obtained are presented for illustration.