The interactive system, NETOPT, may be used to design subtransmission networks, principally at the 33 kV level, adopting an interconnected design philosophy advocated by MANWEB. All secure networks that are designed are stored and can, at any time, be displayed graphically and analysed by a load flow program. A relatively inexpensive, deterministic design process based on Lagrangean relaxation is employed. The cost structure used is completely new. The problem of security specification is overcome by the use of: an existing, statistically derived, security function; a ‘constraint generation’ scheme which adds missing constraints to the problem formulation when required; a collection of other new security assessing procedures. A series of cost cutting features have been added to the reformulated problem to minimise the solution time. Larger problems can be solved using either an interactive or automatic branch and bound scheme.