With the introduction of open transmission access and independent power producers, the present trend is that power systems will have an increasing range of generators which have widely-different inertia constants. This has raised concerns relating to system transient stability, particularly when some generators in the system have low inertias. The paper is devoted to the investigation of this aspect, and reports its findings. The findings of the paper reveal and clarify important points relating to the effects of machine inertia constants on system transient stability. The effects of inertia constants are strongly influenced by other factors relating to pre-fault and fault operating conditions and system configurations. The outcome of the paper brings out key factors that interact with machine inertias in determining system transient stability. These interactions lead to a general conclusion that, for a given system, there is no optimal inertia constant for a particular generator or for a particular group of generators in a system that gives the best system dynamic responses in all of the credible operating conditions and system configurations. (C) 1999 Elsevier Science S.A. All rights reserved.