Abstract A new measure that quantitatively describes the dynamical response of a power system under large disturbances is proposed in this paper. The measure is named relay margin/relay space margin. The intention is to find a set of indicators which are well understood and informative enough for use in a power system control centre environment for dynamic security assessment (DSA). The protective system is included in the mathematical model of the power system. Current methods for DSA proposed in the literature ignore discrete devices such as the relays. It is the practice to update relay settings occasionally, depending on planned changes in the power system structure (new source, new load, new line, periodic checking). Thus the current operating state of the power system can be significantly different from the one for which the relay security are selected. The necessity for inclusion of relay models is illustrated by an example. For the dynamic security assessment of the power system it is not necessary to include all the relays present in the system. A set of relays which may trip for a given contingency can be isolated. This set is named the set of vulnerable relays and a procedure to identify them is proposed. The 10-machine, 39-bus system and 3-machine, 9-bus system are used to illustrate the basic idea of this work.