In this paper we evaluate the probability that a transient fault (TF), multiple or single, affecting a checker of a self-checking circuit, gives rise to an unnecessary error indication (no-harm alarm). A new property (noharm alarm robustness) has been defined that, in case of a fault affecting a self-checking circuit (SCC), guarantees that we can determine whether the fault is affecting the functional block, or the checker itself, and whether such a fault is a transient or a permanent fault. Finally, we propose a possible solution implementing the defined property. Its behavior has been verified by means of HSpice simulations, and we evaluate its cost in terms of area overhead and introduced delay.