The worst cases require that multiple vectors be gated through the circuit until a target internal node has propagated to an observable output. In extreme cases, a fault may be undetectable. The presence of an internal or input fault may not be observable at any circuit output. In this case the fault is considered to be masked. A single fault may be masked as the result of the causes shown in Table 9-1 .
Masked faults are undetectable by definition since the observed circuit behavior is correct.
The occurrence of a second fault may uncover a previously undetectable fault. To be complete, a vector test set must include tests for this case. Faults may be permanent, i.e., considered hard or solid, in which case they may be detected if they are not masked, i.e., are observable. Multiple faults occur when more than one fault occurs at one time. The probability of multiple faults occurring in a circuit is relatively less than the probability of a single fault, but is increasing with the increase in circuit density.