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It is relatively easy to generate a complete single contact fault detection test set Tc for a PLA. However such a test set may fail to detect all multiple faults due to the phenomenon of masking. In previous papers attempting to quantitatively predict the multiple fault coverage capability of a single fault detection test set Tc in PLA's, it was proved that every multiple contact fault in an irredundant PLA is detected by Tc if the multiple fault does not contain any four-way masking cycle. In this correspondence, the masking relations are studied in detail and it is shown that Tc in fact detects a signifilcant percentage of faults with four-way masking. Based on these results more realistic bounds of the coverage capability of Tc are determined. It is shown that the multiple fault coverage ratio of Tc increases with the increasing number m of rows of a PLA and for m = 24 Tc detects 99 percent of all contact faults of size 8 or less.