A test circuit that compares test results between two tests with different local supply voltages is provided. The output of each stage of the logic circuits is stored in a first register of each test circuit. Each test is performed with a critical test vector and a local supply voltage that decreases from test to test. The outputs of successive tests are compared in each test circuit. The tests are performed iteratively with successive reduction in the value of the local supply voltage until at least one stage of the logic circuits produces non-matching results between the first and second register. The voltage immediately before producing such non-matching results is the minimum operational voltage for the local voltage island.