The reliability of testing a normally closed contact in a relay unit may involve the following. Sending a first test signal to the first normally closed contact and detecting a return state of the first test signal sent, sending a second test signal different from the first test signal to the second normally closed contact and detecting a return state of the second test signal sent; and assessing an abnormality when at least one of the detection result for the return state of the first test signal does not satisfy a first predetermined criteria, and the detection result of the return state of the second test signal does not satisfy a second predetermined criteria different from the first predetermined criteria.