Final answer:
The lowest acceptable overvoltage category rating for an ammeter used to test industrial lighting and outdoor conductors should be at least CAT III, but higher ratings may be necessary for outdoor conductors exposed to power lines or lightning.
Step-by-step explanation:
The lowest acceptable overvoltage category rating for an ammeter that will be used to test industrial lighting and outdoor conductors depends on the specific requirements of the application. Generally, for industrial environments, the overvoltage category rating should be at least CAT III. This means that the ammeter is designed to handle transient overvoltages up to 600V. However, for outdoor conductors, especially those exposed to power lines or lightning, a higher overvoltage category rating, such as CAT IV, may be necessary. This rating ensures protection against higher transient overvoltages, up to 1000V.