All testers must be tested on a known live circuit both before and after testing the circuit to be worked on. If you are using the noncontact tester on a bare terminal, it will be ok, but there are cases where they will tell you the conductor/cable is dead when it is infact live. This can occur in tightly twisted conductors, or in cables like NM or SO that have paper inside the outer jacket if the paper is wet. I have even seen this with single conductor USE in wet earth.
PS: I only use my wiggy to test anything that I will be touching with my hands or tools, and I test it before and after on a known live circuit of the same voltage.
Don
