Electronic root apex locators (EAL) do not give correct readings when using through metal or metal ceramic crowns as file touches the metal part of crown. Many methods have been tried by clinicians to get correct readings. I prefer to use small amount of gingival barrier around the exposed metal margin. Flow-able composites can also be used instead of gingival barrier. It is very simple trick and any file can be used thereafter. I have seen few using Teflon tape around file to get correct readings, I found that method cumbersome as you have to wrap all files with Teflon.