Tests LAN jack (RJ-45) wired to 568A/B. standard for telephone line current. RJ-45 test plug is selectable via front panel switch for USOC (telephone) or 568 (LAN) connection. Use to test a LAN jack for telephone current, which might damage an expensive LAN tester. Compact design - fits in shirt pocket. Economical and convenient. One year limited warranty. Instructions: RJ-11 Phone Lines, Plug RJ-11 connector into jack and observe LEDs. Switch position does not effect test. RJ-45 Phone Lines, Set switch appropriately. (USOC for standard phone or 568 A/B for LAN). Plug RJ-45 connector into jack and observe LEDs. Do not plug RJ-11 into RJ-45 jacks. Damage may result. Newer phone lines are often wired to 568 A/B instead of USOC. Some LAN lines have telephone current on previously unused pairs. Pre-testing LAN lines with the Line-Bug 4 before testing with a LAN tester may prevent damage to your LAN tester. Plug RJ-45 connector into jack. Set switch to 568 A/B and observe LEDs. Next, set switch to USOC and observe LEDs. If any LED lights in either switch position, voltage is present.