Outlet testers and voltage detectors: what they can and can't tell you
Plug-in testers and non-contact voltage detectors are fast and cheap, and both have blind spots that are easy to forget at the end of a long day.
Three-light receptacle testers
They're good at catching common wiring faults: open ground, open neutral, open hot, and hot/neutral reversed. What they can't tell you:
- Bootleg grounds. If the neutral has been jumpered to the ground terminal, the tester shows "correct" even though there is no real equipment ground.
- Ground quality. Lights coming on don't show the ground path is low-resistance or that it would carry fault current.
- Some combined faults can read as correct on simple testers.
- Anything about the circuit: wire size, breaker size, overloading or connections in the box.
Write up what the tester showed and recommend evaluation by a licensed electrician for anything abnormal. Avoid saying wiring is "correct" or "safe" on the strength of a three-light tester.
GFCI and AFCI protection
- Use the device's own TEST button first; it is the manufacturer's intended test. The tester's GFCI button is a secondary check.
- Plug-in GFCI testers may not trip a GFCI on a two-wire (ungrounded) circuit, because they create the imbalance through the ground conductor. A non-trip there doesn't necessarily mean the GFCI has failed.
- Identify protected downstream receptacles and note where protection appears missing in locations your applicable standard calls for (kitchens, bathrooms, garages, exterior, etc.). Requirements depend on the code in force when the home was built and local rules.
- Many standards of practice don't require testing AFCI breakers, and some inspectors avoid it in occupied homes because it cuts power to circuits. Follow your standard and your agreement.
Non-contact voltage testers (NCVT)
An NCVT that stays silent doesn't prove a conductor is dead: a weak battery, shielding, or a failed tester give the same silence. Use the live-dead-live check:
- Confirm the tester alerts on a known live source.
- Test the conductor or device in question.
- Confirm the tester still alerts on the known live source.
Inspectors are generally not required to remove devices or work inside live equipment beyond what their standard of practice calls for (such as removing a panel's dead front). When in doubt, don't.