We have used those little three-light testers to check the wiring at wall receptacles for years, but switched to a ground impedance electronic tester lately due to the occasional bad readings that happen with a three-light tester. They are accurate most of the time, but will misdiagnose a problem or call out bad wirng as satisfactory on occasion. And, really, mostly correct is not alright where electrical safety is concerned. 

    The light sequence can indicate satisfactory wiring or reversed polarity when the actual problem is an open ground under cetain circumstances. Also, a bootleg ground—where the ground screw at a receptacle is connected to the neutral wire—can easily fool a three-light tester into showing that the wiring is acceptable. 

    We previously only used our Amprobe INSP-3 ground impedance tester to double-check defects turned up by the three light tester, but now use it exclusively. The three-light gadget is handy, inexpensive, and fits in your pocket. But accuracy is more important.

    Also see our blog post What is a false ground, bootleg ground, or cheated ground receptacle? 

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  To learn more about electrical wiring, devices, and receptacles, see these other blog posts:

 • What is the difference between what trips a GFCI (ground fault) receptacle and a circuit breaker?

• What is the code requirement for GFCI protection for receptacles near a wet bar sink? 

• What is the requirement for a service receptacle outlet for heating, air conditioning, and refrigeration (HACR) equipment? 

• Why is an opening in the wall around the side of an electrical receptacle outlet a safety defect?  

• When was GFCI-protection for kitchen dishwasher receptacle outlet first required? 

• What is allowable voltage range at a wall receptacle outlet in a house?

• When should I replace electric receptacle outlets? 

• Does an electric receptacle outlet in a storage shed require GFCI protection? 

• What are "self-contained" electrical receptacle outlets and switches?

• What is the difference between an electrical receptacle, an outlet, and a plug?

• Does a washing machine receptacle outlet require GFCI protection?

• What is the building code requirement for receptacle outlets at stairs and stair landings?

• Can I remove a 240-volt range receptacle and hard-wire the range? 

• What is a "backstab" receptacle outlet? 

• Does a home inspector remove receptacle outlet cover plates?

• What is the minimum height for an exterior receptacle outlet? 

• When was the current receptacle/outlet spacing of 12-feet first required? 

• When was the three-slot (grounding) outlet/receptacle first required? 

• Why does painting an electric receptacle (outlet) make it unsafe?

• Why are electrical outlets and plugs polarized? 

• How many electrical receptacles (outlets) are required in a hallway?  

• What problems does having too many electric receptacle outlets on a single circuit cause? 

• Is a house required to have outdoor electric receptacle outlets?

   Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.