What the Outlet Tester Means
Interpreting 3-Prong Receptacle Testers
The arrangement of the three lamps on these receptacle testers is different among different manufacturers, but they all have one light to show a voltage difference between hot and neutral slots, another to show a voltage difference between the hot slot and the ground hole, and a third to show a voltage difference between the neutral slot and the ground hole. You could learn the same information by making three tests with the two probes of a neon tester or a voltage tester.
The three lamps of a 3-hole outlet tester will light up for a range of voltages in the neighborhood of 120 volts, but sometimes a receptacle that is unable to run a table lamp can run this 3-hole outlet tester; that is, under a real load the voltage does not remain as high as it tests. So an outlet that doesn't work might even read "correct wiring." Here are some more details regarding outlet tester readings:
|OUTLET TESTER LAMPS||COMMENTS|
|Neutral slot to ground hole||Neutral slot to hot slot||Hot slot to ground hole||-Lamps show voltage difference between receptacle slots or hole.|
|Red||Yellow||Yellow or white||-Usual lamp color|
|1||2||3||-Most common lamp position on tester (left to right).||Alternate order 2-1-3 found with Ideal (not Ideal/Sperry), AMProbe AMPPY, Extech CB10, and Etcon CT101. Alternate order 3-2-1 found with Electrician's Best Friend and DK.|
|COMMON READINGS||-USUAL CONDITION:||QUALIFICATIONS|
|Dark||Lit||Lit||Correct||In some cases the outlet still might not run normal items. A hot ground wire plus reverse polarity could give this reading (rare).|
|Dark||Dark||Dark||Open hot (open=not present)||Can be false if neutral and ground wires are both poor|
|Dark||Dark||Lit||Open neutral||Only true if the ground and hot wires are fairly good|
|Dark||Lit||Dark||Open ground||Only true if the neutral and hot wires are fairly good|
|Lit||Lit||Dark||Hot/neut reversed =Rev. polarity||Only true if all three wires are fairly good. A hot ground wire could give this reading|
|Lit||Dark||Lit||Open neutral with load turned on downstream||Testers call this "Hot and ground reversed," but that would be rare. Besides neutral being open, polarity might be reversed too.|
|UNCOMMON READINGS||-LIKELY CONDITION:||NOTES|
|Dark||Lit||Dim||Poor ground||Poor means a degree of connection but not what it should be|
|Lit||Dim||Dim||Open/poor true neutral with rev. polarity|
--most tester models
|Some testers call this "Hot on neutral with hot open," but that would be rare.|
True means without regard to wire color.
|Lit||Dark||Dark||Same as above|
--other tester models
|Dim||Lit||Dark||Poor ground with rev.polarity||--|
So the three-prong outlet tester can't always be taken at face value. There are other rare conditions that can give the same readings as these twelve (or more), and further Testing can discover these. Having said this, these outlet testers are handy and usually are telling the truth -- if you know how to interpret what they are saying. Learn more and see diagrams about the meaning of the six "common" outlet tester readings at Outlet Corrections.
Your outlet tester may do double-duty as a GFCI outlet tester, simulating a ground-fault by the push of a button on it. There is a limitation here. Unlike a good GFI, which can test itself even without a good ground, the three-prong GFI outlet tester depends on a good ground to purposely trip the GFI; and this is also true when it is used to try tripping the GFI by pushing the tester's button at standard receptacles wired downstream from it.
"Your diagnostic tree combined with an outlet tester helped me find the problem in about 3 minutes." -Alan© 2007-2013 Larry Dimock