Circuit tester question

[nascardoug]

I was called to test a circuit at one of the buildings at a small college in the Fla panhandle. I used one of the plug in testers with the three lights (yellow,yellow,red). I got a strange indication that was not on the chart for problems. All three lights lit in some receptecales and the normal two yellows in other circuits. Has anyone out there had this problem and what can I do to correct??

 

[pqtest]

Re: Circuit tester question

Hello-

What you are reading is two hots in the outlet, and should be confirmed ASAP.

In most cases, this is of course incorrect wiring. However, Article 647 of the 2002 NEC allows what is called technical power for sensitive electronic equipment, which consist of two, 60 volt hot conductors in a receptacle. The receptacle is required to be clearly labeled "Warning- Technical Power. Do not connect to lighting equipment. For electronic equipment use only. 60/120 v 1 phaseAC. GFCI protected." (2002 NEC 647.7(A)(2))

One comment about those three-lamp circuit testers, taken from NFPA standard 70B "Recommended Practice for Electrical Equipment Maintenance":

"Although these test devices can provide some useful and easily acquired information, some have limitations, and the test results should be used with caution. An incorrect display can be considered a valid indication that there is an incorrect situation, but a correct wiring display should not be accepted without further investigation."

You may also want to check out an article I wrote for Occupational Health and Safety magazine in June of 2000 titled "Using Circuit Analyzers". I believe that the article can be found online, and covers different problems with these three-lamp testers.

Hope that Hurricane Francis passed you by safely- my folks are down in Port St. Lucie, about 2.5 miles from original landfall. Who knows how these things work- no damage to their home, but neighbors on either side weren't so lucky.

Best wishes,

Mark

 

[ralpha494]

Re: Circuit tester question

I ran into it once. The neutral opened up and fried a nice lady's boombox. We tested it with a plug-in tester and all three lites came on. The multimeter said 208v across the two slots in the receptacle, 120v from each to ground. Traced it back one box and found the neutral out of the wirenut.

 

[nascardoug]

Re: Circuit tester question

Thanks for all your input.........but the outlet in question is an existing 120V outlet. I took some voltage reading today and found the following

Phase A to neutral.......120V
Phase B to neutral.......120V
Phase C to neutral.......120V

Phase A to ground........208V
Phase B to ground........208V
Phase C to ground........0V

These reading were taken at the panel that supplies the circuit. I also found other outlets in the building with the same problem. This has us very confused, plus we are located in the path of IVAN and we are now preparing for its arrival. We have had no damage from all the other storms. Any help would be appriciated.

thanks,
Doug

 

[petersonra]

Re: Circuit tester question

Originally posted by nascardoug:
Thanks for all your input.........but the outlet in question is an existing 120V outlet. I took some voltage reading today and found the following

Phase A to neutral.......120V
Phase B to neutral.......120V
Phase C to neutral.......120V

Phase A to ground........208V
Phase B to ground........208V
Phase C to ground........0V

These reading were taken at the panel that supplies the circuit. I also found other outlets in the building with the same problem. This has us very confused, plus we are located in the path of IVAN and we are now preparing for its arrival. We have had no damage from all the other storms. Any help would be appriciated.

thanks,
Doug

looks to me like what you actually have is a corner grounded 3 phase with a "neutral" that is ungrounded. Does not seem like a good thing to me.

 

[dereckbc]

Re: Circuit tester question

What is the neutral to ground voltage? I will bet it is something odd.

I had to read this a couple of times, as I couldn't believe the readings. At first thought it sounds like a corner grounded delta, but I have never seen a 208 corner delta before, although it is possible.

I think you have a wiring error. My guess is someone grounded the C phase and left the neutral floating. You would think the C phase transformer fuse would blow, but the earth impedance would not be low enough. Anyway, that is my guess.

 

[hurk27]

Re: Circuit tester question

Check the voltage between neutral and Earth?
Somthing just does not sound logical. As how could he have 120 volts to all three phases and have one phase with 0 volts to ground unless there is a transformer on the load side of the service that has been mis-wired feeding this panel.