Voltage, but not under load

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  #1  
Old 11-25-06, 10:38 AM
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Voltage, but not under load

I have an outdoor lightpost that has stopped functioning. The buried circuit is 30 yrs old. There is a junction box at the base of the light post.

When I disconnect the light post load from the feed in the junction box and measure the voltage across the feeding circuit's black and white wires, I measure 117 volts. But when I reattach the light post load to the feed, the measured voltage drops to zero.

I have replaced the wiring on the light post (from junction box to light fixture). I have also removed the light fixture from the lightpost and tested it on my workbench, and it works fine.

I have also connected the dismounted light fixture directly to the feed cicuit at the junction box (isolating the lamp post wiring), but the light doesn't work. If I measure voltage with my meter, I see the same thing: 117 volts until I connect the load, then the measured voltage drops to zero.

Could this be an indication that there is a break in the neutral in the buried circuit? Or am I misunderstanding something fundamental here?

Thanks for any advice you can provide.
 
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  #2  
Old 11-25-06, 10:45 AM
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you need to prove if the hot wire stays hot under load or if the neutral gets "lost"...you have a bad connection or wire..there is a little bit left which will hold your voltmeter but not enough for a load...
 
  #3  
Old 11-25-06, 10:45 AM
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Question

Your 117 volts... From what two points are you measuring?

H-N.... H-Grd...N-Grd .
 
  #4  
Old 11-25-06, 10:51 AM
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Thanks for your comments.

All measurements are across hot and neutral. This circuit does not contain a third wire for an equipment ground.

Am I correct in thinking there may be a break or some sort of compromise in the underground feed? Is it time to start digging?
 
  #5  
Old 11-25-06, 10:55 AM
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Is it time to start digging?

Sadly, It may be. You could very well have a rotten wire. As stated, enough for voltage but breaking under load.
You seem to have illiminated everything else.
 
  #6  
Old 11-25-06, 10:55 AM
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trace it back to it's source first(switch/photocell/panel) to make sure and isolate the problem to the underground run.. digging stinks and is the last resort once you're positive that the cable is bad.....
 
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