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Old 02-15-2010, 12:57 PM   #1
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Wierd Double Pole Breaker Occurance


Hello, All has been going well for years. Then on a cold day, while using an electric saw in a workshop in an out building, the heat went out and one side of the 100 amp line coming to the out building read 120 volts while the other side read only 20 volts and across the two hot lines there was a reading of zero volts. My 240 volts vanished gone! The red line which reads only 20 volts to neutral appears partially dead. Is it possible that the red line melted somewhere underground between the two buildings or could the dual pole breaker switch be damaged at the main panel in the house? The red, black and white wires are about 1/4 inch thick! No switches were ever thrown on either the box in the house or the box in the out building.
If I read 20 volts at the output of the red line at the main panel in the house, would that indicate a faulty breaker switch right there?

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Old 02-15-2010, 10:33 PM   #2
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Wierd Double Pole Breaker Occurance


The 20 volts is probably an induced voltage in a dead wire that ran juxtaposed with a live wire in a conduit or cable. If a 50 watt incandescent lamp were still plugged in and turned on, you would not measure induced (or phantom) voltage since only a few milliamperes of current leaked through the insulation from a live wire to the dead wire and trying to "use" that electricity drops the voltage to zero. Many digital voltmeters can measure the induced voltage; older voltmeters may draw too much current, simulate usage to some extent, and drop the voltage to zero also.

If you also measure 20 or so volts back at the main panel, then the problem is further "upstream" and could be in or under the breakers that feed the cable to the workshop.

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Last edited by AllanJ; 02-15-2010 at 10:41 PM.
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Old 02-15-2010, 11:02 PM   #3
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Wierd Double Pole Breaker Occurance


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Originally Posted by AllanJ View Post
The 20 volts is probably an induced voltage in a dead wire that ran juxtaposed with a live wire in a conduit or cable. If a 50 watt incandescent lamp were still plugged in and turned on, you would not measure induced (or phantom) voltage since only a few milliamperes of current leaked through the insulation from a live wire to the dead wire and trying to "use" that electricity drops the voltage to zero. Many digital voltmeters can measure the induced voltage; older voltmeters may draw too much current, simulate usage to some extent, and drop the voltage to zero also.
Definitely a possibiliy, basically the exact phenomenon that occured in a story my dad told me years ago.

As I recall the story, an additional set of power lines were installed running from Boston to Washington D.C. My dad was to make some sort of a connection on the D.C. end, and he had been told the line was dead. Not wanting to risk his life on the word of others, he tested the line and found it to have over 600 volts. But when they tried to connect any sort of load to the wires, they would get zero amps. Eventually someone realized that existing live wires were running from Boston to D.C. parallel to this new line and that the existing lines were inducing a voltage in the "dead" lines.
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Old 02-15-2010, 11:35 PM   #4
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Wierd Double Pole Breaker Occurance


Here is a quick way to verify it.,

Go to the main load centre first and verify the breaker by turn it off and back on some case you may have one leg of two pole breaker tripped if not the case then grab a voltmeter or neon test light and check both black et red conductors to make sure you are reading legit 240 volts line to line if so.,,

Then you have a break in underground or one of the junction boxes.

However if you only get 120 volts from one of the two pole and zero volts line to line then the breaker or bussbar tab is toasted but here a major gotcha if you going to pull that double pole breaker turn off the MAIN breaker first before you yank this out otherwise you will face the fireworks if the bussbar want to touch the tub { housing } it will have pretty good burnted concats so either move down to next spare opening or replace the breaker panel depending on age and brand name.

Is your underground feeder is alum or copper ? if alum I am pretty sure it allready corroded and it can get tricky to find the corroded spot and fix otherwise run new underground cable or new conductors.

Merci,Marc
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Old 02-16-2010, 08:18 AM   #5
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Wierd Double Pole Breaker Occurance


At the main breaker box I did get 240 across red and black. Of course I also got 120 from red to white and from black to white. This is where the little beads of sweat began coming out on my forehead. This was telling me that the line between the house and the out building was open somewhere. I have no other lines (telephone, etc.) running along side it.
When the line was originally installed, the contractor made an error and cut the line a bit short. As a result he had to install a junction box at the floor and continue the line from there to the panel (another 5 feet). When I opened the junction box and tapped on the dead line connection, the power to the other side came on.
So it was just a bad connection at a junction box.
Thanks for all your help!!
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Old 02-16-2010, 09:43 PM   #6
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Wierd Double Pole Breaker Occurance


Thanks for letting us know what you found it and I am pretty sure you did proper repair to the splice.

Merci,Marc

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