weird weird weird voltage readings

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Coppersmith

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
Today I was troubleshooting a split phase panel where all the 240 volt circuits were down. The main breaker is bad. I can hear it arcing occasionally. I got some weird readings I don't understand. Perhaps someone can explain.

I got 240 volts across the line side of the main as expected.

When measuring on the load side of the main, either hot to neutral read 120 volts, but hot to hot was zero volts.

I don't understand how this is possible. I would expect one side to be zero and the other to be 120.
 
If the branch breakers were on I would suspect that you have lost a line in the mian breaker and you are reading onr side of your split phase through the loads. If the breakers were off I would suspect witchcraft. You should do a fall of potential test across each line of the main breaker and report back.

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Coppersmith said:
Today I was troubleshooting a split phase panel where all the 240 volt circuits were down. The main breaker is bad. I can hear it arcing occasionally. I got some weird readings I don't understand. Perhaps someone can explain.

I got 240 volts across the line side of the main as expected.

When measuring on the load side of the main, either hot to neutral read 120 volts, but hot to hot was zero volts.

I don't understand how this is possible. I would expect one side to be zero and the other to be 120.
Click to expand...

One side is burned out/disconnected and the other has bridged to it inside the breaker? Do you get 120 to ground at the 240 volt appliances?
 
Ultrafault said:
If the branch breakers were on I would suspect that you have lost a line in the mian breaker and you are reading onr side of your split phase through the loads. If the breakers were off I would suspect witchcraft. You should do a fall of potential test across each line of the main breaker and report back.
Click to expand...
This is correct (except for the spelling :D). The second 120 volts is the good pole of the main breaker feeding through the 240 volt loads. You would also read 240 from line to load on the bad pole of the main breaker.

I always check for line-to-load voltage, which should always be zero across a good breaker or fuse. Also, I always do my troubleshooting with a solenoid tester. I only use a volt meter when I need the actual voltage.
 
Ultrafault said:
If the branch breakers were on I would suspect that you have lost a line in the mian breaker and you are reading onr side of your split phase through the loads. If the breakers were off I would suspect witchcraft. You should do a fall of potential test across each line of the main breaker and report back.

Sent from my SM-N910P using Tapatalk
Click to expand...

Or a failure mode like post #3. One line side is burned out/disconnected and the load side for that line is now faulted to the remaining good load/line combo.
 
It is a 120V service, or got hooked up that way. L1 is jumpered to L2 somewhere. Unless you know for sure that the 240V loads were working earlier.
 
jaggedben said:
It is a 120V service, or got hooked up that way. L1 is jumpered to L2 somewhere. Unless you know for sure that the 240V loads were working earlier.
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I don't believe you are correct. The OP specifically mentioned "240 volt circuits".
 
gadfly56 said:
I don't believe you are correct. The OP specifically mentioned "240 volt circuits".
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Like I mentioned, unless I know that those 240V circuits were ever working, it can't be ruled out. Somebody buys a house thinking it has a 240V service when it doesn't, tries to install some 240V circuits, then calls an electrician when they don't work? I've seen stranger things.
 
Imagine, if you will, a single 120 volt circuit, one hot and one neutral. 120 volts hot to neutral, right?

Now, add, let's say, a 10 ohm resistor to the hot wire, and measure from the loose end of the resistor to the neutral.

Still 120 volts. Why? The resistor is like the line-to-line loads with one end floating, and measuring from there to the neutral.

Now, if the OP were to turn off all of the 2-pole breakers, then he'd measure the expected 120 volts on one line and zero on the other.
 
Thank you folks. Duh, I totally didn't think about the 120v backfeeding from the breakers :dunce::dunce:. Tomorrow I'll be replacing the main. I'll turn off all the circuits and retest to rule out witchcraft.
 
gadfly56 said:
Or a failure mode like post #3. One line side is burned out/disconnected and the load side for that line is now faulted to the remaining good load/line combo.
Click to expand...
very unlikley failure. I remeber hearing recently on ETalk a saying I liked very much and pretty well pegs young me. "when you hear hoofbeats don't look for zebras."

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Coppersmith said:
Thank you folks. Duh, I totally didn't think about the 120v backfeeding from the breakers :dunce::dunce:. Tomorrow I'll be replacing the main. I'll turn off all the circuits and retest to rule out witchcraft.
Click to expand...
Your single pole circuits shouldn't cause this issue - (Assume L2 has the open circuit in the supply) it is that 240 volt water heater that will feed 120 from L1 to L2 and then you still see the 120 to ground that was supplied by L1 on the L2 conductor. It will supply 120 volt loads connected to L2, though they are all in series with the water heater. Place enough load on L2 and you will notice the reduced voltage as a result of series resistance with the water heater.

I mention water heater as it is the main household 240 volt load that will fool you like this, mostly because you don't know when the thermostat is closed and when it isn't. most other 240 volt loads need to be in use (more like attempting to be in use) at the time or you will not get this result. 240 volt electric space heating with manual thermostats would be similar though. Ranges, dryers, air conditioners often are not running and don't cause this problem if not running or attempting to run.
 
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