What happens if you lose half of a 240Vac circuit?

[Ira]

I doubt this occurs very often, but what happens if you lose one of the 120Vac legs going to a 240Vac water well pump? Maybe the circuit breaker malfunctions, or a wire breaks, or there's a problem on the POCO side, or the standby genset has a problem. Does it damage the pump? Is it okay if it only happens for a few seconds before shutting off all power to the pump?

I'm putting in a DPDT relay so I can shut off power to my water well pump if my M1G gets notice of a water leak. It would be easy enough to monitor the voltage (present or not, using a couple of relays) on each leg of the well pump circuit.

Thanks,
Ira

 

[nov0798]

It wont run! nothing will happen to it!

 

[apostolakisl]

The 240 stuff won't work at all as stated above. But there may be some things that tie to one leg for 120 that may still work. Things like the control circuitry might pull off of one leg and close the circuit to common.

The 240 stuff will have an open circuit so even if one leg is intact it will still functionally see zero volts.

A sort of similar but different situation is if you have 3 phase. If one or two legs of the 3 phase go out you will see some of your circuits go out and others will work normal. Anything that draws all three phases won't work though, like AC compressors.

 

[sic0048]

I once lost one of the main power feeds into my house. Most stuff would work fine (lights, TVs etc), but larger power using devices like the garbage disposal, dryer, etc all acted really weird. I would assume that loosing one leg of a 240 circuit would have similar results.

 

[apostolakisl]

There must have been something wrong with the transformer that was affecting both legs. Truly losing one leg and having the second normal is the same as shutting off every other circuit breaker plus the 220's. Also, if your dryer still worked at all you must have had some power on the second leg since killing one leg of 220 to a 220 dryer shuts it off.

Anyhow, something like that must have happened because I can't figure out any reasonable way you would lose one leg except at the transformer. The only other way for that to happen is for one of the two wires to become disconnected and that seems unlikely since a falling tree or whatever would take out both wires or neither wire.

 

[nov0798]

I had a similar problem at my house.
I woke up one morning, and only half of the house worked. I thought it was strange, but then within an hour it was back on. OK so I thought that something was up with the power company.

A week or so passes, and I come home late one night and the half of the house is dead again. So I put a meter on the two incoming legs to the breaker panel, and low and behold, one of the legs is dead.

I call the power company, and they came out and "jumped" the meter, so I would have power for the evening.

Long story short, one of the cables in the ground must have been nicked, and over time it eventually made a path to ground. The power company asked if I had FIOS installed within the last two years, i said yes. They said that they have seen alot of nicked main wires from the installers nicking them with the shovels. im not so sure I beleive this as the mains are way deeper then the FIOS cable. Anyway, they spliced the main feed, and all has been good since.

 

[Ira]

So is everyone in agreement that only having power on one leg to a 240Vac water well pump for an extended period of time will not damage the pump? Keep in mind that this could go on for hours. The pressure switch kicks on because the tank has drawn down. It won't kick off until the pump delivers enough water to bring the tank pressure back up, which won't happen because the pump isn't pumping.

 

[Sandpiper]

So is everyone in agreement that only having power on one leg to a 240Vac water well pump for an extended period of time will not damage the pump? Keep in mind that this could go on for hours. The pressure switch kicks on because the tank has drawn down. It won't kick off until the pump delivers enough water to bring the tank pressure back up, which won't happen because the pump isn't pumping.

Yes, provided one leg is completely disconnected. If one or both legs have reduced voltage for whatever reason, this could damage the pump. The pump will not be damaged if there is full voltage or no voltage, but voltage in the middle somewhere or overvoltage can damage it.

If one leg is completely disconnected, it will have no voltage across the input terminals. Try it and see for yourself. It may, however, have voltage from the input terminal to ground, but this will not hurt the pump.

 

[nov0798]

Double check with the pump manufacturer.

 

[AutomatedOutlet]

120Vac....

 

[nov0798]

240V

 

[mustangcoupe]

I doubt this occurs very often, but what happens if you lose one of the 120Vac legs going to a 240Vac water well pump? Maybe the circuit breaker malfunctions, or a wire breaks, or there's a problem on the POCO side, or the standby genset has a problem. Does it damage the pump? Is it okay if it only happens for a few seconds before shutting off all power to the pump?

I'm putting in a DPDT relay so I can shut off power to my water well pump if my M1G gets notice of a water leak. It would be easy enough to monitor the voltage (present or not, using a couple of relays) on each leg of the well pump circuit.

Thanks,
Ira

IRA,

Are we talking europen 230/240VAC or American standard wiring 208/220VAC split phase? if it is US I would also agree that it should be fine because it is open circuited and there is no potential for voltage as the other leg is disconnected.

Also, if your dryer still worked at all you must have had some power on the second leg since killing one leg of 220 to a 220 dryer shuts it off.

Lou I disagree... Dryers I have seen are typically 120 for the motors, and only use the 220 for the heater elements. Look at gas dryers they only need a 120 outlet. he was just lucky I guess and the leg that went out was not the motor side so there was "strange" indications... I actually had to figure this out when I was going to add a current sensor to my dryer for monitoring. If I picked the wrong leg it would have indicated done when infact it was still spinning and not heating

 

[Ira]

It's good ol' USA standard 120Vac/240Vac single phase stuff. It's a 1.5hp submersible water well pump. Two hots and a ground going to the pump.

Ira