Help me understand this Electrical Problem

[jeanhoyle]

Hopefully someone can help me understand what is happening with my electric.

 

For the past three weekends ( Saturday and Sunday) our 50 amp Surge Guard Model 34850 has been tripped by a high voltage reading on one leg of the 50 amp pedestal at our site. The voltage readout has been as high as 132 volts on l-1 and 128 volts on l-2. When our Surge Guard senses more than 130 volts it will turn off. I understand it is for the safety of my electronics on board.

 

The service here is a 200 amp box with a 50 amp 30 amp and 20 amp outlets.

 

What can cause High Voltage on one leg of our 50 amp service? I can understand what would cause low current but not High Voltage.

 

We turned in a service call to the local electric company and they sent out a technician and he was completely perplexed. He Blamed it on false readings from our Surge Guard. We did convince him to put a monitor device at the meter. It is currently making a recording of the quality of power coming to the meter. Multimeter readings show the same as the Surge Guard.

[Mark and Dale Bruss]

A poor Neutral connection can cause this. The Neutral is in the center between the two Hot Legs (electrically). If the Neutral is poor, a current load on one of the legs will draw its voltage down with the other leg climbing. The rise on the unloaded leg voltage is faster than the voltage drop on the loaded leg. With a good Neutral the changes on one Hot leg are not reflected on the other.

 

The result is the EMS trips on High Voltage. This is one of those problems that a simple meter cannot find because the condition only happens under load.

 

The Neutral gets tied to Ground nearest to power station. If this your residence, the Neutral to ground connection is at the Entrance Power Panel.

 

Don't skip suspecting the receptacle and the plug.

[BigTexRex]

A poor Neutral connection can cause this. The Neutral is in the center between the two Hot Legs (electrically). If the Neutral is poor, a current load on one of the legs will draw its voltage down with the other leg climbing. The rise on the unloaded leg voltage is faster than the voltage drop on the loaded leg. With a good Neutral the changes on one Hot leg are not reflected on the other.

 

The result is the EMS trips on High Voltage. This is one of those problems that a simple meter cannot find because the condition only happens under load.

 

The Neutral gets tied to Ground nearest to power station. If this your residence, the Neutral to ground connection is at the Entrance Power Panel.

 

Don't skip suspecting the receptacle and the plug.

Nice answer.

[jeanhoyle]

That makes sense, but the first time it started was at 2:30 or 3:00 AM. When everything was turned off there should not have been a current load.

[Ranger Smith]

Last time I had that problem it was the power company regulation. If it were a neutral problem one leg would go very high and the other would go very low when a load was applied to the leg. Like 160 and 80.

 

I think it is an issue with your electric provider and you need to call them.

[Kirk W]

A poor Neutral connection can cause this. The Neutral is in the center between the two Hot Legs (electrically). If the Neutral is poor, a current load on one of the legs will draw its voltage down with the other leg climbing. The rise on the unloaded leg voltage is faster than the voltage drop on the loaded leg. With a good Neutral the changes on one Hot leg are not reflected on the other.

This is what electricians call a "floating neutral" and is caused by a difference in resistance between the neutral and one side or the other of your power supply. The two legs of your supply (or phases) are not from different sources but actually are supplied by what is called a center tapped transformer, with one hot leg tied to one end and the other hot tied to the opposite and the neutral is actually connected into the center of the transformer secondary.

When some resistance to current flow is introduced into one side or this source only and not on the other side it causes the neutral to float, or electrically move toward one side or the other of the diagram and thus cause the 240V across the entire circuit to shift and be higher on one leg and lower on the other. When your supply from the power company is on the specified 240V, a minimal amount of this shifting will not be noticed because both legs will remain within the specified +/- 10% but as the source voltage moves toward either limit the result becomes more quickly noticed by your Surge-Guard. The supply from the power company is not actually a constant voltage but is typically at +/- 5v or between 115v and 125v and SG shuts you down at 108v or 132v on either leg.

 

In this example, Va and Vb are each one 120V leg while Vtotal is the 240V supply that in your house would be sent to the electric stove or dryer.

[Kirk W]

I'm going with Ranger Smith in answer #5.Both legs are going high L1--128 and L2 -- 130+.

Both may well be high, and throw in a low level of float you get exactly what he has. But Ranger's is an example that is clearly a situation of float. "Like 160 and 80." A very small amount of float is allowable but as voltages go either high or low the impact on the SG kicks in.