Take the 2-minute tour ×
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It's 100% free, no registration required.

My electric company recently made a repair that involved digging up underground wires near my house that I'm trying to understand.

The symptoms that caused them to initiate the repair were very frequent surges and brownouts inside my house, sometimes caused by an appliance motor turning on, but sometimes with no directly apparent large load to cause it.

While this was happening, I could measure wildly varying voltages (hot to neutral) on each of the two legs of the 220V circuit.

I'm trying to imagine what the cause could have been for this. How does the electric company balance the two legs around ground before sending the power into my house? I assume there's some center-tapped transformer involved somewhere? Would it have been some kind of fault in the transformer? In the quality of the ground reference connected to the center-tap? Something else entirely?

share|improve this question
    
Issues like these often seem to be related to a grounding problem, either at your house or at the utility end of your drop, although a neighbor of ours had a failure in the neutral wire on the buried cable leading to their house, and it had similar symptoms. –  TomG Feb 27 '12 at 4:26
    
You are correct that there is a center tapped transformer. –  Brad Gilbert Feb 27 '12 at 5:54

3 Answers 3

The best way to explain this is to take a look at some circuit diagrams, and do some math.

enter image description here

We know that in a parallel circuit, the voltage across all loads is the same. So we know the voltage across each resistor is 120V, so we can easily calculate the amperage at each resistor.

I=E/R = 120V/1000 ohms = 0.12 amps

Now that we know the current at each resistor, we can find the total current using the following formula.

It = I1 + I2 = 0.12A + 0.12A = 0.24A

We find total resistance in a parallel circuit like this.

Rt = R1 x R2 / (R1 + R2) = 1000ohms x 1000ohms / (1000ohms + 1000ohms) = 1000000 / 2000 = 500 ohms

So Voltage = 120V, Current = 0.24A, and Resistance = 500 ohms on each leg A and B of the of the circuit.

Now if we break (open) the neutral, we have changed the circuit from two 120V parallel circuits to one 240V series circuit. Now we have to change our values, and do some new calculations.

enter image description here

We can use our total resistance calculations from before, to determine the resistance values for each parallel section of the circuit. We can now say we have a 240V series circuit with two 500ohm resistors, so we'll calculate total resistance by adding the resistances. Lets simplify our diagram.

enter image description here

Rt = R1 + R2 = 500ohms + 500ohms = 1000ohms

Next lets calculate the total current.

It = Et / Rt = 240V / 1000ohms = 0.24A

Now we can find the voltage at each combined load.

V1 = It x R1 = 0.24A x 500 ohms = 120V

We can see with a balanced load, we'll still see 120V like normal. But what happens when we have an open nuetral, and an unbalanced load?

enter image description here

Rt1 = 1 / (1/1000 + 1/1000 + 1/1000) = 1 / 0.003 = 333.33ohms

Rt2 = 1 / (1/1000 + 1/1000) = 500ohms

Rt = Rt1 + Rt2 = 333.33ohms + 500 ohms = 833.33ohms

It = 240V / 833.33 = 0.29A

VRt1 = 0.29A x 333.33ohms = 96.6657V

VRt2 = 0.29A X 500 ohms = 145V

As you can see, if you measure hot to neutral on leg A you'll read 96.6657V, while leg B will measure 145V.

What you're seeing is the different characteristics of parallel vs series circuits. In a parallel circuit voltage is constant and amperage varies, however, in a series circuit amperage is constant and voltage varies.

share|improve this answer
    
+1 It's times like this that I wish I took some EE classes when I had the chance. –  BMitch Feb 27 '12 at 15:33
    
Should have taken another class, maybe I would have caught my mistake in the calculations. Fixed. –  Tester101 Feb 28 '12 at 12:52

Sounds as if your neutral wire was broken. The transformer on the pole has a center tap secondary winding. The center lead is grounded and bonded to the neutral. If you loose the neutral at your house, the neutral in your house is trying to return currents through the ground, the earth. The earth is a high resistance path in comparison to a normal neutral path (wire). The current therefore tries to balance between the two hot legs. If the current in both legs were equal, the voltage from each leg to house neutral will be nominally 120 volts. If either leg were to pull more current (very likely) then that leg would have a lower voltage in respect to neutral. The voltage from leg1 to neutral, plus leg2 to neutral will always be the total of the nominal 220 volts. The offset will be a higher voltage on the leg pulling the least current.

share|improve this answer

If the neutral is broken, then it's up to the wiring in your home to balance the two 110 line internally. Any imbalance in the number of running appliances on one leg or another will cause the fluctuations of voltage measured from hot to neutral.

E.g. if you have two lights on hot A, and one light on hot B, the broken neutral will receive more input from hot A. The neutral will end up in phase with hot A, lowering the voltage to the two lights on hot A to the neutral, and increasing the voltage to the single light on hot B to the neutral.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.