Neutral on overhead service broken

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Mr. Wizard

Senior Member
Location
Texas
I am having a raging case of cranial flatulence right now. Can anyone explain why/how/if you lose your sevice neutral you get 240 volts across a receptacle? I went on a service call today, guy complained his appliances were burning up. Checked voltage, line to neutral on his receptacles, and it read 240 volts as opposed to the typical 120 volts. Went outside, checked the panel, all looked o.k. Looked up at the overhead, and due to a big pecan tree, the neutral was broke in half. I called a man who knows all there is to know about all there is to know when it comes to electrical work, and he said that due to the missing neutral, there will be voltaged induced on the neutral side of the plug. He had to let me go before he could get into the theory of it, but it perplexes me. Anyone?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
The reason you're seeing 240 volts is because you meter in now in series with a load on the other hot leg. If under normal conditions, A-N=120 volts and B-N= 120 volts when you lose the neutral your meter reads through the load on B-N through to A. A-B=240 volts


A.........[meter]..........N............{load}...........B
 

brian john

Senior Member
Location
Leesburg, VA
When the neutral is missing you are now series all loads depending on the resistance of the loads involved, depends on the voltage applied across the particular load.
 

SEO

Senior Member
Location
Michigan
If you have 10 amps of load on phase A and 5 amps of load on phase B and the neutral is lost on the pole you will have on a 240 volt system from phase
B to neutral 170 volts and from phase A to neutral 70 volts. This is just an example.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If you have 10 amps of load on phase A and 5 amps of load on phase B and the neutral is lost on the pole you will have on a 240 volt system from phase
B to neutral 170 volts and from phase A to neutral 70 volts. This is just an example.


Seo, are you sure that you numbers are correct? Just looking at the numbers I would think that if one load was 2X the other the voltage drop would be proportionately the same which would give you 160 and 80 volts.
 

Mr. Wizard

Senior Member
Location
Texas
infinity, brian john, thank you for your quick replies. It has been a long while since I have encountered such a problem and drew a blank when trying to figure it out. You guys are the true wizards :D
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
infinity, brian john, thank you for your quick replies. It has been a long while since I have encountered such a problem and drew a blank when trying to figure it out. You guys are the true wizards :D


Not really, just been around long enough to see many things in this business. ;)
 

SEO

Senior Member
Location
Michigan
Seo, are you sure that you numbers are correct? Just looking at the numbers I would think that if one load was 2X the other the voltage drop would be proportionately the same which would give you 160 and 80 volts.

You got me on that one . Very careless math:mad:
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090317-2014 EST

Draw the circuit. A center tapped transformer with 3 wires from the transformer and supplying two loads, one on each side of neutral.

Independent of the size of each load the voltage on each load will be 1/2 of the total secondary voltage. If total secondary voltage is 240, then each half is 120.

Make one of the loads an open circuit. Now all current flow is on 1/2 of the transformer secondary. Open the neutral. What happens? No current flows, but if you put a meter across the open circuit side you will read 240. Put it across the other side and it will read zero.

Next add some load to the side that had no load. Now you have a voltage divider across the 240 V secondary. The voltage at the mid point of the two loads will be determined by their relative impedances. Note: the impedances may not be pure resistors.

Likely what happen was the loads on the two sides of the center tap were quite unbalanced. This caused excessive voltage on one side and thru a cascading set of events essentially burned everything out on the side with the high voltage. Thus, the reason you read 240 and not some lesser value.

This might be reason to have a crowbar protective device that shorts across the 240 output of the of the main circuit protection to cause a trip on an excessive unbalance of the two load voltages.

.
 
Another interesting situation that sometimes arises when the neutral on a service to a building is interrupted.

If the building, [ let say it is a house located on the street with at least one other house between the affected house and the utility transformer supplying the affected house] loses a neutral from the utility supplied transformer and there is copper water pipe in the street supplying the houses, a unique situation could and does arise.
The GEC from both houses will most likely be terminated to the water pipe. So, the house furthest away from the transformer may still work just fine even though the neutral was interrupted.
The neutral current will flow on the GEC from the house that lost the neutral, through the cold water pipe in the street, back up the GEC in the house closer to the transformer and then back to the transformer through the neutral conductor; thus completing the circuit. And this can happen without either of the two occupants being aware of the situation. I have seen this happen many times.
 

Karl H

Senior Member
Location
San Diego,CA
This exact question was my first post on this forum two years ago.
I have to say you guys have gotten a lot better at the explanation
of an Open Neutral (Parallel to Series circuit). Well 480Sparky sent me a
great powerpoint on the subject that I couldn't open for two weeks.
Thankfully,480Sparky was kind enough to do a follow up and help
me open the Power Point. CharlieB had me visualizing charged particles
traveling from one house to another. Which,I must say is my hands down
favorite description of electricity. Good Show! :D
 

Mr. Wizard

Senior Member
Location
Texas
090317-2014 EST

Draw the circuit. A center tapped transformer with 3 wires from the transformer and supplying two loads, one on each side of neutral.

Independent of the size of each load the voltage on each load will be 1/2 of the total secondary voltage. If total secondary voltage is 240, then each half is 120.

Make one of the loads an open circuit. Now all current flow is on 1/2 of the transformer secondary. Open the neutral. What happens? No current flows, but if you put a meter across the open circuit side you will read 240. Put it across the other side and it will read zero.

Next add some load to the side that had no load. Now you have a voltage divider across the 240 V secondary. The voltage at the mid point of the two loads will be determined by their relative impedances. Note: the impedances may not be pure resistors.

Likely what happen was the loads on the two sides of the center tap were quite unbalanced. This caused excessive voltage on one side and thru a cascading set of events essentially burned everything out on the side with the high voltage. Thus, the reason you read 240 and not some lesser value.

This might be reason to have a crowbar protective device that shorts across the 240 output of the of the main circuit protection to cause a trip on an excessive unbalance of the two load voltages.

.

I called the HO and told him what I had found, and for him to call the POCO to get a new service drop. I went on to tell him that after the obvious problem is corrected, there may be other problems come up. Several appliances, tv's, video games, and dvd players were smoked. I'm hoping that everything will be copesetic when the new service drop is installed. gar, are you saying that when the service neutral broke, the now severly unbalanced load caused the voltage to read 240 from line to neutral? That sure seems likely. I honestly can't remember anything being plugged in, but I guess by the time I got there most everything had been unplugged.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090317-2225 EST

Mr. Wizard:

If the neutral is completely open as you indicated, then you have a 240 V source which is floating relative to ground if the transformer center tap is not grounded to earth. Under these conditions and assuming the neutral in the home is connected to earth you will have a voltage relative to neutral and earth, assuming in the house neutral is connected to earth, that will depend upon the relative magnitudes of the loads connected across the 240 V lines.

If the transformer center tap is connected to a ground rod at the transformer, the neutral is broken between the pole and the home, and neutral in the home is connected to earth, then you have a high impedance neutral thru the earth and this will enter into the determination of of the actual load potentials.

My guess is that a lot of items were burned out, opened up, or were unplugged. There may be un-noticed damage in some of these loads.

You may need to check on how things are grounded because the 240 you read seems unusual if there was some ground path in parallel with the neutral.

Drawing out a schematic of the circuit can greatly help you understand the possible voltages that can develop.

.
 

hillbilly

Senior Member
Another interesting situation that sometimes arises when the neutral on a service to a building is interrupted.

If the building, [ let say it is a house located on the street with at least one other house between the affected house and the utility transformer supplying the affected house] loses a neutral from the utility supplied transformer and there is copper water pipe in the street supplying the houses, a unique situation could and does arise.
The GEC from both houses will most likely be terminated to the water pipe. So, the house furthest away from the transformer may still work just fine even though the neutral was interrupted.
The neutral current will flow on the GEC from the house that lost the neutral, through the cold water pipe in the street, back up the GEC in the house closer to the transformer and then back to the transformer through the neutral conductor; thus completing the circuit. And this can happen without either of the two occupants being aware of the situation. I have seen this happen many times.


Me too....well.....not many times, but a few times.:smile:

steve
 

glene77is

Senior Member
Location
Memphis, TN
090317-2225 EST
If the neutral is completely open ...
... then you have a 240 V source which is floating relative ...

and I continue "to the other pole of the transformer".
IF there is any load on one Pole, and
IF OP uses a high-impedance meter,
THEN he could measure 240V all day.

I had this happen to my last house.
I had the sense to keep the loads similar Pole vs Pole, so no great damage.
IMO, turning on the electric oven and A/C and Water Heater mitigated the possible damage.
Comment about this method? :confused:

My friend, a Master Electrician, came over, pulled a permit, called POCO.
He had parts for the 'old' style meter base.
He repaired the blown meter contact while POCO waited 15 minutes. :smile:
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090318-0908 EST

Mr. Wizard:

Thinking more about your 240 V reading.

I believe you probably have more problems than just a single break in the overhead neutral.

1. Establish a ground (earth) voltage reference point. I would like this to be the copper water supply pipe going into the ground and on the ground side of the water meter. If plastic pipe is used for the supply, then outside drive a 10 or 12 inch screwdriver in the ground near the service entrance. To which ever one of these you use for a reference connect a long test lead to one side of your meter. I would suggest the EGC wire in an extension cord as a good test lead.

2. Use a high input impedance, 10 megohms, digital voltmeter, like a Fluke 27 for the following measurements.

3. The 240 V reading implies something wrong in the ground or neutral path from the transformer center tap all the way into the main panel including all required bonding and grounding. Or a very high resistance ground path from the pole center tap to the house ground bus.

4. We know that the overhead neutral is open because you can see this. There should be a parallel path from the transformer center tap to a ground rod at the pole thru the earth to a ground and/or water pipe or other grounding electrode at the house near the service entrance.

This path thru the earth, if it exists, might not be a very low resistance. In my yard this whole path is probably less than 10 to 20 ohms. My water line to ground is probably less than 1 ohm, but the pole ground rod is likely more in the range of 10 to 20 ohms. In other locations this total ground path resistance might be 100 ohms or higher. Suppose it was 100 ohms, then 1.2 A would produce a drop of 120 V. Equivalent to a 150 W bulb.

5. If you have to create a ground reference in the yard, then use insulated boots and use only one hand or insulated gloves. There may be some large potentials somewhere. What do I mean by one hand? Do not do anything that would allow current to flow thru your body.

6. The first procedure is to determine if all the proper bonding of neutral, EGC, ground bus, and grounding electrode exists.

7. If you have copper or iron water pipe inside going to the earth, then measure the voltage from the entry water pipe to the main panel enclosure. You must make good contact to the water pipe and the main enclosure with the test leads or you may read capacitively coupled voltage. Sanding may be necessary to break thru to base metal. In my house this morning it is 2.1 millivolts. I expect you should read a low voltage, but it might be as high 240 V for some unlikely reason.

If this voltage is above 1 V, then there is a very poor connection between the entry water pipe and the main panel.

8. Assuming the voltage of (5) is low, then check the voltage to a neutral in an outlet. If there is no load on the circuit to the outlet, then this voltage should not differ from the measurement to the main enclosure by very much. I suggest that a fraction of a volt would be high. Maybe 0.1 to 0.2 V would be OK. It depends on how the neutral gets to the outlet.

If the voltage seems high, then open the main panel and check to the neutral bus.

9. If any of these voltages measurements are high, then you need to find out why.

If the neutral bus is not bonded to the ground bus in the main panel and in turn to the entry water pipe, then you could have a reason for your previously stated 240 V reading.

10. If you get low voltages between the earth, the ground bus, and the neutral bus, then from the earth reference point measure the voltage to each hot bus in the main panel. These each should be around 120 V. But your 240 V measurement implies that this won't be the case.

11. If you did not have the water pipe as a reference, then you have to use a probe in the ground near the service entrance for a ground reference point. Measurements made to this reference might be somewhat higher than to a water line in the ground.

12. Note: as you make these various measurements it may become clear where the problem is.

13. If relative to earth at the entry water pipe all the voltages are low to EGCs and neutral, then I believe that the transformer center tap is not grounded at the pole for you to get a 240 V reading from a hot to neutral. This could also be possible if the neutral that you used as a reference when you read 240 V does not have a low resistance path back to your earth reference point.

Suppose the pole center tap does connect to a pole ground rod and the pole to house resistance is 100 ohms, that there is no load on one phase, and the load resistance on the other phase is 10 ohms, then the voltage drop on the 10 ohm load is about 120*10/110 = 11 V and the drop from pole center tap to your house neutral is 120 - 11 = 109 V, and the voltage drop from your other phase to neutral is about 240 -11 = 229 V.

14. If copper or iron pipe is used throughout the house, then between the entry water pipe reference point and any conductive plumbing fixture the voltage should be very small, millivolts.

15. If there is a ground rod at the service entrance, then its voltage should be low compared to the ECG and neutral in the panel.

16. If you make voltage measurements in the yard between the house neutral and the pole ground rod and there is no load on one phase and a large load on the other phase (maybe 10 ohms --- a 1500 W electric heater is about 10 ohms), then you should see an increasing voltage as you approach the pole ground rod. The largest changes will be close to the house grounding electrode, and then near the pole ground rod. So across most of the yard the voltage may not change much.

Depending upon the resistance of the pole ground rod to earth in relation to the resistance of the house grounding electrode and the bulk resistivity of the earth will determine what the earth voltage will be at a mid point between the house and pole. Suppose the house to ground resistance is 20 ohms, the earth 10 ohms, and the pole to earth is 70 ohms, then assuming 120 V across this path the drop from the house neutral to mid point is 120*25/100 = 30 V. There are not these discrete steps in resistance, but the assumption makes an easy calculation.

Be aware that you will see a very rapid rise in voltage close to the pole ground rod.

17. Assume that the pole center tap is not connected to a pole ground rod, then you should not see much voltage in the yard relative to the house neutral.

If I remember correctly the maximum voltage in my yard is around 100 MV and that is between my yard and my neighbor, and he is on a different transformer. We have a primary 3 phase delta supply and no primary neutral. The substation secondary may be a Y, but from our perspective it is a delta source.

18. Any troubleshooting of this sort will have the path that you take change as you make measurements that provide new information as to what to check next. From a remote point without first hand information it is hard to conjecture what steps to take.

.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
0903218-0916 EST

glene77is:

If the neutral is open and you simply add loads between the two hot leads (oven and hot water heater), then this makes no change in where the neutral floats. But keep in mind that the ground path, depending upon its relative resistance to the 120 V loads, will have an effect on the balance between the 120 loads.

If the ground path is infinite, then the 120 loads themselves determine the neutral point. So really it is necessary to balance the 120 loads. Thats not easy. So unplug all 120 V loads. Get a generator and power the critical loads, refrigerator and freeze, until the problem is fixed. Unplug the critical loads and plug those into the generator. Do not connect the generator to the home wiring system. This you know, but others may not.

When the neutral has a high resistance it is important to drop all loads immeadiately until the problem is solved. Probably opening the main disconnect is the best starting point.

.
 
Last edited:

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
When the neutral has a high resistance it is important to drop all loads immeadiately until the problem is solved. Probably opening the main disconnect is the best starting point.
Until the POCO comes out to investigate, that is.

My experience is that they generally don't understand that loading is necessary to expose an open-neutral condition.
 
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