Ground currents: Open neutral

I received this kind of mail some time ago (part of the mail posted here with the permission from the writer):

“His cable stopped working so he called the cable company. The cable guy unhooked the coaxial from the main box.

After he unhooked the coax he struck it on the side of the amplifier and it arched severely to the point it burnt the coax. He did this repeatedly as to show the resident he had a problem. He left it disconnected and told the resident to call an electrician. After this, he had major issues in his home. 2 of his circuits were causing a severe ground loop. One circuit was on phase A the other circuit was on phase B. It would cause the voltage on one phase to drop as low as 40 volts and the other phase to go as high as 195 volts.”

“I’m trying to figure out how a coaxial cable could have caused all this problem. Any ideas?”

Based on the mail this sounds like “open neutral” problem on the house power feed.

The coaxial cable did not cause this problem. The original problem was the “open neutral” in the power feed coming to the house.The coaxial cable when it was connected provided a partial “fixup” for the problem acting as some kind of neutral (cable TV coaxial cables are grounded on the pole and at the house to electrical panel ground/neutral bus).

Lots of current flowing on coaxial cable caused the cable TV problems and later the burnt cable. When the cable TV coax was connected, it was hiding the original problem that could have been there for a long time. The coaxial cable acted like a (poor) substitution of neutral wire. Some or most neutral current slows back to ground through the coaxial cable shield instead of service cable neutral.

When the coaxial cable was disconnected you got a typical open neutral situation. When the neutral is not connected, the mains voltages on the both phases vary greatly depending on the loads in them. More load connected to phase means less voltage to it and more to other phase.  When the neutral is connected the system is stabilized and voltage does not vay much.

My advice was helpful and some time later I received another mail:

“His main nuetral was open, the problem went away once it was replaced. In addition his ground was severed at the ground rod. So, the coaxial was the only path the voltage could take. I thank you for your advice.”

Here is some more material on this problem:

A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial applications. In this system you have 3 wires feeding your home off the transformer: two hots and one neutral. The transformer is a 240V center tapped transformer. The neutral is the center tap. The center tap divides the transformer winding in the middle. So between hot and neutral you always get 120V. Neutral is kind of a return wire for 120V circuits. With the neutral you have two hot wires both capable of producing 120V to neutral. The neutral carries the difference in power between the two hots.

An open neutral simply means that neutral, or common wire is not connected somewhere. Basically an open neutral means that the line that regulates voltage feeding into your house goes bad. The result is normally that one leg of your incoming power rises above 120 volts, and the other leg fell below 120 volts. How much the voltages rise and lower depend on the loads in use in the house. Open neutral can lead to all kinds of stange effects.

Obviously a few volts either way is not an issue. Voltages below 120 volts won’t usually bother many loads or things just don’t work. Voltages considerably above 120 volts are the problem: light bulbs will blow and electronic equipment may get fried. The common result is that some lights in the home will burn dimmer than usual and others brighter, that this can be affected by other things in the home going on or off, and light bulbs can blow out and electronics can be damaged. Open neutrals can end in catastrophic fires.

When the main neutral becomes open at some place, the ground rods will try to give an alternate path for 120-volt circuits through the earth, but usually inadequately. So to a large degree the system of circuits is subject to 240 volts, which, depending on what is turned on out on the circuits, tries to run things by using the neutral bar as its path between its two hot main wires.

A resistive neutral is a nasty little problem. It has ultimately the same effects as an open neutral, but is much more subtle. With a resistive neutral, there is a connection, but it is bad. When no current is flowing through the neutral, it appears OK. The more current flows through the neutral, the more potential develops across it by ohms law. Resistive neutrals get worse with time because the place where the resistance is gets heated up. The resistive connection gets hot, it will burn a little further open, and the connection becomes even higher resistance.

The resistive neutral and open neutral problems can be analyzed with multi-meter and some other tools:

Basic analysis: Check the voltage from one hot to neutral, then from the other hot to neutral. If the voltages differ by more than a volt or two, you may have a problem.

You can also make more analyzing by checking how much the voltage varies when you turn on and off heavy single phase loads. If the load variations change the voltages considerably, you have problems.

If you happen to have a current clamp multi-meter you can make some more measurements. Place the current clamp over whole house feed cable (both live and neutral wire). When everything is correct should see in ideal situation zero currents (same amount of current coming to you house and leavign the the same wire). In real life this might not be exactly zero. The changes on the loads in the house (turn some heavy 110V load on/off) should not have any considerable effect on the clamp meter reading.

If you see many amperes of current on the clamp meter, there is for sure something wrong somewhere because some part of the current coming from you transformer has found some other route back to transformer. In this case you can suspect a problem and you should contact the electrical company to check for possible problems on their side. When you make measurements, write down the results you got and how you did the measurements. This information can be good to have around when you contact the electrical company about the problem.

You can also use clamp meter to measure currents flowing on the wire going to house grounding, all metal pipes coming to house and cable TV cable. There should not be any high currents there when everything is OK. If there are considerable currents, there is problems either inside your house or nearby it that should be fixed. Those currents will cause you problems by creating ground loop noise and magnetic fields inside the house.

12 Comments

  1. Mark says:

    This is excellant…Great article.

    Reply
  2. larry s says:

    excellent article, I always try to explain it but not as clearly as you did.

    Reply
  3. How to protect RS-232 serial connections « Tomi Engdahl’s ePanorama blog says:

    [...] a variety of sources. These sources include inter-system ground noise, telephone line surges, and wiring faults. The Protection of RS-232 Serial Connections document explains in detail the special [...]

    Reply
  4. Cheryl says:

    Thank you so much for taking the time to write this article. I have just encountered this issue with a rental home. Our tenants called and said that all the electrical items in the house were going haywire. The surge protectors actually smoked and melted. TV’s smelled like electrical fire. The microwave popped and sparked. I called the electrician and they unplugged everything. The electrician came and tested and found massive fluctuations in volts surging throughout the home. outlets meant to hold 120 were drawing over 200. It was an open neutral and the power company came out and replaced the wire. This happened today. As far as I can tell there were 2 tv’s, one oven, two copiers, one tread mill and a microwave destroyed by the power surges:( This article explains the problem we encountered very well. Thanks again for the info.

    Reply
  5. Anthony says:

    Would a branch circuit neutral have the same effect as a service neutral main?

    Reply
  6. Alan says:

    what I would like to know is could the 120 leg and the neutral return be seen as two 60v legs
    equaling 120v the same as both 120 legs equal 240 or dose the neutral carry a full 120
    back to the trasformer becouse it is grounded there One more question could you run a drill with
    one hot 120 leg and only the safty ground comming from the rod stuck in the ground if that safty
    ground was wired to a receptical the same way the neutral should be

    Reply
    • Tomi Engdahl says:

      “could the 120 leg and the neutral return be seen as two 60v legs
      equaling 120v the same as both 120 legs equal 240″

      No.

      “or dose the neutral carry a full 120
      back to the trasformer becouse it is grounded there”

      Yes. Neutral acts as a return for the current flowing on the live wire when the load is connected between neutral and live wire.

      “One more question could you run a drill with
      one hot 120 leg and only the safty ground comming from the rod stuck in the ground if that safty
      ground was wired to a receptical the same way the neutral should be”

      You could get a drill running by having it’s motor connected between hot 120V leg and ground.
      It would work on old wiring, but I would not recommend using this.

      On some new buildings this kind of connection would not work if the outlet is protected with ground fault interrupter: that protection device sees connection from hot 120V leg to safety ground as a wiring fault – cuts the power immediately.

      Reply
      • Alan says:

        Thank you you guys are the frist people to understand I m not gotng to go out and start rewiring stuff the
        wrong way or out of code I m not trying to reinvent the just understand the spokes a little better so bare
        with me a little bit more
        q1.when a 120v receptical is correctly wired and theres a load {a light bulb } across it if you took a volt
        reading at that same receptical from neutal to ground would the reading be 120v until you turned off the light
        to clairify the light bulb is in a table lamp with it s own on off switch. with the lamp switch on you should have
        120v from black to green you should also have 120v from white to green until you turn the lamp off correct?
        Or would the reading continue to be close to 0v

        q2. Is the reason you can run the drill from only earth grond {no neutral comming from the pole {meaning the
        transformer} is becouse the transformer is is grounded to earth localy and therefor the earth takes the
        place of the missing neutal.

        q3 if you took a 120v leg off of two separate transformers that were a mile away from eachoter
        and brought thoughs two separate legs as normal house suply it shold not matter were you decided
        to neutral from [one of the two or both transformers } becouse you could once again use a local
        earth safty ground at the box {house}

        I hope that I still have a few spokes holding my wheel together after this

        Eather way thumbs up to this site

        Reply
        • Alan says:

          Thank you you guys are the frist people to understand I m not gotng to go out and start rewiring stuff the
          wrong way or out of code I m not trying to reinvent the just understand the spokes a little better so bare
          with me a little bit more
          q1.when a 120v receptical is correctly wired and theres a load {a light bulb } across it if you took a volt
          reading at that same receptical from neutal to ground would the reading be 120v until you turned off the light
          to clairify the light bulb is in a table lamp with it s own on off switch. with the lamp switch on you should have
          120v from black to green you should also have 120v from white to green until you turn the lamp off correct?
          Or would the reading continue to be close to 0v

          q2. Is the reason you can run the drill from only earth grond {no neutral comming from the pole {meaning the
          transformer} is becouse the transformer is is grounded to earth localy and therefor the earth takes the
          place of the missing neutal.

          q3 if you took a 120v leg off of two separate transformers that were a mile away from eachoter
          and brought thoughs two separate legs as normal house suply it shold not matter were you decided
          to neutral from [one of the two or both transformers } becouse you could once again use a local
          earth safty ground at the box {house}

          I hope that I still have a few spokes holding my wheel together after this

          Eather way thumbs up to this site

          Reply
  7. Tomi Engdahl says:

    Grounding – Ground Resistance Measurement, 250.53(A)(2) (26min:27sec)
    https://www.youtube.com/watch?v=Yg6G5VUSsWA

    This real world test will be conducted by Nationally recognized Code Expert Mike Holt and electrical utility professional Joe Symmes using a 3 pole fault potential method, a clamp on meter to check the ground, an Amp meter to measure the current flow in the event of a ground fault and a volt meter and also test the test potential.

    Reply
  8. Tomi Engdahl says:

    Difference between Neutral and Ground wire – Explained
    https://www.youtube.com/watch?v=0MQR-tT7DLg

    Reply

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