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Thread: meter main breaker tripping instead of sub main breaker.... (3 phase)

  1. #1
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    meter main breaker tripping instead of sub main breaker.... (3 phase)

    i have a meter socket main breaker200 amp 3 phase 120/240 in an industrial shop. it has been raining really heavy prior to the call , no problems till rain. ( COINSEDENCE?). overhead feed.

    The meter main breaker is about 80 ft from the sub panel. the sub panel also has a 200 amp main breaker, why would the sub main not trip and the socket meter main trips when using a particular machine in the shop..keep in mind, there has been no alterations or additions to this panel for years, it just started last week.

    havnt did any load readings yet , just changed out the two main 200 breakers and still getting same problem...

    any suggestions well apreciate....

  2. #2
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    It is likely that even if they are the exact same breaker that one of them would trip before the other. Just random.

    I doubt the rain has anything to do with it. Might be that when it rains there are more things on line like sump pumps.
    Bob

  3. #3
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    thought maybe the transformer or poco problem , just odd to me when the main trips but nothing in the sub panel or its main trips....

  4. #4
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    Quote Originally Posted by techniphase View Post
    i have a meter socket main breaker200 amp 3 phase 120/240 in an industrial shop. it has been raining really heavy prior to the call , no problems till rain. ( COINSEDENCE?). overhead feed.

    The meter main breaker is about 80 ft from the sub panel. the sub panel also has a 200 amp main breaker, why would the sub main not trip and the socket meter main trips when using a particular machine in the shop..keep in mind, there has been no alterations or additions to this panel for years, it just started last week.

    havnt did any load readings yet , just changed out the two main 200 breakers and still getting same problem...

    any suggestions well apreciate....
    Could it be getting water into the conduit or sheath between the panels and it is finding a small bad spot in the wire?
    Organized people are people that are just too lazy to look for their stuff

  5. #5
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    Quote Originally Posted by techniphase View Post
    thought maybe the transformer or poco problem , just odd to me when the main trips but nothing in the sub panel or its main trips....
    It happens quite often.
    It often seems very few 'part-time' designers of electrical systems consider coordination of protective devices except for 'emergency' systems.
    Just because you can, doesn't mean you should.

  6. #6
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    Quote Originally Posted by jim dungar View Post
    It happens quite often.
    It often seems very few 'part-time' designers of electrical systems consider coordination of protective devices except for 'emergency' systems.
    Jim, I know all too well that I'm preaching to the choir with you as you know this stuff.
    As such my response is directed to those that are not familiar with ways that they can atainobtainIf you have two devices in series and the instantaneous fault is scene by both a 15at branch breaker and a 100at main breaker it is pot luck which one will trip should the magnitude of the fault be greater than the trip point of either breaker. There is no coordination.
    With commercial/industrial breakers that have electronic trips units with adjustable pickups and delays it is possible to adjust the upstream and down stream breakers such that the up stream breaker has enough delay that if gives the down streak breaker time enough to clear the fault first. Even with breakers that do have basic adjustable magnetic pickups there is no assurance of coordination.
    With the common instantaneous breaker instantaneous is just that, instantaneous. Should there be an instantaneous spike of current that rises to be equal to or greater than the instantaneous pickup of any breaker in series with the fault anyone could trip although we wish that the breaker which is closest to the fault would trip first. Of course there are also breakers that have the ability to communicate with one another to assure that the down stream breaker sees the fault and allows a brief delay for the down stream breaker to trip. If it doesn’t then it trips.

    And we all know a lot of this stuff is price driven. Even though a designer may have the where with all to coordinate a system the people paying for the installation may want to go the cheapest route.

  7. #7
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    but why...why is it tripping when they havnt added any circuits or equipment. this has been happening a week ago and its been okay for years. only when they are running certain machines it does it.

    im going this week to get a load calc for each phase at the main and each phase before the sub main and eliminate that possibility...ill post if i find an issue.

  8. #8
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    Quote Originally Posted by techniphase View Post
    but why...why is it tripping when they havnt added any circuits or equipment. this has been happening a week ago and its been okay for years. only when they are running certain machines it does it.

    im going this week to get a load calc for each phase at the main and each phase before the sub main and eliminate that possibility...ill post if i find an issue.
    This is not magic there is a reason for the tripping, why one circuit breaker over the other has been explained, not isolate the cause of the tripping.

    Megger the feeder,
    Perform FOP across both circuit breakers
    Take amp readings when the machine is starting
    Ideally but expensive do long term current recordings.
    Brian John
    Leesburg, VA

  9. #9
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    Quote Originally Posted by techniphase View Post
    but why...why is it tripping when they havnt added any circuits or equipment. this has been happening a week ago and its been okay for years. only when they are running certain machines it does it.

    im going this week to get a load calc for each phase at the main and each phase before the sub main and eliminate that possibility...ill post if i find an issue.
    Techniphase's information is correct, but it goes deeper. First, a good rule of thumb is 10 times the breaker trip rating for "instantaneous trip". Per Mil spec, instantaneaous is 5 cycles or less. Armed with this information, you need to start troubleshooting. You are referring to the breaker tripping when utilizing certain pieces of equipment. 10x the rating of the 200A breaker is going to be FAR more amperage than the draw of your building, let alone the main's rating. You have either a direct ground fault or a direct phase to phase short. If your 800A is always the one that is tripping, I would expect the short to be on the line side of the 200A breaker, but that is not science, it goes back to the random nature of instant trip that Jim cited. At least on occasion the 200A should trip before the 800A. I would start with some megger readings though. Basically, I am indicating that overload, which you are implying in your initial comments isn't the cause of either breaker tripping.


    I know what I don't know, and I know where to go to find it!

  10. #10
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    Do the identical breakers have adjustable instantaneous settings? Both breakers are seeing the same fault (or inrush) current. If the upstream breaker has a lower setpoint, it will be the first one to trip.

    Most molded case breakers are shipped with the instantaneous trip current set to minimum. Don't be afraid to set it higher if you need to. If you set the upstream instantaneous higher than the downstream, you have a better chance of coordination.

    More importantly, why is the machinery suddenly causing overcurrents? If utility voltage is running low, you can have higher motor currents. Maybe a tap change somewhere is in order. Check your power factor. Capacitors can help to boost motor voltage and reduce running currents.
    V=IR and payday's Friday.

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