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Thread: Main Breaker failure?

  1. #1
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    Main Breaker failure?

    I have a main breaker that failed in the "ON" position. Details are as follows:

    ~A city wide power outage happened at 7:30pm, Wednesday which lasted around 30 seconds.
    ~Client reports a loud noise awoke them at approximately 3am, Thursday. Electrical service was down inside the residence. Homeowner noticed the main breaker handle appeared melted at this time.
    ~Upon my 6:30am arrival I noticed the utility nearly finished replacing a transformer at the pole; evidence of soot was present around the meter socket, stucco surface near the top of the service panel and a thin coating of
    soot was present inside the panel cabinet. The main breaker had a quarter sized hole in the lower side with previously molten components from the breaker strewn along the bottom of the cabinet.
    ~All branch circuit breakers were in the energized position with no sign of trip nor damage.
    ~The splitbolt connections at the riser weatherhead were in excellent condition with no evidence of heat nor oxidation.
    ~Visual inspection of the meter jaw contact points revealed a white residue and discoloration at the primary line B terminal consistent with oxidation and heating. An attempt to tighten the lug revealed a slightly loose connection there. NOTE: the secondary terminal on the same line (after the meter) had a similar residue which was less than the primary side but present nonetheless. The other terminals were in good visual condition and the nuetral connection showed no obvious signs of heat nor discoloration. NOTE: the side of the breaker which exploded corresponded to line A.
    ~All (4) terminals of the main breaker appeared to be in good visual condition showing no sign of loose connection nor heating.
    ~ I was surprised to find that the appliances and various loads connected to branch circuits showed no sign of damage and nothing out the ordinary for any equipment throughout the residence.

    Any ideas about what exactly happened from a technical standpoint would be appreciated. I have some theories but would like to verify my logic as I try to take in the entirety of the information.
    Why did the breaker explode? Why did it not trip/operate? Did the loose connection behind the meter play a part in limiting the energy available to operate the solenoid of the main breaker?

  2. #2
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    My money is almost always on a loose connection from the breaker to the panel buses. This was just its last straw, but it's probably been hot for quite some time.

  3. #3
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    My first guess is a primary-to-secondary fault, presenting a serious over-voltage.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    I knew I read that too quickly. I missed that the breaker "exploded."

  5. #5
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    Quote Originally Posted by LarryFine View Post
    My first guess is a primary-to-secondary fault, presenting a serious over-voltage.
    No damaged equipment beyont that point suggests it wasn't a high voltage event.

    Something inside maybe been heating up - possibly for a long time and it has finally deteriorated to the point there was fault current.

  6. #6
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    I couldn't tell you what the problem was but I also have a hard time believing it was a coincidence the power co. was changing the transformer the same time a main breaker blew apart.

    What are the odds that it is two different problems that happened at the exact same time after who know how many years the panel has been in service? I'd say slim to none.

    I'd be calling the power co. and relaying to them what you found at your customers house and asking them why they were changing the transformer?

  7. #7
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    I'm leaning toward a primary to secondary fault or lightning strike. Severe overvoltage would cause the main to blow instantly internally, thus never tripping, but also not allowing anything (that we know of) downstream to also blow up - if the internals have been blown apart, it wont pass any current nor would it have had time to trip.

    Did they replace the xfmr or just the fuse? They are loud when they blow, and I dont think the PoCo would have a xfmr blow at 330am and have it nearly replaced by 630am.

    Did the meter survive?
    Electricians do it until it Hertz!

  8. #8
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    Not sure what the utility scope of work was. I had my client ask them to pull the meter as I could not guarantee the safety of the service equipment under the conditions. I went to my scheduled job until I could sneak away at lunchtime when I pulled the permit and began the service change process.

    As far as the meter- I never saw it energized so UNK.

    The speculation about the breaker primary-to-secondary fault is insightful as I would think enough of a pulse would get through to either damage something or perhaps trip some of the branch circuit devices....I am trying to imagine the momentary energy scheme within the circuit/main breaker and it seemed intuitive that the failure internal to the main would severely load the neutral connection at least.

    Is this common during severe overvoltage events to have the components fail in such a way as this with no/unnoticeable damage downstream? We get very little lightning here but the power outage had me wondering what had occurred...I hear the local WalMart lost their entire refrigeration section.

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    If the main basically blew up, why would there be damage downstream?

    Unlike an over current event there would not be a build up, just Bam!

    Breakers are only going to trip on current or heat, not voltage.

    I lean towards this with Larry and Fletch.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Quote Originally Posted by jumper View Post
    If the main basically blew up, why would there be damage downstream?

    Unlike an over current event there would not be a build up, just Bam!

    Breakers are only going to trip on current or heat, not voltage.

    I lean towards this with Larry and Fletch.
    But a high enough voltage can lead to excess current!

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