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Thread: Main Breaker failure?

  1. #11
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    If breaker was closed and you suddenly had a surge of voltage because some high voltage line contacted the customer service voltage lines - you can't tell me the voltage won't get past that closed breaker. It might trip in the process but it is not holding everything back.

    Every incident I have ever been around where high voltage contacted low voltage there is usually damaged electronic items at very least throughout customer premises. Even when transmission voltage line falls on local high voltage distribution line. Main breakers almost never trip nor does anyone notice any damage to them unless they would happen to be almost right at the point where the high voltage fault occurred.

    OP said there was a city wide power outage - unless it is a really small town, whatever happened at/near this residence probably did not take out the entire town.

    resistance in that breaker maybe took some time but eventually heated it up enough to cause damage. Maybe it deteriorated to the point a fault developed, and that fault condition lasted long enough to take out the transformer.?

  2. #12
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    With a solidly grounded neutral at the premise, voltage rise due to MV line shorted to LV line is so small as not to cause damage to equipment downstream. I aaume a 4 pole main breaker to explain what happened. Due to fault in POCO transformer, primary shorted to secondary and a high current flew through breaker neutral to ground. The breaker tried to open but the fault current exceeded its rupturing capacity and so it exploded. The OP may confirm it is a 4 pole breaker so that the above explanation is valid.

  3. #13
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    May 2005
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    Speaking of the 4-pole 200a main, I once had the opportunity to ask a manufacturer about the current-sensing mechanism of such a breaker. I was told that only two of the breaker bodies sense the load current; the other two are only there to carry and interrupt half of the current. I guess that means that the two current-sensing bodies are actually 100-amp breakers with the other two being parallel switches.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  4. #14
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    Quote Originally Posted by Sahib View Post
    With a solidly grounded neutral at the premise, voltage rise due to MV line shorted to LV line is so small as not to cause damage to equipment downstream. I aaume a 4 pole main breaker to explain what happened. Due to fault in POCO transformer, primary shorted to secondary and a high current flew through breaker neutral to ground. The breaker tried to open but the fault current exceeded its rupturing capacity and so it exploded. The OP may confirm it is a 4 pole breaker so that the above explanation is valid.
    Typical dwelling and other 120/240 single phase services only use two pole main breakers in the US and we only run the ungrounded conductors through them.

  5. #15
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    Quote Originally Posted by kwired View Post
    Typical dwelling and other 120/240 single phase services only use two pole main breakers in the US and we only run the ungrounded conductors through them.
    I don't know if it's called a "4-pole" breaker or not, but what he is describing is a typical 200A main breaker in some ITE panels. It's the size of four spaces, and in a backfeed configuration, with 2-pair of connections.

  6. #16
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    Solvang, California, USA
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    Forum can't seem to upload the photo...I will try again later but the stamped Cat No is TQD22100. 100amp, 240volt, 2-pole.

  7. #17
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    Solvang, California, USA
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    This is a GE product. They are common in my area and were installed en-masse here.

  8. #18
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    Quote Originally Posted by MAC702 View Post
    I don't know if it's called a "4-pole" breaker or not, but what he is describing is a typical 200A main breaker in some ITE panels. It's the size of four spaces, and in a backfeed configuration, with 2-pair of connections.
    Some might look like a 4 pole unit, but are listed as a 2 pole unit.

    Due to fault in POCO transformer, primary shorted to secondary and a high current flew through breaker neutral to ground.
    They are not connected to the neutral conductor either.

  9. #19
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    Quote Originally Posted by kwired View Post
    Some might look like a 4 pole unit, but are listed as a 2 pole unit...
    Agreed. They only do the only two poles there. He probably meant to say 4-space. Regardless, it appears not to be the breaker used here.

  10. #20
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    Feb 2012
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    Quote Originally Posted by Robrilo View Post
    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?
    The soot and evidence of an explosion in the panel seem like a sure sign of a high voltage condition. Most likely why they were changing the transformer. Socket jaw condition may be unrelated. The widespread outage cause may have also sent high voltage to the primary/secondary of that transformer. Overload or bad connections don't normally produce soot behind a meter socket and explosions in a breaker panel. Hard to say why nothing in the house was damaged. Just lucky? Or maybe they just haven't discovered it yet.
    Last edited by meternerd; 01-19-18 at 11:15 PM.

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