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Author Topic: Way way way off topic electrical question  (Read 2003 times)

David Allred

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Way way way off topic electrical question
« on: July 06, 2015, 08:26:55 AM »

This could easily be applied to a power distribution discussion.

Are the reaction times on breakers generally same, regardless of amp rating, or are lower amperage ones "faster", or does it vary brand to brand?

July 4th, sitting at home watching a movie.  AC compressor starts up, I hear that pop-snap of a breaker tripping, and the thermostat goes dark.
My heat pump system has a 100 amp main in the panel and a 60 amp and a 40 amp (I assume for the emergency heat) outside on the unit.

The 100 amp breaker was tripped.  The 60 amp was also tripped as I might expect.  Obviously the draw topped 100 amps, but logically shouldn't the 60 amp be a little quicker and not allowed the 100 amp to trip? 

The AC is still working.
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TJ (Tom) Cornish

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Re: Way way way off topic electrical question
« Reply #1 on: July 06, 2015, 08:54:48 AM »

This could easily be applied to a power distribution discussion.

Are the reaction times on breakers generally same, regardless of amp rating, or are lower amperage ones "faster", or does it vary brand to brand?

July 4th, sitting at home watching a movie.  AC compressor starts up, I hear that pop-snap of a breaker tripping, and the thermostat goes dark.
My heat pump system has a 100 amp main in the panel and a 60 amp and a 40 amp (I assume for the emergency heat) outside on the unit.

The 100 amp breaker was tripped.  The 60 amp was also tripped as I might expect.  Obviously the draw topped 100 amps, but logically shouldn't the 60 amp be a little quicker and not allowed the 100 amp to trip? 

The AC is still working.
There is a forum section for electrical stuff, btw.

Trip time is inversely proportional to the amount of overcurrent.  Generally breakers in the same series will have similar characteristic trip times, but different breaker types, brands, or series may be different from each other.

If you have a fault of say 200A that comes from something shorting, the current rises to 200A almost immediately.  In the case of a short, there really isn't significant time spent at say 80A that would cause the 60 to trip first, so I would expect both breakers to go at the same time.  The reverse wouldn't surprise me either though, as breakers are fairly low-tech devices, and tolerances are fairly wide.

The length of the wiring between the panel and the device can have a dampening effect, as the cable resistance serves to limit the current to some degree.  If your device is very close to the panel, it is a lot easier to have a high-current fault.

Why did your breaker trip?
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David Allred

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Re: Way way way off topic electrical question
« Reply #2 on: July 06, 2015, 09:24:56 AM »

Why did your breaker trip?

I don't know.  It started right up as soon as I reset the 60 amp on the unit (though there was an arc pop sound from the unit).  I turned it right off and then back on with no pop.  Could a grasshopper or other creature create enough resistance to do it?  I smelled no burning nor saw any smoke.
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Cailen Waddell

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Re: Way way way off topic electrical question
« Reply #3 on: July 06, 2015, 08:24:31 PM »

Outdoor units have a contactor that is not always particularly well protected.  Bugs could get in there, or other debris, and you arc'd them out when resetting.  Also possible you have a capacitor going bad.   Compressors generally have a capacitor start capacitor run single phase motor.   

I'm no expert though, just relaying what I've experienced with my rather old unit. 

Generally there is power for the compressor, power for the blower and power for the emergency heat strips.  Sometimes the blower power is also the heat strip power. 


Sent from my iPhone using Tapatalk
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Stephen Swaffer

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Re: Way way way off topic electrical question
« Reply #4 on: July 06, 2015, 11:24:18 PM »

Breakers do indeed react differently by brand (as well as series, etc)-one brand in particular is notorious among electricians (at least in my circle) as being easy to "weld with."  Not a recommended practice by any means, but I (and many other sparkies) have been known to locate breakers by tripping them (usually in industrial/commercial settings)-recently I attempted to do just that and tripped a 100 amp main instead of a 20 amp branch breaker.

I have also personally seen breakers as big as 150 amp 480 V 3 phase that cannot be turned off-obviously unable to trip.  I wouldn't think it unreasonable to think that some might stick and thus trip slower or at a higher amp rating.  Certainly not a bad idea to exercise them every year or two.

I would tend to think that maybe the contactor stuck for some reason.  I would turn off power, verify power is off, then after making sure power is off, manually exercise the contactor-maybe just a bit of rust or dirt that can be worked out of it.

Also, as Caileen said could be a capacitor-or even a bad spot in the insulation on a wire-and the short vaporized enough copper during the pop you heard to -at least temporarily-fix it.
« Last Edit: July 06, 2015, 11:29:36 PM by Stephen Swaffer »
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Steve Swaffer

Jeff Robinson

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Re: Way way way off topic electrical question
« Reply #5 on: July 07, 2015, 12:07:34 AM »

Breakers do indeed react differently by brand (as well as series, etc)-one brand in particular is notorious among electricians (at least in my circle) as being easy to "weld with."  Not a recommended practice by any means, but I (and many other sparkies) have been known to locate breakers by tripping them (usually in industrial/commercial settings)-recently I attempted to do just that and tripped a 100 amp main instead of a 20 amp branch breaker.

I have also personally seen breakers as big as 150 amp 480 V 3 phase that cannot be turned off-obviously unable to trip.  I wouldn't think it unreasonable to think that some might stick and thus trip slower or at a higher amp rating.  Certainly not a bad idea to exercise them every year or two.

I would tend to think that maybe the contactor stuck for some reason.  I would turn off power, verify power is off, then after making sure power is off, manually exercise the contactor-maybe just a bit of rust or dirt that can be worked out of it.

Also, as Caileen said could be a capacitor-or even a bad spot in the insulation on a wire-and the short vaporized enough copper during the pop you heard to -at least temporarily-fix it.

"Selective coordination" is an important specification in hospital and other 'mission critical' systems. Usually it defines the inverse time curves of specified groups of breakers so that as long as the feeder nameplate trip value is 2.5 times higher than the downstream unit, the outage does not trip an upstream device. Example: a worker creates a bolted fault and the 100kA peak current takes out the facility 3,000 amp GFCI main breaker quicker than the local 100A branch. A Very Bad Thing for hospitals and industrial processes that cannot stop without losing (hundreds of) thousands of dollars in production losses and equipment repairs. Sometimes the result of someone tweaking adjustments without understanding why the main's instantaneous setting has a .1 second delay. Last month I sold a 1200A expansion cabinet for a switchboard I sold 10(+) years ago and this was a consideration (Camoplast in Emporia).

Jeff Robinson
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Re: Way way way off topic electrical question
« Reply #5 on: July 07, 2015, 12:07:34 AM »


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