Singlepole is tripping the main

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hdnvn

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I have a customer that needs an explanation.
He has a subpanel that is fed with 60 amps off of his 200 amp main panelboard. The tenant was cooking on an outdoor outlet fed from the subpanel behind her grill and the outlet shorted. The result was the 60amp main for the subpanel tripped, not the 15 amp breaker feeding that branch circuit. The Main panel is an older Crouse and Hinds, the sub is GE that had just been installed recently...I would like to explain to the homeowner why this occurs...Older breaker versus newer? Is it about the type of short that occurred?
 
I will let the techie guys explain it but it does occasionally happen where the larger breaker or fuse will go before the smaller one. Certainly mor common with a larger fuse and a smaller breaker.

I once had a 20 amp circuit short and it blew a 400 amp fuse.....
 
The larger OCPD was faster acting.

Not all breakers have the same trip curve.
 
It's also very difficult to coordinate molded case breakers in the instantaneous range.
 
Once saw an electrician cause a short in a 1/2HP 480V drive motor... its breaker did not trip, the 2000A main did - almost instantly.

I think mgookin nailed it with 'different trip curves', or like Dave wrote, the 60A was already close to max load so it tripped first.
 
Two things:


1. the 15 amp is defective

2. The fault current was high enough to exceed the magnetic trip value of the 60amp breaker.


Question, who is the maker of this panel?
 
Typical miscoordination of breakers issue. Age doesn't matter, it's all about how they are designed by the manufacturer. The attached picture shows why the main may have tripped before the 15A breaker. The areas that are shaded red and blue represent the sensing boundary of each breaker. Basically, if a fault happens anywhere within those boundaries the breaker will see it and trip. The chart reads amperage on the x-axis vs. time in seconds on the y-axis. So for example, point (1000, 0.1) means that there was a fault of 1000 amps that lasted for 0.1 seconds. Remember, whichever breaker gets its boundary crossed by the fault current first will trip. If the fault crosses both breakers boundaries at the same time then either of them may trip first.
 

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The correct answer has already been given. Basically in a short circuit the fastest acting breaker will trip first if the short circuit current exceeds the ratings of all breakers involved. (I'm not taking in to consideration ones with adjustable trip, that changes things.)
 
Pharon said:
It's also very difficult to coordinate molded case breakers in the instantaneous range.
Click to expand...
Most likely this is the issue.

Every household breaker has two trip settings, a thermal trip that reacts to a long(ish) slow increase in load current, and a magnetic trip that only responds to very high very fast increases, aka a "short circuit". Most likely the failure at the outlet was a short circuit. The 60A main in that sub would have had a magnetic trip set for 600A, the 15A would be set for 150A. But if the instantaneous current jumped way higher than that, you can get into the range of operating times where they overlap.

For example the 15A couldtrip between 0.1 and .5 sec at 10X the rating (1250A), but musttrip in .1 sec at 40X (600A). But that mustvalue is within the .01 second couldtrip range of the 60A breaker. So if the fault current immediately jumps to 600A, it would be a race to see which one reacted faster. In this case, it was the 60A because .01 sec is 10 times faster than .1 sec., even though the 15A breaker WOULD have tripped no MORE than .99 seconds later.
 
Jraef said:
Most likely this is the issue.

Every household breaker has two trip settings, a thermal trip that reacts to a long(ish) slow increase in load current, and a magnetic trip that only responds to very high very fast increases, aka a "short circuit". Most likely the failure at the outlet was a short circuit. The 60A main in that sub would have had a magnetic trip set for 600A, the 15A would be set for 150A. But if the instantaneous current jumped way higher than that, you can get into the range of operating times where they overlap.

For example the 15A couldtrip between 0.1 and .5 sec at 10X the rating (1250A), but musttrip in .1 sec at 40X (600A). But that mustvalue is within the .01 second couldtrip range of the 60A breaker. So if the fault current immediately jumps to 600A, it would be a race to see which one reacted faster. In this case, it was the 60A because .01 sec is 10 times faster than .1 sec., even though the 15A breaker WOULD have tripped no MORE than .99 seconds later.
Click to expand...

Exactly what I am thinking, and if this was an older breaker the magnetic trip of the 15amp breaker may equal that of that 60amp breaker.
 
iwire said:
But nothing wrong with it as coordination is not required for this type of work.
Click to expand...

No, nothing is wrong here. A breaker tripped even if it wasn't the desired one. I was merely trying to explain how this happened by showing a miscoordination. As others have said it is not always possible for every breaker to be perfectly coordinated.
 
jeremy.zinkofsky said:
No, nothing is wrong here. A breaker tripped even if it wasn't the desired one. I was merely trying to explain how this happened by showing a miscoordination. As others have said it is not always possible for every breaker to be perfectly coordinated.
Click to expand...

Just trying to be clear because it seems to me the OPs customer believes there is a problem that needs an explanation or correction. :)
 
mgookin said:
The larger OCPD was faster acting.

Not all breakers have the same trip curve.
Click to expand...


^^^ This.
I had a 100A subpanel main that would hold but the 400A feeding it was a Square D vh or something like that. Regular QOB in the subpanel. A 20A breaker would take the whole thing down.
Awesome........
 
What are the mag trip ratings of both the 60 & 15A ocpd ?

Please keep in mind the nrtl's seem to not care.......

~RJ~
 
Ok, that made me laugh Bob :lol:

seriously, try imposing the mag trips over each other

it's rather enlightening....

~RJ~
 
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