CB trip time

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petey_c

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Since I work in a government facility, they decided that it would be best to have a contractor come in and remove some unused 120VAC circuits. He decided that he didn't need to check to see if the outlets were still live (they were a whole 6" away) and turned pliers into strippers. Minutes later, while pulling a "dead" cable (or so he thought) he cut another live cable. It didn't trip the 20A branch CB, or the 100A CB feeding that panel. Instead it tripped the 150A CB further up the food chain. It knocked out 19 radar displays and other equipment. This work was being done on midnights and nothing bad happened. If this happened during a period of high traffic, it could have been disasterous. The engineers explained that it was just the breaker with the fastest reaction time tripping first. Can someone point me in the right direction for having the smallest breaker tripping first? (faster reaction time, etc...) Regards, pete
 

zog

Senior Member
Location
Charlotte, NC
Since I work in a government facility, they decided that it would be best to have a contractor come in and remove some unused 120VAC circuits. He decided that he didn't need to check to see if the outlets were still live (they were a whole 6" away) and turned pliers into strippers. Minutes later, while pulling a "dead" cable (or so he thought) he cut another live cable. It didn't trip the 20A branch CB, or the 100A CB feeding that panel. Instead it tripped the 150A CB further up the food chain. It knocked out 19 radar displays and other equipment. This work was being done on midnights and nothing bad happened. If this happened during a period of high traffic, it could have been disasterous. The engineers explained that it was just the breaker with the fastest reaction time tripping first. Can someone point me in the right direction for having the smallest breaker tripping first? (faster reaction time, etc...) Regards, pete
You need to provide much more info on the breakers. Obviously the smaller breaker should trip first. However, there are different types of trips and some breakers may or may not react the same to different conditions. To get you in the right direction search for "Selective coordination of overcurrent protective devices"

Now the real question is why would a gov facility allow a contractor to do live work with so much liability at stake. IBM just had to pay several million bucks to a contractor that was injured working at thier facility because they did not pre-qualify him for following OSHA and 70E safe work practices. I know, 120V, bid deal, but the fact this happened at all should be a wake up call to your facility, Next time it might be an electrical system with real energy that can do some serious damage.
 

jim dungar

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Staff member
Location
Wisconsin
Occupation
Engineer
The engineers explained that it was just the breaker with the fastest reaction time tripping first. Can someone point me in the right direction for having the smallest breaker tripping first? (faster reaction time, etc...) Regards, pete
This has to do with lack of 'selective coordination' which is required in NEC 708 critical operation locations (as well as 700 and 701 emergency systems).

Breakers operate based on the amount of current versus the length of time. Each breaker as a Time Current Curve (TCC) which needs to be compared to the TCC of its upstream breakers. If the TCC's overlap, there may not be any selective coordination. But, to be really accurate the amount of current flowing through the fault needs to be considered in the evaluation.
 

charlie b

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Location
Seattle, WA
Occupation
Electrical Engineer
In simpler terms, when you have a lot of amps flowing through the two breakers (and keep in mind the current has to go through the 150 amp breaker on its way to the 20 amp breaker), both breakers are going to get the command to open. One will obey that order first, and we often cannot predict which one it will be. Whichever breaker opens first will terminate the event, so that the other breaker may remain closed.

You might expect the smaller breaker to go first. That is only true for a small amount of current. If the fault current were to be only 100 amps, the 20 amp breaker will trip, and the 150 amp breaker will not. If the fault current were 200 amps, there is a good chance (but not a certainty) that the 20 amp breaker will trip, and the 150 will not. But if the fault current were 2000 amps, it could be either breaker that goes first.
 

petey_c

Member
ZOG, I'm away from my normal workplace at a school for about 10 days. More info when I get back. This work was being done on midnights; that's about as low traffic/minimal risk as we can get. The contractor didn't take basic precautions and used the wrong method for removing/cutting cables. He is no longer welcome at our facility.
charlie b, ron, jim dungar thanks as well. pete
 

jwjrw

Senior Member
Just a question when you say "remove some circuits we dont need"
Were the circuits in the way of something or what?
I guess Im asking is this someone deciding we dont use that outlet lets pay to have it removed when no real reason existed?
As in wasting tax dollars and accomplishing anything?
 

pfalcon

Senior Member
Location
Indiana
You might expect the smaller breaker to go first. That is only true for a small amount of current. If the fault current were to be only 100 amps, the 20 amp breaker will trip, and the 150 amp breaker will not. If the fault current were 200 amps, there is a good chance (but not a certainty) that the 20 amp breaker will trip, and the 150 will not. But if the fault current were 2000 amps, it could be either breaker that goes first.
Nice example. What if the 150A breaker already had 120A on it when the 100A fault occurred?
 

petey_c

Member
jwjrw, We're in the process of changing out old equipment and installing some new. When we remove the old racks we don't want power cables dangling from cable trays. We install the new eq. in available spaces and remove the old, creating more space. We keep the new eq. until about 10 years after it becomes obsolete. This time it's not just to spend money. pete
 
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