Electrical Myth
While searching through the forums recently, I came across two posts in which an old electrical myth is repeated- "Circuit breakers will trip at 80% of their rating." For any breaker which has not deg...
Forum Discussion
goes vertical at 100%??? say what???
Do you know how to read the trip curve?
The left edge is the lower limit of overcurrent and is measured in "multiples of rated current", the right edge is the acceptable amount of overcurrent. I.E. the curve on pg 19...
Looking at the bottom of the curve you can see the breaker will handle between 9 times the rated current and 1000 times the rated current for 1 A/C cycle (16.67mS).
For times between 16.67mS and 300 ms the breaker can handle between 9 times it's rated current and ~22 times it's rated current before blowing. See the variability starting? In other words the breaker can handle 9 - 22 times it's rated current for up to 300 mS before throwing.
At that point it begins to thermal react to the over current and drops down to 1.25 times over it's rated current at 50 plus seconds. I.E. A 100 Amp breaker could withstand between 125 amps and 200 amps for 50+ seconds before it finally throws. You have to consider that in sizing the breaker for the wire gauge, and you have to derate it down for the expected operating temp.
You do not want to go around thinking breakers are absolute sensors.
Do you know how to read the trip curve?
The left edge is the lower limit of overcurrent and is measured in "multiples of rated current", the right edge is the acceptable amount of overcurrent. I.E. the curve on pg 19...
Looking at the bottom of the curve you can see the breaker will handle between 9 times the rated current and 1000 times the rated current for 1 A/C cycle (16.67mS).
For times between 16.67mS and 300 ms the breaker can handle between 9 times it's rated current and ~22 times it's rated current before blowing. See the variability starting? In other words the breaker can handle 9 - 22 times it's rated current for up to 300 mS before throwing.
At that point it begins to thermal react to the over current and drops down to 1.25 times over it's rated current at 50 plus seconds. I.E. A 100 Amp breaker could withstand between 125 amps and 200 amps for 50+ seconds before it finally throws. You have to consider that in sizing the breaker for the wire gauge, and you have to derate it down for the expected operating temp.
You do not want to go around thinking breakers are absolute sensors.
