MyCleveland
Senior Member
- Location
- Cleveland, Ohio
Does anyone know of any manufacturer that publishes TCC's with the current (X-axis) displayed in MULTIPLES of the OCPD? If, yes please share and I will try and track them down.
TCC's X axis scaling
- Thread starter MyCleveland
- Start date
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
Schneider/Square-D QO breakers have time-current curves with the x-axis in multiples of the rated current on a log scale.
JoeStillman
Senior Member
- Location
- West Chester, PA
- Occupation
- Electrical PE
If you have a TCC scaled only to one device, how will you use it to compare with other devices?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
TCCs based on the trip element performance and not the specific longtime value of the element allows one TCC to be used for several different handle ratings.
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
And when the curves for different handle ratings are on the same plot, where they depart from each other in the magnetic trip region they are individually labeled per their handle rating. It appears that the magnetic trip level in amperes is the actually the same for these breakers with different ratings, but their curves depart from each other because they are normalized to their individual handle rating.
JoeStillman
Senior Member
- Location
- West Chester, PA
- Occupation
- Electrical PE
But if they were scaled according to the handle ratings, breakers of different ratings but the same instantaneous rating would not overlap, even though the instantaneous is the same.
What are curves like that used for?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The instantaneous trip performance is often the same regardless of the handle rating, within the same family of trip ratings. This means it is hard to coordinate breaker within the same frame size or family.
JoeStillman
Senior Member
- Location
- West Chester, PA
- Occupation
- Electrical PE
I get it. I just imagine it would be hard to coordinate ANYTHING if the curves all have a different horizontal scale. What are these used for?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You simply need to make the math work to adjust each curve to be on a common basis.
For example, adjust the curves do the 4X 200A lines up with the 2X 400A.
In the old days (80s) the Square D TCCs had each had alignment arrows so you could easily overlay the onion skins on a light table.
Of course today's software packages do all of this for you.