I have several 800A Micrologic 2 MCCBs with only three adjustable settings Ir,tr and Isd. Ir is set at 0.7, tr is set at 4s at x6 Ir and Isd is set at 5. Is this not incompatible? My reckoning is that the Ir is 560A, tr holds an overload of 3360A for 4s but the short time pick up pulls the breaker out instantaneously at 2800A. All the devices are similarly set. There are no issues as a result but just wondering.
OK, using the settings Lyle has, rather than the ones in the pics lifted from the 'how to' guide.
Let us consider higher and higher currents being taken
At less than 0.7 of 800 A 560A , nothing trips and it stays on for ever. On the curves this is region 1 and all current to the left of it.
At any higher current, the region 2 part of the curve applies, so there is a constant time *current product, with a slope equivalent to 4 seconds to reach 6In
- so we know it should fire in 16 seconds at 1.5In (840A), 8 seconds at 3 In ( 560*3= 1680A), and intermediate points on that curve need a bit more thought to calculate. *
Except, we never get right up to the 4 seconds point because when you exceed 5In, another bit of logic cuts in and it fires near instantly, region 3 so we never see anything slower than about 5 seconds, as any fault current above 2200A the fast action takes precedent.
* the data sheet curves use log paper so y=1/x is a straight line. Personally I like log paper, it makes my 'assume the horse is a sphere' sort of maths look almost reasonable.
It is a fun if rather childish exercise for students to draw something, perhaps a picture of Mickey Mouse or Donald Duck, on a linear grid, and then replot the same points on log lin, or log log scales - it makes different bits of the date look more or less signifiant, and hammers home the importance of looking at the axes.
OK, using the settings Lyle has, rather than the ones in the pics lifted from the 'how to' guide.
Let us consider higher and higher currents being taken
At less than 0.7 of 800 A 560A , nothing trips and it stays on for ever. On the curves this is region 1 and all current to the left of it.
At any higher current, the region 2 part of the curve applies, so there is a constant time *current product, with a slope equivalent to 4 seconds to reach 6In
- so we know it should fire in 16 seconds at 1.5In (840A), 8 seconds at 3 In ( 560*3= 1680A), and intermediate points on that curve need a bit more thought to calculate. *
Except, we never get right up to the 4 seconds point because when you exceed 5In, another bit of logic cuts in and it fires near instantly, region 3 so we never see anything slower than about 5 seconds, as any fault current above 2200A the fast action takes precedent.
* the data sheet curves use log paper so y=1/x is a straight line. Personally I like log paper, it makes my 'assume the horse is a sphere' sort of maths look almost reasonable.
It is a fun if rather childish exercise for students to draw something, perhaps a picture of Mickey Mouse or Donald Duck, on a linear grid, and then replot the same points on log lin, or log log scales - it makes different bits of the date look more or less signifiant, and hammers home the importance of looking at the axes.
