Plot LV Circuit Breaker Time Current Curve Using Equations 1

[jpatricksss]

How to plot the time-current curve from the LV circuit breaker parameters to determine tripping times?

What are the standard features of the time current curve and what properties of the CB do you need to know to plot?

E.g. I want to plot the time-current curve for In = 100 Amp CB to BS EN 60947 (I presume the Standard sets the limits for variables depending on what Type it is for example Type A).

Thanks guys.

 

[DTR2011]

I am not 100% sure about LV breakers and BS standards, however in the relay world, the older E/M relays do not have math equations to define the curves (directly from mfg).

There are many SW programs that can take a scan of the curve and you can pick points from there.

Again this is how things work for older protection relays, that do not have an equatn to define them.

 

[Zogzog]

Not sure why you want to "plot" a TCC, why not just look at the actual TCC?

You will have different curves depending on the trip functions and settings of your breaker are.

 

[PHovnanian]

Its best done with a lookup table (if you are doing it in s/w, for example) and then interpolated. There are quite a few different functions that are combined to produce a modern circuit breaker's trip characteristic so that a simple function doesn't exist.

I've done it (curve fit) for fuses with reasonable accuracy. But then fuses are simple animals compared to breakers.

The basic min melt fuse current vs time function is something like:

I(t) = I0 * ( 1 + ( t/b )^m )

where I0 is the long time min melt current and m and b are parameters derived by curve fitting a series of test values or points off a mfg's fuse curve. In real life, its probably more involved than that. But this produces curves that closely match many mfg's published curves.