Time Current Curve Equations for MCBs

Hi all,

Searching for equations to plot time-current curves for low voltage Type B,C,D MCBs (using Standard values)."
asked 3/31/2012

3 Answers

You may be right and there may be equations. From my reading of the standard it does not appear to be like this; I would could be wrong though.

Before answering yesterday I did some searching and was not able to find anything with equations. For the next couple of weeks I’ll keep the question at the front of my mind, and if I come across anything related to this I’ll post it here.

If you find anything in the meantime, it would be great if you can give feedback as well.
answered 4/2/2012 Steven McFadyen 240
Steven McFadyen
I believe it is possible to plot standard time-current curves based on the limits quoted in the Standards.

There is software which plots these curves with no specific reference to a manufacturer.

I'm trying to find out the method on how this is done.
answered 4/1/2012 battler. 2
Interesting question. I don’t remember coming across equations and generally look at the curves in the catalogue (or assume my software has the correct curves). The fact that there are ranges, I would interpret as every manufacturer is different and you need to look at their individual curves:

Curve B tripping: 3 to 5 times the rated current (In)
Curve C tripping: 5 to 10 In
Curve D tripping: 10 to 20 In

The actual curve may also vary depending on if you are looking at ac or dc, frequency , etc.

I have also just had a quick flick through the IEC 60898 standard, which gives limits in which the breaker shall operate (for example a C type at 5In should operating between 0.1 and 15 seconds for In<32A), implying that there are no standard curves.
answered 10/4/2011 Steven McFadyen 240
Steven McFadyen

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