problem with calculus

You may get better response to questions if you attach a worksheet (upper right corner: choose extended editor).

Do Mathcad has a limit with calculation?

Yes, there are limits, at least with numeric evaluation. Numeric evaluation has an upper limit around 10^307 (biggest number is 2^1024-1). I am not sure about the lower limit.

Symbolic evaluation provides much more precision, if needed.

Unless otherwise stated I assume you are using Mathcad 14 or 15. By double clicking the result (0 in your case) you open the Result Format menu (Format/Result from menu line) and if you choose 17 decimal places, you see the result even with numeric evaluation, but you can also see some sort of rounding error.

Symbolic evaluation is precise, but it seems you cannot get it to show 10^-28.

Edited: Just tried it with Prime2 and ... result as expected - chapeaux!

The "zero tolerance" is set to 15 by default in Mathcad 11 (and up, I guess), meaning that all numbers between -10^-15 and 10^-15 are displayed as 0.

In Mathcad 11, if you click somewehere on the sheet, not on an expression, and choose the menu command:

Format => Result...

You get a form with some tabs, Choose the "Tolerance" tab, and set the Zero Tolerance to its maximum (307), to display all numbers outside of the range -10^-307 ... 10^-307 as they are instead of as 0.

Success!

Luc