There are a few approaches (in general, nothing specific to Amtech) - reduce the impedance upstream - e.g. increase the cable size (although that may not be possible if the problem is at or close to the intake), reduce the rating or type of the breaker, or switch to a different approach for ADS - e.g. RCD or earth leakage trips.

In theory you could go down allow >5s disconnection times and add supplementary bonding to keep touch voltages below 50V - although that's unusual, likely to raise a lot of eyebrows, and typically isn't very practical in other than small contained situations.

As a rule of thumb though, if Zs is too high for typical breakers in a TN system and c.p.c.s aren't substantially smaller than the live conductors, then you're very likely to be seeing voltage drop issues too - in which case a bigger review of the design might be in order.

   - Andy.

you mention external  loop impedance - by which I assume the DNO network, which is not part of your design. What is the external earthing arrangement ? If it is TNCS/PME/MEN then as Andy notes if hte impedance is too hihg to trip the breaker, then the supply will also suffer voltage drop problems as well - or in other words you have a breaker that is too large for the available supply. If it is TT, then of course it will be high and the incomer or panel needs to incorporate some kind of earth fault trip (which may still be set to several amps) or RCD.

For TN-S it is possible though unusual in practice for the external earth loop resistance  to be higher than the L-N loop and the default values may reflect this.

Is this a software only problem, or are the figures you are using in your calculation based on site survey or test results ? Ideally use the right figures for the site, or treat as TT and add earth fault protection.

Mike