Amtech Software Update Zs Calculation Issues.

I don't use Amtech, but it strikes me you might be relying on it, as if it is always the last word in design, rather than as just some software that automates some rather repetitive calculations, you could at least in principle, do by hand, perhaps more slowly, with the relevant data sheets in front of you.

In these tricky cases, would it help to ask how would you be dealing with it, if you were in fact calculating manually ?

I suspect you would simply say - 'designed to BS7671 (2020 or whatever)' rather than 'designed to BS7671 (latest edit) ' which in fact contractually is very common. Certainly for big jobs that last several years, some change of regulations in mid-build is almost inevitable, and the legal types hedge this by fixing at handshake time exactly which edition of which regulations apply to the whole programme of works or to sub-sections of it.

It is almost never that stuff that has already started needs to  be undone and redone just because the calculations use the old assumptions ...

The other part of your question seems to be 'is there a legacy mode' that allows you to check designs that should have complied in the past. That I don't know, and is really a question for the software authors.

In general , in the field the problems of a high  Zs can be reduced by making sure that conduit, or cable trays etc are in parallel with the CPC, or by running an additional one along side, and the shock risk can be mitigated by use of RCD or earth fault relay, if the design allows. Or failing that,. as you note, the nuclear option is to spec shorter, fatter cables  or a smaller fuse or MCB  !!

How sure are you of the actual Zs at the origin ?

Mike.

changed how they calculate Zs of a circuit in line with IET guidance that came out in 2022? The change mainly effects cables run in parallel over longer distances

Which change is this? I don't recall BS 7671 itself changing in that area for a while, is this something more subtle about how parallel conductors are arranged (flat, trefoil etc.) and the resulting impedances?

   - Andy.

Kieran Small said:

Amtech software update issued September 2025 which has changed how they calculate Zs of a circuit

Is this actually a subtle bug fix and all previous calculations in some particular "special case" (aye, right) were slightly incorrect?

Maybe the apparent regulation update (have they stated which reg?) was actually a clarification, and made plain the method of calculation and the previous software geeks had misunderstood?

In 'big company' software it's quite easy for the code implementation be no where near those who actually understand what is being calculated, so the coder may have misunderstood what the wording says.

I heard a story of a rotary angle encoder being fed to the sine/cosine formulae without any appreciation that the encoded angle was in fractions of a revolution, rather than radians. SI was the standard, so the encoder must produce radians, obviously. Similar misunderstanding happen all over the place. You could have 'cross section' being interpreted as area in one place and diameter in another and gauge in a third.

So Google AI to the "rescue" ? >lmgtfy "Amtech group Zs software calculation bug fix"

Specific bugs fixed related to Zs calculations in past releases include:

• Incorrect k value: A bug was fixed where certain 300mm cables were obtaining an incorrect k value from regulatory tables used for calculations.
• Adjustable RCDs: A bug was resolved where changing the Earth Fault settings on an adjustable RCD did not correctly update the maximum Zs figure in the software.
• Adiabatic calculation time: A bug was fixed where the errors and warnings dialog displayed an incorrect disconnection time used for the Adiabatic calculation. 

<

So, some possibilities.