There are indeed assumptions - one small lighting circuit on a board fed by a hundred amp submain, you can indeed assume the source impedance is negligible compared to that of the final circuit. However circuits that supply one or two big loads maybe not.

Another way to think about that is how the design voltage drop has been partitioned, as that is also in effect the conductor resistances.

Actually measuring of very low impedance loops - without dedicated test rigs that uses representative fault currents anyway, becomes a bit fruit machine once you get into the many kA end of things, and can be dominated by meter leads and the cleanliness of connections.

So what to do ?

Well it depends how well you need to know, and what for - usually we are ensuring that the fuse or breaker operates promptly for a fault at far point, where a rounding up resistanace and down in current is needed, but the other case is to check that  the let through energy is not so high that cables will be damaged with a fault near the origin, when the safe assumption is to round the current up and resistance down.

Not all texts are clear for which of those two situation the assumptions are appropriate.
In reality it all needs to be taken into account, but taking into account may be to decide that ' it's not the dominant uncertainty, I'll ignore it for now'

Mike

There are indeed assumptions - one small lighting circuit on a board fed by a hundred amp submain, you can indeed assume the source impedance is negligible compared to that of the final circuit. However circuits that supply one or two big loads maybe not.

Another way to think about that is how the design voltage drop has been partitioned, as that is also in effect the conductor resistances.

Actually measuring of very low impedance loops - without dedicated test rigs that uses representative fault currents anyway, becomes a bit fruit machine once you get into the many kA end of things, and can be dominated by meter leads and the cleanliness of connections.

So what to do ?

Well it depends how well you need to know, and what for - usually we are ensuring that the fuse or breaker operates promptly for a fault at far point, where a rounding up resistanace and down in current is needed, but the other case is to check that  the let through energy is not so high that cables will be damaged with a fault near the origin, when the safe assumption is to round the current up and resistance down.

Not all texts are clear for which of those two situation the assumptions are appropriate.
In reality it all needs to be taken into account, but taking into account may be to decide that ' it's not the dominant uncertainty, I'll ignore it for now'

Mike