For situations where it is required, such as high fault levels, you do not want any graph for <0.1s - you get the actual I²t values from the MCB manufacturer and put those into the equation.
Faults in final circuits are seldom of any relevance for those, unless the fault occurs near the origin, as the circuit impedance will be large compared to the supply impedance.
Upstream fuses do have a significant impact on the fault levels, as fuses heat up when current flows through them, which increases their resistance and reduces the current.
Even over a fraction of a second, the heating effect of the fuse element (whether it actually fails or not) can reduce the actual fault current significantly - far below what you might expect from calculations just based on the impedance and theoretical disconnection time.
That is one reason why 6kA circuit breakers are permitted on smaller installations regardless of what the fault current actually is (or might be in theory).
10 metres of 2 core 16mm² supply cable is 0.023 ohms, and that is already down to the 10kA level before anything else is considered.
When measuring Ze, any result of <0.1 ohms should probably be disregarded - equipment simply isn't that good for values lower than that, despite plenty of devices showing 2 decimal places and tiny values.
For those places which have high fault levels (e.g. large transformer in the basement), Ze must be determined by calculation, primarily from such things as the transformer specification, cable type / length etc.
Whether that was due to the 60A fuse being old and possibly overloaded in the past wasn't clear, but it was an old installation (40+ years).
The supplier fuse had already been replaced before I got there.
Faults in final circuits are seldom of any relevance for those, unless the fault occurs near the origin, as the circuit impedance will be large compared to the supply impedance.
Upstream fuses do have a significant impact on the fault levels, as fuses heat up when current flows through them, which increases their resistance and reduces the current.
Even over a fraction of a second, the heating effect of the fuse element (whether it actually fails or not) can reduce the actual fault current significantly - far below what you might expect from calculations just based on the impedance and theoretical disconnection time.
That is one reason why 6kA circuit breakers are permitted on smaller installations regardless of what the fault current actually is (or might be in theory).
Won't ever happen on a domestic installation.
10 metres of 2 core 16mm² supply cable is 0.023 ohms, and that is already down to the 10kA level before anything else is considered.
When measuring Ze, any result of <0.1 ohms should probably be disregarded - equipment simply isn't that good for values lower than that, despite plenty of devices showing 2 decimal places and tiny values.
For those places which have high fault levels (e.g. large transformer in the basement), Ze must be determined by calculation, primarily from such things as the transformer specification, cable type / length etc.
I have seen one - a L-N short on a ring final only a couple of meters from the origin, circuit fuse 30A 3036 rewireable. Both the 30A fuse and the 60A supplier fuse had failed.
Whether that was due to the 60A fuse being old and possibly overloaded in the past wasn't clear, but it was an old installation (40+ years).
The supplier fuse had already been replaced before I got there.
