Hi there,  trying to piece it together without knowing the installation, or that breaker family, or the software, terribly well, so please forgive my opening with a few "obvious" questions.

Has a Zs of 80 milliohms been measured as round loop at the far point of the circuit this breaker supplies ? 

(and measured with what kit, and how well does that tally with the expected resistance for the length and size of cable involved?  I ask that because it reads you are only in or out by a few milliohms here, and to get a reading of that sort of precision can be rather fraught - just cos the meter has that many digits does not always mean that the last few digits have more  value than pulling the handle on a  fruit machine in the arcade depending how it was done )

This is an adjustable breaker, but it is wound up to the max, which is 250A ? (so we expect the 'instant trip' to be some multiple of that)  So Ir is set to 1,  and is Im also adjustable and if so what is it fixed at or adjusted to ? (Im is the "magnetic" trip multiplier Ir is the thermal one )

If for example the Im multiplier is set or fixed to  10, then you need 2.5kA to give an instant trip , and that looks rather like one of your graphs  is suggesting, and for fault current below that, the time will be some seconds.

The supply is TN something and there is no earth fault relay or RCD anywhere, so you are relying on this breaker to do the deed for LE faults as well as overloads ?


So if all these things are true, then with 70 milliohms and 230V L-E your PSSC is more like 3.2KA,  and with 80 milliohms more like 2.8kA so really you should be OK - but nowadays folk allow for lower line voltage  and instrument tolerances etc, and decide the PSSC may be a bit lower, and after that correction, then it fails, but only just.


You could re-measure, especially if the cable  calcs show it should be a lot less.  (is there really 20 volts drop on full load that would really suggest Zs reading is true ?)

You could wind the I max down by 10% and see if anyone notices.  Or if you have control over Im, just wind that in a bit.

You could investigate if there is any mileage in additional earth paths to lower Zs

You could  consider the addition of an earth fault relay.

Hi there,  trying to piece it together without knowing the installation, or that breaker family, or the software, terribly well, so please forgive my opening with a few "obvious" questions.

Has a Zs of 80 milliohms been measured as round loop at the far point of the circuit this breaker supplies ? 

(and measured with what kit, and how well does that tally with the expected resistance for the length and size of cable involved?  I ask that because it reads you are only in or out by a few milliohms here, and to get a reading of that sort of precision can be rather fraught - just cos the meter has that many digits does not always mean that the last few digits have more  value than pulling the handle on a  fruit machine in the arcade depending how it was done )

This is an adjustable breaker, but it is wound up to the max, which is 250A ? (so we expect the 'instant trip' to be some multiple of that)  So Ir is set to 1,  and is Im also adjustable and if so what is it fixed at or adjusted to ? (Im is the "magnetic" trip multiplier Ir is the thermal one )

If for example the Im multiplier is set or fixed to  10, then you need 2.5kA to give an instant trip , and that looks rather like one of your graphs  is suggesting, and for fault current below that, the time will be some seconds.

The supply is TN something and there is no earth fault relay or RCD anywhere, so you are relying on this breaker to do the deed for LE faults as well as overloads ?


So if all these things are true, then with 70 milliohms and 230V L-E your PSSC is more like 3.2KA,  and with 80 milliohms more like 2.8kA so really you should be OK - but nowadays folk allow for lower line voltage  and instrument tolerances etc, and decide the PSSC may be a bit lower, and after that correction, then it fails, but only just.


You could re-measure, especially if the cable  calcs show it should be a lot less.  (is there really 20 volts drop on full load that would really suggest Zs reading is true ?)

You could wind the I max down by 10% and see if anyone notices.  Or if you have control over Im, just wind that in a bit.

You could investigate if there is any mileage in additional earth paths to lower Zs

You could  consider the addition of an earth fault relay.