Thank you for all the replies, I have revisited the test forms and allowed some minor adjustments to take into consideration the test equipment used, using the accuracy percentages from the manufacturers data and the calibration test sheets. Although this small amount hasn't made a massive decrease it does allow on the larger MCCB's that have adjustable magnetic settings for these to be reduced allowing the circuit to comply. I have done the calculations for the client to what they would need to be set at and the client can now check whether these settings will be suitable in regards to the downstream loading and potential start up currents etc.

They are the units I was referring to Mapj1, there is only 2 circuits which fail currently on these type of units and both are out by a reasonable amount, the first one can be changed to the 80A MCCB with 640A fixed magnetic which then squeezes a pass, this would mean that the current unit at 15secs would be achievable but given the downstream load would be sufficient on an 80A MCCB then this feels the better option, even if they may not agree. 

The last one is to far out and in my opinion likely designed wrong, i have suggested that a switch fuse be connected somewhere along the circuit run where the Zs would be achievable to the switch fuse and then installing fuses to BS88 to allow for a higher Zs to be achieved at the DB.

Currently on a 100A MCCB with a max Zs at 100% of 0.228 allowed 0.18 at 0.8. The measured Zs is 0.33 so way off the mark even with tester in accuracy, this one was also tested by myself with my tester that was calibrated only a couple days before the job so I am confident in this result (it is also around the result gained 3 years ago but the failure was not picked up). A switch fuse with 80A fuses at 5s disconnection time would allow 0.55 at 100% and 0.44 at 0.8. 100A fuses would squeeze a pass but the downstream load does not require a large supply and the wiggle room would be nice.

It will now be down to the client to decide what they wish to do. 

Most of the testing would of been cold, thermal imaging at the time of testing does not suggest that the circuits were running much above ambient, between 20 & 30 degrees for the submains. 

Thank you for all the replies, I have revisited the test forms and allowed some minor adjustments to take into consideration the test equipment used, using the accuracy percentages from the manufacturers data and the calibration test sheets. Although this small amount hasn't made a massive decrease it does allow on the larger MCCB's that have adjustable magnetic settings for these to be reduced allowing the circuit to comply. I have done the calculations for the client to what they would need to be set at and the client can now check whether these settings will be suitable in regards to the downstream loading and potential start up currents etc.

They are the units I was referring to Mapj1, there is only 2 circuits which fail currently on these type of units and both are out by a reasonable amount, the first one can be changed to the 80A MCCB with 640A fixed magnetic which then squeezes a pass, this would mean that the current unit at 15secs would be achievable but given the downstream load would be sufficient on an 80A MCCB then this feels the better option, even if they may not agree. 

The last one is to far out and in my opinion likely designed wrong, i have suggested that a switch fuse be connected somewhere along the circuit run where the Zs would be achievable to the switch fuse and then installing fuses to BS88 to allow for a higher Zs to be achieved at the DB.

Currently on a 100A MCCB with a max Zs at 100% of 0.228 allowed 0.18 at 0.8. The measured Zs is 0.33 so way off the mark even with tester in accuracy, this one was also tested by myself with my tester that was calibrated only a couple days before the job so I am confident in this result (it is also around the result gained 3 years ago but the failure was not picked up). A switch fuse with 80A fuses at 5s disconnection time would allow 0.55 at 100% and 0.44 at 0.8. 100A fuses would squeeze a pass but the downstream load does not require a large supply and the wiggle room would be nice.

It will now be down to the client to decide what they wish to do. 

Most of the testing would of been cold, thermal imaging at the time of testing does not suggest that the circuits were running much above ambient, between 20 & 30 degrees for the submains.