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Line impedance query

me1899
Hi 


I am looking at a job where a supply to a replacement CT scanner has to be calculated.


The current scanner is fed by a cable with line impedance 0.06 at source end of the sub panel that feeds it.


The new scanner must have a line impedance no more than 0.06 so basically I can't use the way on the current panel.


The client engineer has said to run from the intake and provide a cable calc to prove the line impedance is OK for the new scanner feed (this is D and B).


I am using AMTECH and was wondering if there was a way to output line impedance or if I need to show a calculation another way?


The client engaged a firm to do line impedance tests, and their report shows values measured between L1-L2, L2-L3 and L1-L3, but looks like they did testing at the sub panel and not at the intake.


This is basically like PSCC right? So I can get PSCC from AMTECH, divide by root 3 and work out impedance from there?


Am I on the right track? Assistance would be much appreciated.


Regards


Anisur
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  • 0
    I agree OMS he usual advice applies "if all else fails read the instructions"!


    Andy I think you are using the rule of thumb calculation from Appendix 14 of the good book. They will always come out on the very high side for safety.


    For a bolted 3 phase fault the maximum PFC would need to be measured with the installation cold. The PFC will be the open circuit phase voltage, so for a transformer taped to 433V divice by route 3 = 250V divided by the impedance of one winding added to the impedance of one line conductor. The calculated figure assumes the HV supply can supply unlimited current (which it cannot) and ignores any current limiting effect of any upstream circuit protection.


    To get a closer actual measurement in the field measure the fault current phase to phase with a decent loop tester and divide the reading by 0.87. 


    In this case I thing the requirement for very low supply impedance values is for supply voltage stability. I am thinking the voltage will be relatively stable for normal running current then short high current demands when the images are being taken will drop the voltage, depending on the supply impedance, leading to blurred images.


    If the running loads and peak load are known and the manufacturer gives voltage data then Amtech can do the rest as it will give supply impedances for the chosen cable and will calculate volt drop.
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  • 0
    I agree OMS he usual advice applies "if all else fails read the instructions"!


    Andy I think you are using the rule of thumb calculation from Appendix 14 of the good book. They will always come out on the very high side for safety.


    For a bolted 3 phase fault the maximum PFC would need to be measured with the installation cold. The PFC will be the open circuit phase voltage, so for a transformer taped to 433V divice by route 3 = 250V divided by the impedance of one winding added to the impedance of one line conductor. The calculated figure assumes the HV supply can supply unlimited current (which it cannot) and ignores any current limiting effect of any upstream circuit protection.


    To get a closer actual measurement in the field measure the fault current phase to phase with a decent loop tester and divide the reading by 0.87. 


    In this case I thing the requirement for very low supply impedance values is for supply voltage stability. I am thinking the voltage will be relatively stable for normal running current then short high current demands when the images are being taken will drop the voltage, depending on the supply impedance, leading to blurred images.


    If the running loads and peak load are known and the manufacturer gives voltage data then Amtech can do the rest as it will give supply impedances for the chosen cable and will calculate volt drop.
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