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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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    I assume that this is the sort of CT machine that takes X-rays (rather than the magnetic kind of scanner, that used to be called nuclear magnetic resonance until folk got spooked by the idea that the atoms they are made of have nuclii  ?)

    The reason for checking is that the magnetic type require  cable layouts that keep the flow and return currents together.


    I'd be a bit wary of the claim of problems of interference, as this normally has more to do with cable routes and what else shares the supply, rather than the impedance per-se.

    Having said that,  in practice the current this machine draws will be quite bursty, so self interference where the voltage droops when it is time to take a picture may be an issue.)  Asking for a very low source impedance may just be a way of forcing installers to use a dedicated short fat cable.


    Line impedance is related to PSSC you are right,  - but note that for PSSC for fault survival without exploding the fuses, you are normally looking for the maximum of L-L or L-N or L-E fault states - normally L-L wins, but not always for all arrangements. Here the issue is the lowest current case, rather than the highest.

    Like Dave, I'd also be very wary of the accuracy of low value readings - a few milli-ohms is the sort or change you get with how well things are polished before they are bolted, and tests made with spiky probes are often just a fruit machine at that sort of level, and do not forget to include the impedance of the fuses themselves - a fuse that at 150A dissipates say 20-30 watts (not unusual for a BS88 style thing) is another milliohm or two, and you have very little margin.


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  • 0
    I assume that this is the sort of CT machine that takes X-rays (rather than the magnetic kind of scanner, that used to be called nuclear magnetic resonance until folk got spooked by the idea that the atoms they are made of have nuclii  ?)

    The reason for checking is that the magnetic type require  cable layouts that keep the flow and return currents together.


    I'd be a bit wary of the claim of problems of interference, as this normally has more to do with cable routes and what else shares the supply, rather than the impedance per-se.

    Having said that,  in practice the current this machine draws will be quite bursty, so self interference where the voltage droops when it is time to take a picture may be an issue.)  Asking for a very low source impedance may just be a way of forcing installers to use a dedicated short fat cable.


    Line impedance is related to PSSC you are right,  - but note that for PSSC for fault survival without exploding the fuses, you are normally looking for the maximum of L-L or L-N or L-E fault states - normally L-L wins, but not always for all arrangements. Here the issue is the lowest current case, rather than the highest.

    Like Dave, I'd also be very wary of the accuracy of low value readings - a few milli-ohms is the sort or change you get with how well things are polished before they are bolted, and tests made with spiky probes are often just a fruit machine at that sort of level, and do not forget to include the impedance of the fuses themselves - a fuse that at 150A dissipates say 20-30 watts (not unusual for a BS88 style thing) is another milliohm or two, and you have very little margin.


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