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Determining Max Zs For Time Current Curve

Former Community Member
Former Community Member
How do you determine the max Zs when the manufacturer does not list or specify the maximum permitted earth fault loop impedance? TN system.


https://library.industrialsolutions.abb.com/publibrary/checkout/GES-6119D?TNR=Time%20Current%20Curves%7CGES-6119D%7CPDF&filename=GES6119.pdf
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    yes. Decide how fast you need it to break, safety of life at 240V, 110V   whatever, will give you 0.4 seconds, 0.2 seconds or sometimes 0.1 seconds. Run your finger along that timeline, and read off the current multiple on the 'always breaks' side of the shaded region.

    This is the fault current you need, to always be sure to open the circuit in that time.

    Then use ohms law to see what resistance gets that fault current from your supply voltage (assume supply could be as low as 80% of nominal)

    That's it.

    M.
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  • 0
    yes. Decide how fast you need it to break, safety of life at 240V, 110V   whatever, will give you 0.4 seconds, 0.2 seconds or sometimes 0.1 seconds. Run your finger along that timeline, and read off the current multiple on the 'always breaks' side of the shaded region.

    This is the fault current you need, to always be sure to open the circuit in that time.

    Then use ohms law to see what resistance gets that fault current from your supply voltage (assume supply could be as low as 80% of nominal)

    That's it.

    M.
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