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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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    Use the longest time that may occur and assume you need a fault current that exceeds the 'all trip' condition when considering how low Zs needs to be in respect of ADS for safety of life, or it may not prompt trip when you need it to.


    Use the shortest trip time and least tripping current that may fire the device  when designing a hierarchy of breakers  trying to ensure that downstream devices trip first and this one does not.

    Hint, with breakers you often cannot achieve this nested discrimination as easily as with fuses.

    Mike
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    Use the longest time that may occur and assume you need a fault current that exceeds the 'all trip' condition when considering how low Zs needs to be in respect of ADS for safety of life, or it may not prompt trip when you need it to.


    Use the shortest trip time and least tripping current that may fire the device  when designing a hierarchy of breakers  trying to ensure that downstream devices trip first and this one does not.

    Hint, with breakers you often cannot achieve this nested discrimination as easily as with fuses.

    Mike
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