SCCR rating 5 KA

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Davebones

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North Of Tampa Fl
Had some equipment come in with 5ka sccr rating . It will be fed from a bus with available fault current of 30,000 amps . We will be feeding the machine with a 100 amp bus switch fused at 100 amps . The 100 amp fuses ( LLSRK-Id ) fuses show let-thru current as 4,100 amps . Is this correct for meeting the 5KA sccr rating of the machine ?
 
Provided the 100A fuse is also rated for short circuit current of 30kA. But if its short circuit current rating is any higher, say 50kA, its let-through current may exceed the SCCR rating of the equipment.
 
Davebones said:
Had some equipment come in with 5ka sccr rating . It will be fed from a bus with available fault current of 30,000 amps . We will be feeding the machine with a 100 amp bus switch fused at 100 amps . The 100 amp fuses ( LLSRK-Id ) fuses show let-thru current as 4,100 amps . Is this correct for meeting the 5KA sccr rating of the machine ?
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Maybe, maybe not. It does depend on what you are trying to 'protect'

In general you want to see some type of tested series-combination of devices. There are two elements of fault current that need to be considered when evaluating SCCR. The maximum level of current called I-peak (Ip) and the length of time that current flows, called I-squared T (I?t).

The current limiting effect of upstream protect devices should almost never be used to 'increase the fault rating' of downstream protective devices without testing or supervision (e.g. a standard like UL 508A).

Current limiting effects, let-through current, can be used to protect non-active components like bus bars.
 
Now that equipment is starting to show up with the SCCR rating just trying to make sure we understand how to protect it . This is just putting up a bus switch and fusing it for the equipment . I know a lot of the older equipmwnt no one looked at this . Just install a bus switch and put in fuses ...
 
Most likely you will need to add impedance to the circuit in order to lower the fault current.

Either the panel builder has already taken into account the 'let through of fuses', and yet still only got to a 5kA SCCR, or they never bothered to make a detailed evaluation and simply stuck a 'default' value on their panel.
 
Can you use CL fuses based upon their let through without doing combination/series testing?
Otherwise consider adding an air core reactor sized to add impedance to limit the fault current.
 
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