protective device coordination

[rrzzan]

Hi Guys,

Does the TCC Clipping current of a fuse necessarily be equal to the short circuit current?

Thanks.

 

[magoo2]

I'm not sure what you mean by clipping current.

The TCC or total clearing current of a fuse is determined by test and is presented as a time-current curve.

The short circuit current is a system parameter. The system impedances determine how much short circuit current will flow.

On a time-current curve, the available short circuit current is usually less than the limits of the fuse TCC curve.

 

[dpc]

Define what you mean by "clipping current".

 

[obiwan9394]

The short circuit clearing capability of a fuse is determined by the manufacturer's rating and the type of fuse you are dealing with. I am familiar with Medium Voltage fuses only. Distribution fuses are tested to X/R ratios of 8-12 and Power fuses are tested to X/R ratios in the range of 25. Thus, power fuses can handle more assymmetry during the interruption process. You must be sure that the fuse can clear both the available symmetrical fault current and the assymmetric fault current (DC offset). This is required becuase most fuses will operate within the first few half cycles of current, where assymmetrical current is present.

 

[Kiribanda]

rrzzan,

If I am not mistaken, the term "Clipping Current" is normally used by software programs used for protection coordination(ETAP, SKM). It is the maximum short circuit or ground fault current shown in the current axis as an arrow. It is just a reference value shown for the person who is evaluating the coordination curves. So, always try to use standard terms.

In my opinion, depending on your study and the objective, the clipping current of fuse may or may not be equal to the short circuit current. Remember, coordination study is an "art work".