Short circuit study - Defining Impedance for Ground fault calculation

[jesusrperezd]

Hi all,

There is a 480-277V (3ph 4w) feeder from a wye solid grounded system. I am analysing the available faults on this circuit at 20%, 50%, and 100% of the circuit length.

For the Ground fault simulation, I have the option to add a "Fault impedance" in ohms to the calculation. I understand this value considers the impedance of the fault contact plus the return path to the source. It makes sense to consider a value to be realistic. The fault results vary significantly with impedance from 0 to 5 ohms.

A folk told me that he always uses 0.5 ohms, another one said he prefers to do it with 0 ohms to obtain the largest value.

I would like to have a consistent and logical method to analyse these cases. I believe this is crucial for coordinating protective devices and defining settings. My initial thought is to consider a value that represents the return through the EGCs, considering the sizes and length from the fault to the source.

What are your thoughts on this?

Thanks!

 

[Strathead]

I am just an electrician, but wouldn't an "available fault" be based on a direct short circuit to represent to worst case at each location and as such be 0 ohms?

 

[jim dungar]

You want the maximum fault current when selecting OCPD AziC and equipment SCCR, in order to not have problems if the fault resistance was less than you calculated.

You consider the impedance of the fault when performing arc flash calculations and when comparing relay settings to see if they provide adequate coordination.

 

[D. Castor]

Assuming a fault impedance can be useful in determining if the ground fault protection will detect the fault. I've never considered it for 480 V calculations - only for medium voltage utility distribution.

 

[jesusrperezd]

You want the maximum fault current when selecting OCPD AziC and equipment SCCR, in order to not have problems if the fault resistance was less than you calculated.

You consider the impedance of the fault when performing arc flash calculations and when comparing relay settings to see if they provide adequate coordination.

Thanks you Jim. how would you estimate thar impedance?

 

[jim dungar]

Thanks you Jim. how would you estimate thar impedance?

I cant remember the specifics. I have used the IEEE books and the white papers from the protective relay manufacturers.