IEEE 80 Method

[downinit]

Hello,

I have been trying to get an understanding of IEEE 80 and using it to determine if step / touch potentials are below safe levels.

There are a few details I am unsure of, and hope someone here can help with.

1) When calculating the grid current Ig, using the zero-sequence fault current 3Io, for a resistance grounded system.. is it correct to use for example 3Io=100A if the system is low resistance grounded to limit the fault current to 100A, even if there is 25kA available fault current?

2) The fault current division factor Sf...

In the case I am considering, there is a 5kV/600V transformer T1 feeding an MCC. This transformer is fed from a substation a few hundred feet away (with a 27kV/5kV transformer T2 delta/low res grounded wye). The ground grid at the transformer T1 / MCC area is to be connected to the ground grid at the substation with a #2/0 or so ground wire, and therefore in a fault condition there is a copper path from T1 area back to the secondary windings of T2 through the NGR.

Knowing these facts, would it be safe to assume Sf be pretty small? It would seem like only a small portion of the fault current would flow through the ground.

3) Also, just out of curiosity, what is gained by making Rg <5ohm in this particular case? It would seem the lower the value of Rg here, the more fault current that would flow through earth, and therefore higher earth potential rise? Perhaps I have misunderstood something here.

I would definitely appreciate someone with more experience with this to expand my knowledge on the subject! Please let me know if my thinking is wrong.

Thank you.

 

[waross]

The ground grid at the transformer T1 / MCC area is to be connected to the ground grid at the substation with a #2/0 or so ground wire, and therefore in a fault condition there is a copper path from T1 area back to the secondary windings of T2 through the NGR.

Are you considering a fault on the 5kV system or a fault on the 600V system?
The maximum step or touch potential on the 600V system will be 347V. With a resistance grounded system the maximum touch or step potential will be 347V minus the voltage drop across the NGR.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

The GPR, step-, and touch-voltages will be negligible for a 600 V ground fault because of the resistance grounding and the metallic path from the MCC to the transformer. As Bill said, you need to check a 5 kV system ground fault. There may also be transferred potential from the substation ground grid to the MCC area. The grounded MCC enclosure will rise to the GPR voltage with respect to remote earth. If there is a point where someone can stand with his feet at a lower voltage point on earth and touch the MCC enclosure, there may be a touch-voltage problem. This isn't likely to be a problem if the MCC is indoors on a reinforced concrete floor. If it is outdoors, you may want to consider a perimeter ground wire around the MCC to keep the earth surface voltage near the same level as the MCC/ground grid voltage.