IEEE 80 Grounding Grid Design vs. NEC 250

[ThePunisher]

In NEC 250, provision of ground rods is focused mainly on the fact that they are there facilitate dissipation of static voltages and lightning induced surges. The system is provided with equipment grounding conductors (EGCs) to establish an effective path for the ground fault to return to its source. In the code, it is also emphasized that the ground (via rods or grounding electrodes) is NOT an effective ground fault current path and if current does go through it due to no EGCs, it will may not trip the OCPD and will create potential gradients.

In IEEE 80 on substation grid design on the other hand, it is assumed that a ground fault will get injected into the grounding grid (where the equipment grounding is bonded directly to the grounding grid). The ground mesh I designed in such a way that it would result to smaller "scallops" of potential gradients based on the largest ground fault current injected into the grid (not to mention that this ground fault current is further reduced into smaller grid currents).

a). However, what if the substation is Code compliant such that EGCs are installed and bonded between the transformer secondary and the main switchgear wherein an effective current path is established and the conductor is sized to carry the full ground fault current. Can we safely assume that a single loop grid encircling the building will suffice?

b) Will it be right for me to say that IEEE 80 grounding grid design is mainly focused on the deliberately designing the system so that the ground fault current is (of if) injected to the grid will not cause any intolerable touch/step voltages or potential gradients as it is based on a design that the "earth" is being used as an effective current path?

 

[dpc]

IEEE 80 exists because the earth is not a good conductor. If it was, no ground grid would be necessary.

 

[bacon4life]

a) Not as a general statement. A typical medium voltage multi-grounded distribution circuit has a neutral conductor sized to carry full ground current. However, during a fault, a substantial portion of the current may flow through the earth. For distances of multiple miles, the earth impedance may be lower than impedance of the neutral/ground wire. Over short distances, the earth is a poor conductor compared to a wire. In your specific application, you may be able to use the current split factor guidance in IEEE 80 to show that most of the current flows through the EGC.

 

[wroggent]

The NEC does not want you to rely on an earth as a ground fault return path; this doesn't mean your grounding grid should not be properly designed per a standard such as IEEE 80.

 

[Kiribanda]

ThePunisher,
IEEE-80 is for designing ground grids for Utility connected substations where the ground fault return current
is taken back to the Utility transformer through soil (depending on the chosen current spilt factor). If one can
prove that the return current is 100% taken through the overhead shield wire, then it resembles the EGC specified in
NEC250 where a solid galvanic path (EGC & the neutral conductor) are provided to the ground fault current back to
the Utility transformer.