Earth Mat (Grid) Design for a 275/33kV Main Station

Dear Lyn,

Thank you for the links. I was actually looking for more insight into the EF calculations. Like what are the assumptions as to the value of parameters used and why it's so. The EF calculation would also need to be practical as too conservative a value will lead to overdesigning and wastage. An example would be great.

Simply divide the voltage by the earth loop impedance, Mildred.

Fault level it has to be the initial symmetrical short-circuit current and this does not depends on duration. The rated [thermal] short-time withstand current of an equipment it depends indeed on duration.

So, it is not the system prospective short-circuit level for GPR, touch and step potential calculation.

Usually-as per IEC 60076-5 recommendation, for instance-for 300 kV highest voltage for equipment [ Um] the system short-circuit apparent power could be 30000 MVA and the short-circuit current: Ish=30000/sqrt(3)/275=63 kA

However you have to take into consideration the Sf factor -as per IEEE 80/2000-since part of short-circuit current will not flow through the grounding grid but it will return to the sources through static wires , cable shields or else.

"...The EF calculation would also need to be practical as too conservative a value will lead to overdesigning and wastage..." Sorry, when it comes to safety you have to always err on the side of conservative by using the worst-case values; i.e., those that result in overdesign and (apparent) wastage, so if you have a range of values you must choose the one that represents the most extreme conditions.

If you try and use a more reasonable value, say one that doesn't get exceeded 90% of the time, you may find yourself facing the following dialogue: Attorney for the Plaintiff, "Your Honor, the design engineer has already admitted under oath that he chose materials and construction techniques that our expert witnesses testified only covered 90% of the possible conditions that could be expected at the accident site. In other words, in the interest of saving some incremental expense he knowingly exposed my client to the finite probability of risk of injury, and should therefore be held responsible for all the costs associated with his injuries, loss of income, and treble damages for negligence."

On the other hand, if you take the absolute worst case (but more expensive) scenario you might hear: Attorney for the Defense, "Your Honor, my client has testified that he followed all the (inter)nationally accepted engineering safety codes relevant to the installation, and in all cases his design has withstood the scrutiny of the Plaintiff's expert witnesses who as a group testified under oath that they would have done nothing differently. Therefore the defendant should be released from any further culpability in this matter."

Which chair would you rather be sitting in? Penny-wise can be very Pound-foolish.