Project Overview:

 The initial design for a 22kV 1500kVA 3-phase padmount substation inside a school premises was certified by Essential Energy with a separate earthing arrangement. To ensure safety, especially given the proximity of children with easy access to the substation's outer body, the client decided to install a metallic fence and a fire wall for the main switchboard. This was to limit access of children, as the substation was initially exposed with no fence in place.

These modifications necessitated the recertification of the original plan, as the metallic fence introduced potential hazards during an earth fault event. Essential Energy requested a detailed risk assessment of the earthing system to evaluate the associated risks and ensure compliance with safety standards. The assessment aimed to identify and mitigate hazards posed by earth potential rise (EPR) and other related risks at and around the substation site.

Earthing Arrangement:

20 copper rods 3m in length and fence connected to the earthing system.

Step & Touch Plots:

Touch Potential Calculation:

Step Potential Calculation:

Testing:

Fall of Potential Testing:

A fall-of-potential test was conducted to measure the grid impedance.

The design grid impedance was 0.24 ohms.

Using the 62% method, nine test values were recorded, and the average grid impedance was calculated to be 0.186 ohms, which is well below the design value.

Step & Touch Testing:

Step and touch testing was undertaken.

The Maximum Step & Touch Potential at nearby structure was 2.4V & 57.13V, well under the allowable step and Touch voltages.