Mike Dolt Enterprises
Mike Dolt Enterprises
User: John Scapillato
Asked on: Friday, April 29, 2011 at 10:47 am EDT
Answered on: Tuesday, May 14, 2024 at 4:47 am EDT
Question: I have seen some utility standards which use ground rings around pad mount distribution transformers. I've read that this configuration will reduce step and touch potentials but I am failing to see how without a full grid being installed under the pad. I understand ground rings can be used for stations or equipment houses to create equipotential planes so that as a fault occurs the GPR seen by all equipment will be the same. But what is the purpose for a single transformer? Other than tying the multiple grounds together to reduce the overall electrode resistance. Also, I am wondering what depth should the ring be installed? Does greater depth of the ring conductor reduce protection against the step or touch potential? Or are we just trying to bury the conductor below the frost line. That's a mouthful, so thanks in advance to any who can help make this clear for me.
Purpose of Ground Rings for Pad-Mount Transformers
Ground rings around pad-mount distribution transformers serve the following purposes: Reduction of Touch Potentials
- Although a full grid under the pad would be more effective, a limited ground ring can still provide some protection against touch potentials.- In the event of a fault, the ground ring acts as a low-resistance path for current to flow away from the transformer, reducing the potential difference (touch potential) between the transformer and the ground.
Reduction of Step Potentials
- Ground rings around pad-mount transformers are similar to those used around substation ground grids.- They create an equipotential zone around the transformer, which reduces the potential difference between two points on the ground surface near the transformer (step potential).
Depth of Ground Ring The depth of the ground ring is typically determined by the following factors: Local soil conditions and frost depth Allowable maximum step and touch potentials Greater depth
Generally reduces the effectiveness of the ground ring in reducing step and touch potentials, as the current is more likely to spread vertically through the soil rather than horizontally towards the ground ring.
Frost line
The ring conductor should be buried below the frost line to prevent heaving and damage to the ring.
Other Benefits Tying Grounds Together
- Ground rings provide a common point for connecting all grounding conductors associated with the transformer, such as the transformer case, neutral conductor, and lightning arrestor grounds.
Lowering Electrode Resistance
- The multiple grounding connections provided by the ground ring effectively reduce the overall electrode resistance, improving the grounding system's performance.
Recommendations
For optimal performance, the following recommendations are generally followed: Use a copper ground ring with a minimum size of #2 AWG. Bury the ring at a depth of approximately 18 inches or below the frost line. Connect all grounding conductors to the ground ring using exothermic welds or other approved methods.* Ensure that the ground ring is properly maintained and inspected regularly. Flag for review