Grounding Questionby Ted Smith on 10/18/17
From Chris Hartzel: There were a few of us who had some question about the most recent homework that was due yesterday. We felt some of the answers were not correct. Here's an example: Q. What is the primary purpose of the grounded conductor? A. Provide a second voltage without a second transformer B. Connect the grounded conductor to a single reference point, the earth C. Provide a low resistance path for fault current to get back to the overcurrent protective device to ensure that it trips and opens the circuit to stop the fault current D. Ensure that fault current is transferred into the earth and dissipated.
Excellent question Chris. This is one of the things that often causes confusion with electricians. It goes to the heart of grounding theory and practice. The grounded conductor's primary purpose is to provide a second voltage without a second transformer. For example, in a single phase system like a house, the grounded conductor is tapped into the center of the secondary winding on the transformer and allows you to get 240 volts between the secondary ungrounded conductors and 120 volts from either of the secondary ungrounded conductors to the grounded conductor. The same is true if a three phase wye system, where the grounded conductor is connected to the end point of each of the transformers allowing 208V between any of the ungrounded conductors and 120V between an ungrounded conductor and the grounded conductor. In situations where a second voltage is not needed, we don’t utilize a grounded conductor. For example, in a three-phase motor or feeding a three phase transformer. We simply run three ungrounded conductors and an equipment grounding conductor to those systems because we don’t need a second voltage there.
In order for this transformation and use of the grounded conductor to work properly, it must be connected in such a way that it has a solid, common reference point. Otherwise the transformation is unstable. For this reason we have to connect the grounded conductor to the earth and do it in a way that we ensure it will not be undone in the future.
In order to do this we use the grounding electrode system which is compromised of grounding electrodes and grounding electrode conductors. This system is used to connect the grounded conductor to earth at the main service or on the secondary side of three phase transformer where a new grounded conductor is created.
The third and final part of the grounding system is the equipment grounding conductor. This was added in the 1950s because our use of the grounded conductor to give us a second voltage also created a safety issue. It made the earth a potential return path for current back to its source. In order to keep fault currents from using the earth as a potential path, we began connecting the electrical devices and equipment and metal parts of buildings that were close to electrical equipment to a common conductor and bringing that conductor back to the main service and at that point connecting it all together with the grounded conductor and the grounding electrode system. That way, in the event of a fault, some small amount of fault current will still utilize the earth as a return path but the majority of the fault current will use the equipment grounding conductor as a return path and get the majority of that fault current to the overcurrent protective device and trip it.
I hope that helps. Please feel free to call me with any questions.