The grounding system in a typical home consists of the equipment grounding conductors and grounding electrode that are connected to a common bonding point with the neutral from the incoming power, the ground connection to the telco, Cable TV, Satellite Antenna, gas and water pipes.
Electricity always takes the path of least resistance back to the source. Ohms law states the electric current through a conductor between two points is proportional to voltage and inversely proportional to its resistance. It can also be said that the voltage between the two points is directly proportional to the current flowing through the conductor and its resistance. Assuming a phase to case fault occurs to equipment that is at the end of a 50 foot 18 AWG branch conductor supplying 120V at 10A. The equipment grounding conductor will carry a 10A fault current until the circuit breaker opens. From Ohm’s law, a voltage between the fault and the source termination will develop on the conductor. With a resistance of 6.385 ohms per 1000 feet, resistance of the 50 foot 18 AWG equipment grounding conductor between the fault and source is 0.319 ohms. The voltage or touch potential between two points will be 3.19 V.
The Risk & Dangers of Electrical Currents
Current passing through the human body develops a potential that can affect the person in various ways. It can be felt as a slight tingling sensation to causing tissues to burn, ventricular fibrillation, or death. A human body has a resistance that ranges from 100,000 to 300 ohms depending on the skin surface, that is calloused, soft, dry, wet, or sweaty. Assume the same fault occurred in a piece of equipment without an equipment grounding conductor or ground return. And a person is touching the conductive case of the equipment with one hand and touching building steel on the other. The touch potential that may develop going through the person’s hands is 116.81V. The current determines the physiological effects on the body. In this example, the person will experience the effect of 1.2mA to 389mA of current flowing through the body. The person may barely perceive the current or have some tingling sensation a 1.2mA to ventricular fibrillation or death at 389mA.
In the first example, the 120V/10A source will take the path of least resistance via the equipment grounding conductor and expose personnel to a potential of 3.19V on the conductive surface of the case. At 3.19V, the current that may flow through the person is 0.03mA to 10.6 mA. The person may feel the current enough to react and let go of the touch at 10.6mA. In the second example, the person exposed to a 116.81V potential can be harmless or lethal.
In any equipment fault or electrical storm, the fault or induced currents created will travel on any conductive path such as equipment, telephone, set-top boxes, or faucets placing people nearby in harm’s way. Electrically connecting the equipment and services to a common bonding point provides the path of least resistance for the fault or induced currents to bypass the human body as they return to the source. As can be seen in the second example, the equipment grounding conductor reduces the touch potentials by 97%.
We all know the primary purpose of grounding is to protect personnel from electric shock or electrocution. The grounding system works by providing the fault or lightning-induced currents a low resistance return path instead of the high resistance of a touch/human body. In a properly grounded system, a fault in equipment returns the current to the breaker panel via the equipment grounding conductor. A fault on the line side of the panel uses the neutral conductor on the feeder line and grounding electrode as the return to the utility transformer. Induced currents during an electrical storm will use all electrical connections to the common bonding point and the grounding electrode to return to earth.