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Earth Loop

earth loop Ground Loop or Earth Loop
The Neutral and ground system
In the US: The Neutral wire runs from pole to pole across the electric grid.
When the Neutral is installed on top of the pole, above the other wires, it's called the static line, and protects the grid from lightning strikes by redistributing voltage to the earth through ground connections at each pole.
When the Neutral is installed below the hot wires on the pole, it is simply called the Neutral or System Neutral.
The Neutral is connected to the ground wire at each pole.
The Neutral serves the grid by bonding (via approved electric connection) all Neutrals together, and bonding all ground wires into a single large ground array or earthing array.

The Neutral wire is a function of the ground. For example, Neutral arrives at the local substation from the power plant and connects onto an array of earth grounds. The Neutral from the power plant connects to a grounded steel pole. The Neutral wire can terminate at that point as long as the steel pole is connected to the ground or earth array.
Under the substation is an array of ground rods that interconnect all the equipment in the substation and with the incoming and outgoing Neutral wire, and untimately bonds the main generator to earth.
The Neutral-ground system leaving the substation will arise from this ground array, and reappear on each pole leaving the substation. The Neutral leaving the substation does not always connect directly to the Neutral wire that arrives at the substation, as observed in one connection at localpower plant.
Grounding codes vary by type of soil in each location. The Neutral is a function of the grounding system. And the ground system is bonded together by the Neutral wire.
The Neutral can arise anywhere there is appropriate grounding. At the local power plant in south Texas, with dense, coductive soils that offer relatively low resistance to ground, I observed a Neutral wire that started at the first transmission tower. That Neutral was not connected to the power station switchyard or the power plant. Instead it originated from the switchyard ground array which extended out to the transmission tower. All other towers observed had an overhead Neutral wire that connected to steel structures in the switchyard, and all the steel structures appeared to be interconnected by Neutral wires.

Both Neutral and ground are necessary for safety and reliability, to absorb overvoltages, surges, lightning strikes etc by providing direct pathway to earth.
The system is further protected by circuit breakers at each substation, including the power plant substation. If lightning hits the static line at a transmission tower or power pole, and the voltage is greater than the grounding can absorb, the overvoltage jumps to the hot wires, travels to the substation or power plant, causing a circuit breaker to trip at the substation.

At the end user location, the Neutral connects to the transformer neutral connection, depending on the service requirement.
For example a residential service is typically 120-240 volt single phase. The Neutral and 1 hot wire connect to the primary side of the transformer. At the same time, the Neutral is connected to a ground wire that travels down the pole to earth. The ground wire also connects to the steel transformer can at the point where the Neutral goes to the house. The transformer is grounded by the ground wire. Inside each transformer are two coils of wire: the primary and the secondary. The Neutral from the power company connects to the primary coil, and the secondary coil. The secondary coil on a residential transformer is actually composed of two coils, with a connection in the middle that gives rise to the Neutral wire that goes into the house, and is bonded to ground. Therefore the transformer creates it's own Neutral. The Neutral from the pole connects to the Neutral going to the house. Therefore the Neutral going into the house is connected to the metal can, thus giving continuous Neutral-Ground back to the power company generator.
The Neutral wire connects inside the home's circuit breaker panel, and the Neutral busbar is bonded to the ground busbar, and the ground busbar is connected to the outdoor ground rod.
Another example is a 3-phase service with 3 transformers and 6 coils. In some applications, the Neutral is directed connected to the Neautral going into the end-user location, assuming a Neutral is part of the wiring configuration.
 

Finally we arrive at explanation of earth loop.
Ground wires, like all wires have resistance to electricity that can slow the flow of electricity. This resistance can be measured.
The ground wire must have sufficiently low resistance, or a short circuit might cause overload on breaker panel, circuit breaker and wire. An overload can damage the service panel and create hazardous wiring conditions.
Each breaker has a rating for amps, but also a rating for ground resistance. For example, a breaker cannot absorb a direct lightning strike, or it will burn out. That's an extreme example of too much voltage, but the earthing or ground system plays a role for protecting circuit breakers.

When a short circuit occurs, the overload should move rapidly through an unobstructed ground path and be absorbed in the earth via the ground rod. At the same moment, the breaker also absorbs the overload and trips. If the resistance is too high on the ground wire, that affects heat on the breaker and wire. This demonstrates that the ground wire assists the breaker. And of course the breaker is essential for protecting the grounding as well.

The overall grounding at each location (home or business) can be tested for earth loop impedance (resistance). Impedance measures how much the ground is impeded or slowed by poor earthing.

There is no simple explanation for measuring earth loop impedance, but the general theory is as follows: the local ground rod at each business or home is electrically bonded to all other grounds back to the main power plant via the Neutral wire. The entire ground system is an array that runs from power plant to end user. There is a resistance present in the total ground array, that affects each individual ground connection at each installation. The ground loop intends to measure the effect of this resistance at each location by measuring the total ground resistance at each location.

The total ground loop or earth loop offers impedance. The impedance can be calculated, but it is usually advisable to buy proper measuring device.
Impedance can vary at different locations and is caused by many factors. If the loop resistance is too high, the business or home may require more ground rods or better location of ground rod so that the ground can correctly protect the circuit breaker and breaker can protect the wire and electric appliances.
Measure earth loop resistance

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