grounding
grounding
(OP)
I'm wondering what the mechanism is for fault current traveling into the earth via a grounding electrode from the grounding conductor in a system. Is the current set up due to a potential difference, hence traveling back to it's source via the earth? Or is it more like lightning, where the ground acts like a sink to discharge a charge that is higher than ground potential?
If possible, please explain with respect to the supply transformer from the utility. It seems to me the earth is not a very reliable conductor. Thanks.
If possible, please explain with respect to the supply transformer from the utility. It seems to me the earth is not a very reliable conductor. Thanks.





RE: grounding
RE: grounding
RE: grounding
Since the earth, with all its variations IS the basic "0" from which the potentials are measured, grounding the neutrals create the same "0" for all measurements around. On small distances the earth's potentiality does not change too much. But on dispersed installations, we create a specific Equipotentiality for each and deliver current by PEN system.
RE: grounding
For most systems, this is either a wye-connected transformer winding or a wye-connected generator. It could also be a zig-zag grounding transformer.
In any event, the earth is not really a sink. It is just functioning as a return conductor. So if there is no path back to the source, no fault current will flow. If you have a good ground wire, very little current will flow in the earth. If there is no ground wire, all the current will flow in the earth.
Earth resistivity can vary drastically, so fault currents vary drastically.
As with any conductor, current flow creates voltage drop. So for high fault currents, there can be appreciable voltage difference between two ground points located some distance apart. This is why substation engineers worry about step and touch potential and communications engineers worry about ground potential rise on phone lines.
Regards,
dpc
RE: grounding
RE: grounding
For instance, I think generators are grounded to earth as is the center-tap (neutral) of the secondary of the final transformer which feeds residences. In Canada, it is a requirement to ground this neutral at the meter base and at the first disconnect/service panel. With a low resistance path already available for fault currents via a copper conductor, why would they also ground the same neutral to a ground electrode or water pipe ground?
It would seem the earth is not a good alternate path unless, as one person said, the ground is wet and conductive. Since the neutral is also grounded at the transformer on the service pole, I am assuming a conductive path is available from the grounding electrode at the residence to the ground at the pole. I don't understand the need for both an electrode ground and a neutral. If the earth presents a high impedance with respect to the neutral, the fault current will take the path of least resistance.
That prompted me to ask if another mechanism might be available to conduct fault currents. I know lightning is an electrostatic discharge and doesn't require a return path. Is it possible the earth can act in a similar capacity, as a sink, or have a large capacitor-like effect? After all, a return path through earth is not constrained to a narrow path as in a copper conductor.
thanks
RE: grounding
The bonding of the neutral to earth at a service entrance is done for similar reasons and because the electrical system ground in a residence should not be dependent on the utility system ground, since not all utility grounding practices are the same. In typical 120V branch circuits, the green ground wire provides an alternative path for fault current should the neutral wire continuity be lost. It also reduces the shock hazard to someone contacting a neutral wire that has lost its continuity back to the transformer neutral. Another concern is the possibility of metal piping accidentally becoming electrified. By grounding the water pipe, this helps maintain the piping system at the same potential as the system neutral.
As far as the path of current in the earth, it does tend to diffuse out away from the metal electrodes. Current is highest in the volume of earth closest to the electrodes. Current density decreases as the distance increases. I don't think there is any capacitance effect.
In the case of lightning, the lightning itself is the return path for equalizing the enormous voltage difference between the clouds and the earth. But lightning is an impulse event and behaves quite a bit differently than a typical 50/60 Hz system.
RE: grounding
Also, if the transformer secondary’s connection to ground is lifted for whatever reason, the secondary conductors may rise to a fairly high voltage with respect to ground, increasing to approach the high-side (primary) voltage, chiefly by electrostatic induction. Unlike the case with ground faults, only milliamperes of current flowing in the bonding jumper are needed to keep potential difference to a low (and usually safe) value.