grounding 1

[gord99]

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.

 

[cbarn24050]

Hi, the earth is a good conductor due to high water content, there is a path to ground because the local transformer's neutral point is grounded.

 

[buzzp]

I am not up to speed on grounding. The earth is a poor conductor of electricity. This much I do know. I also know that from location to location the earth impedance varies greatly. There has to be a potential difference for current to flow and there must be a return path (via back to source). These are fundamental requirements for current flow. This is why enclosures and such are grounded to the earth, in case enclosure gets energized it has a return path via the ground. The same rule applies to fault currents. The transformer neutral is grounded for this reason, safety.

 

[Nemrod]

There is another reason for grounding the neutral:
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.

 

[dpc]

For a ground fault, the fault current leaves the phase conductors and seeks a path to return to its source.

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

 

[BJC]

A ground fault in an AC system seeks the lowest reactance path back to the source. That in most cases is the grounded "star" of the transformer or the neutral of the derived system. The lowest reactance path is the one that encloses the least area between the fault current and the return path. Most of the fault current in an AC ground fault will return in the conduit ( assurming you have metalic conduit) enclosing the faulting conductor or the ground wire in the circuit. This is all explained in the IEEE green book.

 

[gord99]

I appreciate all the replies. I'm fully conversant with the basic principles of electricity/electronics, I'm looking more for the theory of grounding with respect to a transmission system as it affects the end user.

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

 

[dpc]

One of the reasons for connecting the neutral to ground is to fix the maximum voltage that the system insulation will experience. With no earth reference, the neutral will float and this can create voltages with respect to earth that are significantly higher than the phase to neutral voltage, causing insulation failures.

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.

 

[busbar]

dpc gave a good description. Minor comments— the 'main bonding jumper' generally facilitates sufficient current in a phase-to-ground fault; intended to operate the upstream overcurrent device. To rely on ground rods/electrodes {‘earth return’} for this on low-voltage systems is an unreliable and dangerous practice, but is accepted and carefully engineered in >600 volt systems.

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.