Solidly grounded Wye system
the solidly grounded system is the most common and one of the most versatile. The most commonly used configuration is the solidly grounded wye, as it supports single phase to neutral loads.
The solidly grounded star system layout can be illustrated by considering the neutral terminal of the star system layout to be grounded.
First, the system voltage relative to earth is set by the phase-to-neutral winding voltage . Because some parts of the power system, such as equipment chassis, are earthed and the rest of the environment is essentially at earth potential as well, this has big implications for the system.
This means that the line-to-earth insulation level of the equipment must be as great as the phase-to-phase voltage, 57.7% of the phase-to-phase voltage . It also means that the system is less sensitive to phase-to-ground voltage transients. Second, the system is suitable for supplying line to neutral loads.
The operation of a single phase load connected between one phase and neutral will be the same on all phases because the phase voltage amplitudes are equal.
This system layout is very common, both in use as 480 Y / 277 V and 208 Y / 120 V , as well as on most utility distribution systems.
While the solidly grounded wiring system is by far the most common system typically grounded, star connection is not the only arrangement that can be configured as a solid ground system.
Solidly grounded Delta system
The delta system can also be grounded. Compared to the solidly grounded star system of Figure 1, this system grounding arrangement has a number of drawbacks. The phase-to-earth voltages are not equal and therefore the system is not suitable for single-phase loads. And, without correct identification of the phases, there is a risk of shock, as one conductor, phase B, is earthed and could be misidentified.
This arrangement is no longer in common use , although there are still a few installations where this arrangement is used.
The triangle arrangement can be configured in another way but has the advantages of a solidly founded system. Although the arrangement it appears that this system is suitable for three-phase and single-phase loads, provided that the single-phase and three-phase load cables are separated.
This is commonly used for small services that require both 240 VAC three phase and 1 20/240 VAC single phase .
Note that the phase A voltage to earth is equal to 173% of the phase B and C voltages to earth. This arrangement requires the coiled BC to have a thread in the center.
A common feature of all three grounded system shown below, and grounded systems in general, indicates that a short to ground will result in a large amount of short circuit current.
This condition is known as a ground fault. The voltage on the faulty phase is lowered and a large current flows in the faulty phase because the fault phase and impedance are low.
The voltage and current on the other two phases are not affected. The fact that a solidly grounded system withstand a large earth fault current is an important characteristic of this type of system grounding and affects the design of the system. Statistically, 90-95% of all shorts in the system are ground faults, so this is an important topic.
The occurrence of an earth fault on a solidly earthed system requires the fault to be remedied as quickly as possible . This is the major drawback of the solidly grounded system compared to other types of system grounding.
A solidly grounded system is very effective in reducing the possibility of transient voltages between line and ground .
However, to do this, the system must be effectively grounded. A measure of system grounding efficiency is the ratio of the available earth fault current to the available three phase fault current. For efficiently grounded systems, this ratio is typically at least 60%.
Most utility systems providing services for commercial and industrial systems are solidly grounded. Typical utility practice is to ground the neutral in many places, usually at each line pole, creating a multi-conductor neutral system. Since a separate earth conductor is not connected to the utility line, the earth resistance limits the circulating ground currents that can be caused by this type of earthing.
Because Separate Ground Conductors Are Used In a commercial or industrial installation, multi-conductor neutrals are not preferred for the power systems of such facilities due to the possibility of circulating ground currents.
Neutral, multi-base entities in NEC jurisdictions, such as commercial or industrial facilities, are in fact prohibited in most cases by the NEC. Instead, a single grounding point is preferable for this type of system , creating a uni-grounded or one-point system.
In general, the solidly grounded system is the most common. It is required when single phase phase-to-neutral loads are to be supplied and has the most stable phase-to-earth voltage characteristics.
However, the large earth fault currents that this type of system can withstand, and the necessary equipment, are a disadvantage and can affect the reliability of the system.