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An Overview of Wye and Delta Solidly-Grounded Systems
An Overview of Wye and Delta Solidly-Grounded Systems (photo credit: forums.mikeholt.com)

Solidly-grounded Wye System

The solidly-grounded system is the most common system arrangement, and one of the most versatile. The most commonly-used configuration is the solidly-grounded wye, because it will support single-phase phase-to neutral loads.

The solidly-grounded wye system arrangement can be shown by considering the neutral terminal from the wye system arrangement to be grounded. This is shown in Figure 1 below.

Solidly-Grounded Wye System arrangement and voltage relationships
Figure 1 – Solidly-Grounded Wye System arrangement and voltage relationships

Several points regarding Figure 1 can be noted.

First, the system voltage with respect to ground is fixed by the phase-to-neutral winding voltage. Because parts of the power system, such as equipment frames, are grounded, and the rest of the environment essentially is at ground potential also, this has big implications for the system.

It means that the line-to-ground insulation level of equipment need only be as large as the phase-to-neutral voltage, which is 57.7% of the phase-to-phase voltage. It also means that the system is less susceptible 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 any phase since the phase voltage magnitudes are equal.

This system arrangement is very common, both at the utilization level as 480 Y/277 V and 208 Y/120 V, and also on most utility distribution systems.

While the solidly-grounded wye system is by far the most common solidly-grounded system, the wye arrangement is not the only arrangement that can be configured as a solidly grounded system.


Solidly-grounded Delta System

The delta system can also be grounded, as shown in Figure 2 below. Compared with the solidly-grounded wye system of Figure 1 this system grounding arrangement has a number of disadvantages. The phase-to-ground voltages are not equal, and therefore the system is not suitable for single-phase loads. And, without proper identification of the phases there is the risk of shock since one conductor, the B-phase, is grounded and could be mis-identified.

This arrangement is no longer in common use, although a few facilities where this arrangement is used still exist.

Corner-Grounded Delta System arrangement and voltage relationships
Figure 2 – Corner-Grounded Delta System arrangement and voltage relationships

The delta arrangement can be configured in another manner, however, that does have merits as a solidlygrounded system. This arrangement is shown in Figure 3. While the arrangement of Figure 3 may not appear at first glance to have merit, it can be seen that this system is suitable both for three-phase and single-phase loads, so long as the single-phase and three-phase load cables are kept separate from each other.

This is commonly used for small services which require both 240 VAC three-phase and 120/240 VAC single-phase.

Note that the phase A voltage to ground is 173% of the phase B and C voltages to ground. This arrangement requires the BC winding to have a center tap.

Center-Tap-Grounded Delta System arrangement and voltage relationships
Figure 3 – Center-Tap-Grounded Delta System arrangement and voltage relationships

Ground Fault

A common characteristic of all three solidly-grounded system shown here, and of solidly-grounded systems in general, is that a short-circuit to ground will cause a large amount of short-circuit current to flow.

This condition is known as a ground fault and is illustrated in Figure 4. As can be seen from figure 4, the voltage on the faulted phase is depressed, and a large current flows in the faulted phase since the phase and fault impedance are small.

The voltage and current on the other two phases are not affected. The fact that a solidly-grounded system will support a large ground fault current is an important characteristic of this type of system grounding and does affect the system design. Statistically, 90-95% of all system short-circuits are ground faults so this is an important topic.

Solidly-Grounded System with a ground fault on phase A
Figure 4 – Solidly-Grounded System with a ground fault on phase A

The occurrence of a ground fault on a solidly-grounded system necessitates the removal of the fault as quickly as possible. This is the major disadvantage of the solidly-grounded system as compared to other types of system grounding.

A solidly-grounded system is very effective at reducing the possibility of line-to-ground voltage transients.

However, to do this the system must be effectively grounded. One measure of the effectiveness of the system grounding is the ratio of the available ground-fault current to the available three-phase fault current. For effectively-grounded systems this ratio is usually at least 60%.

Most utility systems which supply service for commercial and industrial systems are solidly grounded. Typical utility practice is to ground the neutral at many points, usually at every line pole, creating a multi-grounded neutral system. Because a separate grounding conductor is not run with the utility line, the resistance of the earth limits the circulating ground currents that can be caused by this type of grounding.

Because separate grounding conductors are used inside a commercial or industrial facility, multi-grounded neutrals not preferred for power systems in these facilities due to the possibility of circulating ground currents.

Multi-grounded neutrals in NEC jurisdictions, such as commercial or industrial facilities, are actually prohibited in most cases by the NEC. Instead, a single point of grounding is preferred for this type of system, creating a uni-grounded or single-point grounded system.

In general, the solidly-grounded system is the most popular, is required where single-phase phase-to-neutral loads must be supplied, and has the most stable phase-to-ground voltage characteristics.

However, the large ground fault currents this type of system can support, and the equipment that this necessitates, are a disadvantage and can be hindrance to system reliability.

Reference: System Grounding – Bill Brown, P.E., Square D Engineering Services

About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

6 Comments


  1. Jackie Culver
    Jan 02, 2015

    Thank you, for the much needed information. I do have a question when reading a transformer with a meter how do you test between phases and your neutral to determine whether it’s a wye our a delta transformer and on which side hi our low ?


  2. ASIM
    Dec 23, 2014

    thank you for this informative article, what are the delta grounded system over wye one?
    and it the delta grounded system suits three phase loads?


  3. Amar Joshi
    Dec 02, 2014

    What are the basic differences between IEC60364 and AS3000? Can IEC60364 be acceptable globally or regional standards are better?


  4. Claudette D. Pusing
    Nov 30, 2014

    Thank you for tackling this topic…this would surely be a great help in my work environment.. In a distribution utility.


  5. SUBIR KUMAR DAS
    Nov 28, 2014

    It’s true that star pont solidly grounded is better from voltage stability point of view. But if not properly maintained there will be catastrophic impact during over loading conditions. So all time there must be a support system to maintain resistance almost to ground level. I always suggest star point solid earth circuit to the system must be twice the phase/line conductors.


    • Steve Neunhoffer
      Nov 28, 2014

      While agreeing with Mr Subar’s comments, I would like to suggest that just doubling the conductor size may be insufficient for keeping touch and step potentials below lethal levels.

      While it is easy to generalise, part of ant rigorous design must include touch and step potential calculations for individual installations.

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