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What Would Be The Worst Type Of Three Phase Faults (And WhyIt Happens)

When Three Phase Faults Occur…

In a three phase power system, the type of faults that can occur are classified by the combination of conductors orbuses that are faulted together. In addition, faults may be classified as either bolted faults or faults that occurthrough some impedance such as an arc. Each of the basic types of faults will be described and shown in Figure 1.

It must be noted that in a majority of cases, the fault current calculation required for the selection of interrupting andwithstand current capabilities of equipment is the three phase bolted fault with zero impedance.

Let’s go through each of the four three phase faults //

1. Three phase bolted faults

2. Bolted line-to-line faults

3. Line-to-line-to-ground faults

4. Line-to-ground faults

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Figure 1 – Designation of short-circuit categories

1. Three Phase Bolted Faults

A three phase bolted fault describes the condition where the three conductors are physically held together with zeroimpedance between them, just as if they were bolted together. For a balanced symmetrical system, the fault currentmagnitude is balanced equally within the three phases.

While this type of fault does not occur frequently, its results are used for protective device selection,because this fault type generally yields the maximum short-circuit current values.

Figure 1(a) provides a graphical representation of a bolted three phase fault.

Go back to three phase faults ↑

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Figure 1a – Three-phase short circuit

2. Bolted Line-To-Line Faults

Bolted line-to-line faults, Figure 1(b), are more common than three phase faults and have fault currents that areapproximately 87% of the three phase bolted fault current.

This type of fault is not balanced within the three phases and its fault current is seldom calculated for equipmentratings because it does not provide the maximumfault current magnitude. The line-to-line current canbe calculated by multiplying the three phasevalue by 0.866, when the impedance Z1 = Z2.

Special symmetrical component calculatingtechniques are not required for this condition .

Go back to three phase faults ↑

3. Line-To-Line-To-Ground Faults

Line-to-line-to-ground faults, Figure 1(c), aretypically line-to-ground faults that have escalated toinclude a second phase conductor. This is anunbalanced fault. The magnitudes of double line-to-ground fault currents are usually greater than thoseof line-to-line faults, but are less than those ofthree phase faults.

Calculation of double line-to-ground fault currentsrequires the use of symmetrical componentsanalysis. The impedance of the ground return pathwill affect the result, and should be obtained ifpossible.

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Figure 1b – Bolted line-to-line faults

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Figure 1c – Line-to-line-to-ground faults

Go back to three phase faults ↑

4. Line-To-Ground Faults

Line-to-ground faults, Figure 1(d), are the most common type of faults and are usually the least disturbing to thesystem. The current in the faulted phase can range from near zero to a value slightly greater than the bolted threephase fault current.

The line-to-ground fault current magnitude is determined by the method in which the system is grounded and theimpedance of the ground return path of the fault current.

Calculation of the exact line-to-ground fault current magnitudes requires the special calculatingtechniques of symmetrical components.

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Figure 1d – Line-to-ground faults

However, close approximations can be made knowing the method of system grounding used. On ungroundeddistribution systems, the line-to-ground fault currents are near zero.

Line-to-ground fault current magnitudes indistribution systems with resistance groundedsystem neutrals can be estimated by dividing thesystem line-to-neutral system voltage by thetotal value of the system ground- to-neutralresistance.

Line-to-ground fault current magnitudes indistribution systems with a solidly grounded systemwill be approximately equal to the three phase faultcurrent magnitudes. Determining line-to-groundfault currents on long cable runs or transmissionlines will require detailed ground return pathimpedance data and detailed calculationtechniques.

Reference // IEEE Recommended Practice forCalculating Short-Circuit Currents in Industrial andCommercial Power Systems

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