Ground fault factors

GFF = Ground fault factor (Section 20.6). tTOV = Temporary overvoltage. 

This is to achieve a higher level of fault current to obtain a quicker tripping on fault. It is obtained when the system has a ground fault factor not exceeding 1.4 (Vg < O.SVf), as noted above. A solidly grounded system will provide effective grounding. This system will reduce the transient oscillations and allow a current sufficient to select a ground fault protection. It is normally applicable to an LT system. 

Figure 20.16 Relationship between Rg/X, and Xq/X, to determine the values of ground fault factor kg based on I EC...

In the preceding paragraphs we have analysed the behaviour and characteristics of a system when grounded or left isolated. The ground fault factor (GFF) plays a very significant role in the selection of insulation level (BIL) and its coordination with the different equipment connected on the system. The application of a particular method of grounding would thus depend upon... 

Arc suppression coil or ground fault neutralizer Ground fault factor (GFF) Magnitude of temporary overvoltages Insulation coordination Application of different types of grounding methods (for HT, HV and EHV systems) Important parameters for selecting a ground fault protection scheme... 

These are protection CTs lor special applications such as biased differential protection, restricted ground fault protection and distance protection schemes, where it is not possible to easily identify the elass of accuracy, the accuracy limit factor and the rated burden of the CTs and where a full primary fault current is required to be transformed to the secondary without saturation, to accurately monitor the level of fault and/or unbalance. The type of application tind the relay being used determine the knee point voltage. The knee point voltage and the excitation current of the CTs now form the basic design parameters for such CTs. They are classified as class PS CTs and can be identified by the following characteristics ... 

Ground fault Section 20.1. It is essential to know the grounding conditions to deterniine the OV factor as below ... 

The overvoltage factor, due to a phase fault, may thus not exceed a ground fault (GF) condition. It would, therefore, be appropriate to consider the GF condition instead of a phase fault and. in all probability, it will be adequate to account for this contingency. 

Iq = jiiaximum ground fault current /,j = symmetrical ground fault current D, = decrement factor... 

The combination of the above factors has rendered the nanocrystalline solution competitive, not only with amorphous Co-based alloys, but also with classical crystalline alloys and ferrites. The consequence is a steadily increasing level of applications in magnetic cores for ground fault interrupters, common mode chokes and high frequency transformers. Fig. 14 shows some typical examples. The worldwide production rate meanwhile approaches an estimated 1000 tons/year, and the trend is increasing. The only drawback of the nanocrystalline material appears to be the embrittlement that occurs upon crystallization, which requires final shape annealing and, thus, restricts application mainly to toroidally wound cores. 

The main electrical consideration is safety. The electrical service in the kitchen and bathroom must incorporate ground-fault monitors as required by local codes. Additional consideration involves the accessibility of switches and outlets. Lighting is also a factor in meeting the needs of the visually impaired. 

A properly installed groimding system protects against ground faults, static discharge, and lightning. The resistance, and so the effectiveness, of the ground path is affected by five factors, almost all of which can be controlled ... 

The rated voltage factor and the corresponding rated time Not applicable Depends upon the system grounding. Refer to Table 15.4 Depends upon the system fault conditions and generally as 1.9 Table 15.4... 

An electrical earth system comprises the provision on the supply system of an earth connection to facilitate earth fault current flow, the connecting of all exposed metalwork within the installation to a common grounding terminal and the connection of this terminal to earth. For a complete installation the principal factors which need to be considered, are ... 

Here Ze is the impedance of the supply side of the earth fault loop. Tfie actual value will depend upon many factors the type of supply, the ground conditions, the distance from the transformer, etc. The value can be obtained from the area electricity companies, but typical values are 0.35for TN-C-S (protective multiple earthing, PME) supplies and 0.8n forTN-S (cable sheath earth) supplies. Also in the above formula, R is the resistance of the line conductor and R2 is the resistance of the earth conductor. The complete earth fault loop path is shown in Fig. 3.32. 

The deformation and stability of an artificial slope are usually affected by many factors, such as geometric boundary conditions, geological structures, rock structures and engineering properties, ground stress, and hydrogeological conditions (Willie and Mah 2004). Among the factors that induce slope failure, relatively large and weak faults are the most common. 

HGURE 23.3 Fault In an undeiground coal mine (photo by G. Molinda) TABLE 23.5 Factors In undeiground coal mine faii of ground fatalities ... 

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