Generally if several machines are operated in parallel, and only one machine grounded, this effect is accentuated in the grounded machine. For example, consider four generators operating in parallel as shown in Figure below. The reactances shown are in percent based on the individual machine ratings. 
The phase-sequence-reactance diagram is shown by Fig.(b) from which Z0 = 2.2; Z1= 2.2 and Z2, = 2.2. The sequence components for a line-to-ground fault are 
The three-phase fault current of an individual generatol is Xg/X1= 100/8.8 = 11.4 times full load. Consequently, in the case illuskated, grounding only one generator will cause that generator to carry 22.8/11.4 or twice the current it would have on a three-phase fault. Since mechanical stresses are proportional to the square of the current, they would be equal to four times the three-phase short-circuit stresses. It will be observed that the line-togroundfault current is always greater than the threephase fault current, and that the situation becomes partitularly serious as the number of paralleled machines increases. In an installation of this kind, some form of neutral impedance device is a necessity.also when two generators are operated in parallel at generated voltage, there is the possibility of circulating harmonic currents. This is true whether the neutrals are interconnected or not. The two conditions necessary for the: flow of harmonic current are: the prcsence of a resultant harmonic voltage, and a path for the flow of current. It is important to note the term