306 ELECTRICAL EQUIPMENT 



Whether the generator neutral should be grounded or not 

 depends on the operating conditions. If an uninterrupted 

 service is the most essential consideration, the system should not 

 be grounded, while if it is more desirable to limit the voltage 

 strains, imposed by grounds, it may be advisable to ground the 

 neutral, thus limiting the stress to the Y-voltage. Grounding may 

 also be advisable where selective action is desired on a number 

 of outgoing feeders, especially underground, so that individual - 

 feeders may be disconnected even in the case of grounds. 



The use of a resistance in the grounded neutral of a system 

 offers the advantages of limiting the current which flows through a 

 ground on one phase, and thereby eliminates the danger of mechan- 

 ical destruction due to the excessive currents at the dead short- 

 circuit, which would occur with a ground on one phase of a system 

 with the neutral grounded without resistance. Such a grounding 

 resistance, however, abandons the advantage of the dead grounded 

 system that the voltage between lines and ground can never exceed 

 the Y voltage. It is, therefore, not permissible where the appa- 

 ratus cannot safely stand the delta voltage of the line. This is 

 the case with low- voltage generators feeding a line or cable through 

 step-up auto-transformers. With dead-grounded generator neu- 

 tral the voltage between generator and ground is fixed, but with 

 resistance in the neutral ground, a dead ground on the high- 

 potential phase puts nearly delta voltage of the high-potential 

 circuit on the low-potential generator, and thereby seriously 

 endangers it, if the step-up ratio of the auto-transformer is 1:2. 

 If it is higher there is every likelihood that the generator will be 

 destroyed. The same is the case when connecting together trans- 

 mission systems of different voltages through auto-transformers. 

 In any case, if auto-transformers are not considered safe, trans- 

 formers must be used. 



A grounding resistance should have a value high enough to 

 limit the neutral current, but still low enough to insure that, if a 

 ground occurs in one phase, it will permit a sufficiently large cur- 

 rent to flow in the neutral to open the protective circuit-breakers. 

 Non-inductive resistances are preferable to reactances, since they 

 eliminate the danger of high-frequency oscillations beween lines 

 and ground through the generator reactance in the path of the third 

 harmonic, by damping the oscillation in resistance. The ground- 

 ing of the neutral through a reactance may, therefore, be 



