TRE.ynS I.\ PRACTISE AS AFFF.CTING COORDINATION 179 



inten-al the breakers controllini;- the faulty pliase would he reclosed 

 automatically. 



Single-phase switching has not progressed beyond a preliminary 

 consideration of its possibilities. If applied in situations of proximity, 

 the residual voltages and load currents while one phase of a three- 

 phase grounded neutral system is momentarily open circuited may con- 

 stitute a problem in coordination. 



Under 'ground Construction. — The use of underground construction 

 in distribution systems is seldom economical but is increasing in high 

 load density districts and in some residential areas primarily due to 

 requirements for civic improvements and the relieving of surface con- 

 gestion. The reduced influence on communication circuits of such 

 underground circuits as compared to overhead construction, is too 

 well known to need repeating here. Coincident with the more recent 

 developments in underground distribution certain special situations 

 have brought about the development of underground cable suitable 

 for use in high-voltage transmission circuits, inclusive of 132 kv. 

 Underground installations involving these transmission voltages are 

 highly special, comparatively few in number and small in extent. 

 However, they have a definitely favorable effect upon coordination 

 problems withing the territories surrounding them. 



Aerial cable construction for both distribution and transmission 

 circuits has been used to a limited extent and has a definitely beneficial 

 effect upon coordination matters. Whether this type of construction 

 will be extended in the future is not evident. 



Grounding of Distribution System Neutrals. — One of the difficult 

 tasks encountered in distribution systems is that of obtaining adequate 

 grounding of primary and secondary circuits. Because of this difficulty 

 the establishment of neutral networks grounded at many points has 

 become a practise. In most cases in the past, two separate neutral 

 networks have been provided, one for the primary and one for the 

 secondary system. However, in several localities these two separate 

 neutrals have been combined into a common-neutral arrangement pro- 

 viding in this way an increased multiplicity of ground connections to 

 both the primary and secondary neutral conductors. Further exten- 

 sion of the use of this system is probable. This arrangement intro- 

 duces features of interest from the coordination standpoint, because 

 of the increased opportunities for the How of currents through the 

 ground. Experience and investigations so far, however, indicate that 

 with adequate attention to coordination this arrangement is comparable 

 in its effect on neighboring communication circuits, to other types of 

 distribution systems. 



