INDUCTIVE COORDINATION 77 



substation only although sometimes low-voltage lightning arresters 

 were placed on it at various points in the distribution network to aid 

 in its stabilization in case of a break in it. In some instances, at the 

 time of the installation of the four-wire system, the primary neutral 

 was connected at various points in the network to driven ground rods 

 thus resulting in a multi-grounded neutral system. In at least one 

 instance the neutral conductor was not solidly grounded even at the 

 substation, it being connected to ground through lightning arresters. 



The experiences of the power companies with the multi-grounded 

 neutral were generally favorable. It was found to be more reliable and 

 to embrace some simplifications over other distribution methods. 

 While the early multi -grounded neutral arrangements were obtained 

 by making connections to ground along the primary neutral conductor 

 and interconnecting it, at service transformers, to well-grounded 

 secondary neutrals a further simplification in the arrangement was 

 readily apparent. 



It will be noted in Fig. 1 that this interconnection of the primary and 

 secondary neutrals resulted in two grounded neutrals on the pole line 

 in all sections where the secondary neutral existed. In extending the 

 multi-grounded neutral arrangement or in reconstructing existing 

 portions of the network, these two neutrals were combined into a 

 single well-grounded conductor continuous in all portions of a feeder 

 area and often continuous in all parts of a substation area or of several 

 contiguous substation areas. This arrangement, called the "common- 

 neutral," which was first extensively applied in Minneapolis* by Mr. 

 S. B. Hood, resulted in certain savings in equipment and relief of 

 congested pole heads and in a neutral network most effectively 

 grounded since all secondary neutral grounds were thus made avail- 

 able, in addition to any driven grounds along the pole line. 



The operation of this system in Minneapolis showed many advan- 

 tages in the protection of secondary networks from the effects of voltage 

 rises under abnormal conditions. In addition a paper presented in 

 1925 by Mr. Hood ^ pointed out that over a period of three years the 

 rate of transformer failure was reduced to 8/10 of 1 per cent per annum. 

 This excellent performance in transformers arose undoubtedly from 

 the fact that with the "common-neutral" or interconnected neutral 

 arrangements the lightning arresters are connected directly around the 

 transformers. Later studies showed that the connection of the light- 

 ning arresters directly between the primary conductors and secondary 

 neutral provides a degree of protection which cannot readily be ob- 

 tained in any other way.^- '• *• ^' ^' '' 



* Prior to applying this system in Minneapolis, Mr. Hood introduced it at 

 Toronto, Canada. 



