IEEE C62.92.4-1991 (R2002) pdf free.IEEE Guide for the Application of NeutralGrounding in Electrical Utility Systems,Part lV-Distribution.
IEEE C62.92.4 three-wire unigrounded systems generally result in the least investment when the load is mostlythree-phase. On the other hand,costs favor multigrounded neutral systems when the loads are mostly single phase.Several factors must be given consideration to determine if the system should be grounded or ungrounded.
One factor to consider is the geographical area and the isokeraunic level in which the system will be operating.Thiswill have considerable effect on the type of surge arresters required and the degree of direct-stroke protection that isconsidered necessary.In an area having a high isokeraunic level, it may be advantageous to use a static wire or anoverhead shield wire for direct stroke lightning protection.The primary neutral conductor can very easily be used forthe shield wire and may also function as a common neutral conductor with the secondary system.However, with theuse of aerial cable for secondary circuits, there will be some duplication of neutral conductors that may not beobjectionable compared with the reduced outages and reduction of trips for fuse replacement and repair of lightning-damaged equipment. Also, the number of ground connections on the neutral conductor per mile of line must beconsidered.
Generally, a minimum of four ground connections per mile is considered adequate for a fourwire multigroundedneutral system, but, with the use of a static or overhead shield wire, it may be necessary to install grounds on everypole.
Another method of providing lightning protection that is affected by the type of grounding is the use of surge arrestersinstalled on each conductor at intervals of approximately every 394 m (1200 ft).The voltage rating of the arrester isdirectly related to the effectiveness of grounding employed on a particular system. For example , the arresters on athree-wire three-phase ungrounded system would need a voltage rating suitable for the phase-to-phase voltage.On theother hand, on an effectively grounded four-wire three-phase circuit, the arresters may have a voltage rating of only75% of the phase-to-phase voltage.This may also allow the use of equipment having a lower Basic Insulation Level(BIL).
The use of multigrounded neutral distribution systems that satisfy the requirements of effectively grounded systemsprovides several areas in which reduced costs can be realized.
1)The National Electrical Safety Code (NESC)[B2] permits neutral conductors that are multigrounded on
systems of 50 kV and below to be attached directly to the structure surface. The same neutral conductor maybe used for both the primary and secondary systems.
2)Lower surge-arrester voltage ratings can be used.
3) Lower surge-arrester voltage ratings may permit lower equipment BIL, especially at the higher distribution
voltages.IEEE C62.92.4 pdf download.