WP21v1 Neutral Wire Facts and Mythology

Neutral Wire Facts and Mythology Revision 1 by James Spitaels Introduction 2Characteristics of the 3-wire system 2Neutral wire mythology4The origin of the 3-wire system5Conclusion 7Resources 8 Click on a section to jump to it Contents White Paper 21 This technical note discusses many common misun-derstandings about the function of the neutral wire and its relation to power problems.The subjects of dedi-cated lines,phase reversal,isolation transformers,and grounding are addressed.Various myths are described and criticized.Executive summary white papers are now part of the Schneider Electric white paper libraryproduced by Schneider Electric s Data Center Science Center Data Center Science Center DCSCSchneider-E Neutral Wire Facts and Mythology Schneider Electric Data Center Science Center White Paper 21 Rev 1 2 In the normal three wire single phase power system found in most offices,the neutral wire is one of the three connections at the wall outlet.Some semi-technical articles and advertis-ing literature for power protection products attribute special properties to this wire.Fre-quently misleading claims are made regarding the function of this wire and how it is used by electronic equipment.Sometimes,power protection equipment is sold on the basis of claims involving special treatment of the neutral wire.A typical office wall outlet has three electrical connections,which are the hot,neutral,and grounding wires.All office equipment requires only the hot and neutral wires to function.The third or grounding wire is connected to exposed metal parts on the equipment.Within the building,the grounding connections of all electrical receptacles are wired to one another and are connected to the water piping.This ensures that all electrical equipment with exposed metal parts has these parts electrically connected to each other and to exposed metal fixtures in the building such as water fixtures.The wiring system is diagrammed in Figure 1:The hot and neutral wires are interchangeable as far as the equipment is concerned.Both are power carrying wires.One of the power carrying wires is grounded at the source for reasons of safety(see the appendix:the origin of the 3-wire system).The only reason why the two wires are differentiated(hot vs.neutral)is to identify which of the wires is grounded(the neutral wire).The 3-wire system that the user sees is actually derived from three phase distribution,which uses a 5-wire system.In the 5-wire system,there are 3 hot wires,1 neutral wire,and 1 grounding wire.The common 3-wire receptacle uses only one of the 3 hot wires.In North America,minicomputers frequently are designed to use a 208 V 3-wire system.In this 3-wire system there are 2 hot wires and 1 ground wire(no neutral wire).In this case the two hot wires are the power carrying wires.In Europe,the normal 3-wire receptacle is symmetrical so that the neutral and hot wire connections can be swapped by simply rotating the plug.Introduction Figure 1 The 3-wire system Characteristics of the 3-wire system Neutral Wire Facts and Mythology Schneider Electric Data Center Science Center White Paper 21 Rev 1 3 “Grounded”vs.”grounding”In North America,much of the confusion regarding grounding has its roots in the National Electrical Code of the United States.In the Code,the receptacle ground connection is referred to as the grounding conductor while the neutral connection is referred to as the grounded conductor.The subtle difference between ing and ed has led many to erroneously interchange these terms in the literature.The chief error,which occurs,is that real problems which are caused by problems with the grounding wire such as inter-system ground noise are improperly attributed to the grounded wire.The result is that users sometimes purchase power protection equipment that is not designed to solve the problem that they are attempting to prevent(Inter-system ground noise is discussed in White Paper 8,Inter-system Ground Noise:Cause and Effects).The most common example of this mistake is the purchase of a plug-in isolation transformer to solve a problem with ground loops or inter-system ground noise.Isolation transformers have absolutely no effect on these problems because they are required to pass the grounding wire straight through.Isolation transformers are frequently represented as providing an isolated ground.What they actually provide is an isolated neutral or grounded wire.This provides no benefit in solving grounding problems,which are the primary cause of power problems.Special issues in North America The neutral and hot wires are interchangeable and reversible insofar as the operation of equipment is concerned.This leads to the questions as to why in North America one of the power prongs is slightly different in size from the other.A natural but wrong conclusion is that proper“polarization”is important to the proper operation of equipment.The actual answer as to why plugs are polarized in North America is found in a ubiquitous a