628 ELECTRICAL EQUIPMENT 



Cable should be supported every four feet in vertical runs and 

 every three feet in horizontal runs, while for tubing the distance 

 between the insulators may be increased to about 10 feet. When 

 dealing with large conductors carrying heavy currents, care 

 should be taken, as explained under the section of " Current Lim- 

 iting Reactors," to rigidly support them so that they will not be 

 torn from their supports when severe short-circuits occur. 



Cables in Ducts or Conduits. It is not always convenient 

 or desirable to run all of the conductors exposed for several reasons. 

 There may be no suitable place to support such cables. The 

 congestion may be so great that it would be hazardous in other 

 respects. They may be subject to mechanical injury. They 

 may be in a bad location from a " safety first " standpoint. If 

 therefore, for any of the above reasons it is undesirable to run 

 conductors exposed, then they may be run in conduit or ducts and 

 may be provided with a protecting weatherproof braid or lead 

 sheath as the occasion demands. It should be borne in mind that 

 if the lead sheath is omitted the conduit or ducts should be thor- 

 oughly drained to some pit so that water cannot remain in them. 



Iron conduit should not be employed on alternating currents 

 unless all conductors of the circuit are in the same conduit. The 

 general practice is to use iron conduit up to about two inches in 

 diameter, above which fiber conduit is generally used. 



This type of conduit is formed in cylindrical shape from fiber 

 or wood pulp under pressure. The pulp is thoroughly saturated 

 with a bituminous compound so as to kill any vegetable matter or 

 bacteria which would tend to promote decay. 



It has been found that the majority of all initial cable troubles 

 are directly traceable to some injury to the lead casing when being 

 drawn into the duct, due to the roughness of the walls, and the 

 cement which has seeped through the joint and formed cutting 

 edges after hardening. Cable troubles are also due to stray cur- 

 rents leaking through the joints, as a result of improper installa- 

 tion and the impossibility of securing proper alignment. These 

 objections, however, are eliminated by the use of fiber conduit, 

 due to the smooth interior and water-tight joints. Unlike joining 

 tile conduit, the connection made with fiber conduit is ideal, 

 affording perfect alignment, without the use of mandrels or dowel- 

 pins, and not having to use cement, mortar or burlap at the joints. 

 It is also true that fiber conduit is impervious to moisture, gases, 



