OVER-VOLTAGE PROTECTION 609 



absorbing capacity, enables the arresters to discharge continuously 

 for long periods, a very valuable feature of these arresters. The 

 tanks are of steel with welded seams. 



The location and arrangement of aluminum lightning arrester 

 installation depend greatly upon the station layout. In general 

 the arrester should be installed as near as possible to the apparatus 

 or station to be protected. The ideal arrangement would be to 

 have the tanks and horn gaps installed as a complete unit just 

 inside the station. For lower voltage equipments this is feasible, 

 as the arcing at the gaps is not severe even in abnormal cases. 

 Above 27,000 volts, this practice is usually questionable and it is 

 recommended that the horn gaps be installed outside the building, 

 with leads tapping the line near its entrance to the station, the 

 object, of course, being to isolate any arc from the station appara- 

 tus. The tanks, cones and transfer device may be installed inside 

 of the station in a suitable compartment. This requires the use 

 of an extra set of either wall or roof entrance bushings in addition 

 to those used for the line entrance leads. 



Wherever horn gaps are mounted inside the building sufficient 

 clearance should be allowed over them. The exact distance to 

 be allowed depends upon the voltage and the nature of the material 

 or apparatus under which the horns are installed. If there are 

 cables, wires, buses, or any material which would be damaged 

 by fire, considerable distance should be allowed. On the other 

 hand, if there are only concrete and iron beams of the floor or 

 roof a much smaller clearance is permissible. Normally there is 

 no appreciable arc at the gaps, but in abnormal cases where the 

 film has been allowed to get out of order, the arc might be of 

 considerable size. Where there are no buses or inflammable 

 apparatus, the following are the minimum clearances from the 

 tops of horns to be allowed: 



Feet. 



Up to 16,100 volts 3 



16,100 to 37,900 volts 4 



37,900 to 75,000 volts. 6 



Above 75,000 volts, the horn gaps should never be placed 

 indoors. 



The horn gaps for arresters for 27,000 volts and above are 

 supported on a pipe framework which is so designed as to permit 

 mounting OP either wooden or steel towers, or, if desirable, on the 



