488 ELECTRICAL EQUIPMENT 



the insulators. This subject of insulator design has been studied 

 very carefully during the past few years and great improvements 

 have been made, but they have as yet an apparent deterioration 

 causing breakdowns from time to time. Together with atmos- 

 pheric disturbances in districts frequented by lightning storms, it 

 makes the transmission line a vulnerable part of the system and 

 the largest percentage of troubles is caused, thereby. Apparatus 

 troubles are furthermore often traced directly to line troubles as a 

 secondary cause from arcing grounds, surges, etc. 



The secret of success in relay protection is speed. That is, 

 the faulty sections should be cut out so rapidly as to prevent the 

 synchronous apparatus connected to the system from falling out 

 of step and stopping. The time limit for this differs, however, 

 depending on the stability of the apparatus and where the short- 

 circuit occurs. The closer to the machines, the shorter the time 

 before they drop out. 



The longer an arcing ground hangs on, the more damage 

 it will do in breaking insulators and melting off the transmission 

 wires. The arc is very small to begin with, but increases rapidly 

 in size and should therefore be quickly cleared so as to cause as 

 little damage as possible. 



Interruptions can, in many cases, be traced to the customer's 

 own fault. For example the motor breakers may be set at such 

 low-tripping value, that if the power of the system should mo- 

 mentarily drop off and come on again, the heavy current rush 

 would trip the breaker and disconnect the machine. To provide 

 against such interruptions the breaker need, of course, only be 

 set for a sufficiently high value. Similarly, with motor breakers 

 provided with low-voltage releases, which would cause the motor 

 to be cut off from the system on any momentary voltage drop unless 

 provided with a time-limit device. Such relays should therefore 

 be avoided as far as possible if strict continuity of service is 

 essential. 



The time in which a fault might be cleared depends naturally 

 on how quickly the switches may disconnect the faulty section. 

 This in turn depends on the rapidity of the switch action, and on 

 the characteristics of the relay which is used for closing the trip- 

 ping circuit of the oil circuit breaker. 



Due to the inertia of the moving parts it is, of course, impos- 

 sible for a breaker to open instantaneously, and it requires approx- 



