ARCING OF ELECTRICAL CONTACTS IN TELEPHONE SWITCHING CIRCUITS 539 



rupted, it is followed by a recharging process to a new arc initiation 

 voltage when a second arc is initiated. Under certain conditions, the 

 second arc may be initiated at a lower voltages than the first arc due to 

 residual effects of the first arc which may alter the conditions in the gap. 

 This effect is discussed later. 



A transient on break with a series of interrupted arcs is shown in 

 Fig. 3. The first arc was initiated at 230 volts and a gross field of 2.5 X 10^ 

 volts/cm. All the following arcs were initiated at the spark breakdown 

 potentials in air corresponding to the separations involved. Fig. 4 

 shows a transient where the arc w-as sustained with occasional interrup- 

 tions. 



In addition to arcing, one may obtain glow discharge. Fig. 5 shows a 

 transient where glow discharge predominates. Glow initiation and 

 glow-arc transitions are discussed in a later Section. 



Fig. 6 shows the methods used for current and voltage measurements. 

 As indicated, direct voltage measurements at the contacts were avoided 

 to eliminate the unnecessary complications of the measuring circuit. 



INTERRUPTED ARCS 



Conditions for Obtaining Interrupted Arcs 



A breakdown from a voltage Vai into an arc corresponds to a rapid 

 voltage drop at the contacts from Vai to the arc voltage v. For most prac- 



50 

 -6 



TIME,t, IN 10~^ SECONDS 



Fig. 3 — -Typical contact voltage transient on break of an inductive circuit. 

 Pd contacts in atmospheric air, E = 50 volts, L = 0.2 henry, R = 950 ohms and 

 C = 510 X 10^'- farad. Velocity of contact separation = 40 cms/sec. 



