ARCING OF CONTACTS IN TELEPHONE SWITCHING CIRCUITS 1243 



spread in the minimum arcing current as given in the same reference, 

 Table V. 



(3) Contact activation and hmiting circuit conditions for arcing: In 

 Reference 3, it was reported that the Hmiting inductance for active 

 contacts was greater by more than 2 orders of magnitude than for clean 

 contacts. By calculation, Equation 5(a), 



[(^)active/(i')cleanlLimit = 600. 



It was also reported that for C = 10~^ farad and Fo = 50 volts the 

 limiting inductance observed for active silver was between 10"^ and 

 10"^ henry. By calculation. Equation 5(a), the limiting inductance for 

 the same conditions is about 4 X 10~^ henry. 



(4) Contact activation and arc initiation time: According to Equation 

 3 the initiation time of the steady arc is directly proportional to the 

 arc initiation current. For the same circuit conditions, therefore, active 

 contacts should have a shorter period of arc initiation. This result 

 seems to contradict some published observations* where it was pointed 

 out that the voltage drop into an arc was shorter for clean contacts 

 than for activated contacts. A number of transients, furnished by the 

 authors, were carefully examined. It was observed that in most cases 



Table III — Computed and Measured Limiting Voltages for 

 Establishing a Steady Arc Between Clean Silver Contacts* 



* A few observations were not included in this table. These correspond to the 

 cases where the calculated initiation times of the arc were of the order or greater 

 than the time of physical closure of the contacts. As expected, the observed volt- 

 ages were consistently higher than the computed voltages. 



^ L. H. Germer and J. L. Smith, Arcing at Electrical Contacts on Closure, Part, 

 III. J. Appl. Phys., 23, p. 553, 1952. 



