ARCING OF ELECTRICAL CONTACTS IN TELEPHONE SWITCHING CIRCUITS 543 



LLI 

 (i) 

 < 



O 

 > 



I- 



o 

 < 



f- 

 z 

 o 

 u 



300 



50 



TlMEjt, IN 10"^ SECONDS 



Fig. 8 — Contact voltage transient with interrupted arcs on break of an in- 

 ductive circuit. Pd contacts in atmospheric air, E = 50 volts, L = 0.010 henry, 

 R = 40 ohms and C = 900 X lO""^^ farad. Velocity of contact separation = 40 

 cms/sec. 



Initiation of Reversed Arcs in One Discharge 



In one breakdown from a voltage Vai it is commonly observed that a 

 succession of reversed arcs may be obtained. It was shown in equation 3 

 that the residual condenser voltage Vn progressively decreases, numeri- 

 cally, with the number of arcs n. Following the interruption of the first 

 arc, the condenser voltage is — | Vi | and the contact voltage is +v, the 

 arc voltage. The capacity C \vill then recharge the local capacitance at the 

 contact through a small lead inductance /. If the circuit resistance is neg- 

 lected, the maximum voltage the contact will acquire is — (2 | Di | + v). 

 If this equals or exceeds the original arc initiation voltage Vai , a, second 

 arc is obtained. For illustration, consider a breakdown initiated at Vat 

 = 300 volts and y = 14 volts. From Equation 3, Vn was calculated for 

 the first four arcs at r/z = 0.0 and 0.2 The corresponding maximum 

 contact voltages acquired after each arc were also calculated and the 

 results are given in Table I. For r/z = 0, column 3, one may obtain, ac- 

 cording to this simple circuit consideration, more than 4 arcs, actually 

 5. For r/z — 0.2, which is a reasonable practical value, only 2 arcs may 

 be obtained, column 5, since following the second arc the maximum 

 voltage attained at the contacts is only 256 volts which is less than the 

 initial arc initiation voltage. 



It is possible in some cases, however, to obtain a few more arcs than 



