ARCING OF ELECTRICAL CONTACTS IN TELEPHONE SWITrillNG CIRCUITS 541 



tcrmiuated when the curiciit drops to the minimum arcing current /, 

 It is e^•i(^eIlt that the condition for ohtainin<>; an interi'uptcvl arc is: 



/o — {Vai — v) 



< In 



(1) 



It may t)c pointed out tiiat surface contamination, such as organic 

 acti\ation, tends to decrease both /^ and V aC ■ According to equation 1, 

 one may conchide that contact surface contaminations usually tend to 

 cause a transition from an interrupted arc transient to a steady arc 

 transient. The latter is usually associated with appreciably higher 

 (Miergy dissipation between the contacts and much lower contact life 

 due to erosion. 



L R 



^WT' WV 



V-OSCILLOSCOPE 



yV\ \ 





:iox C 



Fig. 6 — Voltage and current measuring circuit. 



Residual* Voltage Folloiving an Interrupted Arc 



At the interruption of the first arc the voltage at the contact is v, 

 the arc voltage, and the voltage at the capacitor C, Figure lb, is vi 

 which is usually negative. If the local contact circuit is non-dissipative, 

 the residual voltage is vi = 2v — Vai . For a dissipative circuit with a 

 resistance r corresponding to the frequencies involved: 



h = v - (Vai - v)e-''"''-'"'' (2)t 



for an oscillating circuit, as is usually the case, where z = {l/Cf^. 

 The capacitor C at v\ will then recharge the local contact capacity c, 

 c « C, through the inductance I. If the voltage attained at the contacts 

 is sufficient and the conditions in the gap and at the contact surface are 

 favorable, a reversed arc may be re-initiated, as previously discussed. 

 This process may repeat several times and the residual voltage y„ will 

 change sign and decrease progressively. At the end of n arcs, it can be 

 shown that the residual voltage Vn is given by: 



* The term "recovery has also been used in the literature, 

 t Equation 2 and 3 are valid only for small values of r/z. These are ai)pro.\ima- 

 tions of the more general expression given by Germer." 



