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Measurement of arc voltage across opening switch contacts

Citation

Hamilton, James Hugh (1928) Measurement of arc voltage across opening switch contacts. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-02182005-095331

Abstract

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The circuit breaker or switch is a piece of apparatus of prime importance in electrical engineering practice. Its value is evident at once on reflection that in the last analysis the control of electric power is primarily dependent on this part of the electrical system. The modern circuit breaker has taken the form of two or more separable contact immersed in an insulating fluid, usually oil. However, in the past few years the separation of contacts and the interruption of currents in vacuum has been successfully demonstrated and possesses distinct advantages.

In connection with switching problems a difference between opening alternating current circuits and direct current circuits is encountered in that in the former case the current passes thru a zero value twice each cycle. Since the alternating case is the more important in modern practice (because alternating currents and voltages are employed on all large power and distribution systems) this case only will be discussed in this work.

It would seem that the ideal method for electrical circuit interruption would be to separate the switch contacts very rapidly at the instant the current passes thru a zero value for at this time the energy stored in the magnetic fields linked with the circuit would be nil. If at this instant the voltage were also zero and if the contacts were opened with sufficient speed, an arcless switching operation would result. This is true only for the unity power factor case, however, and furthermore the contacts would have to be separated a reasonable distance in a time interval of the order of 0.0001 second. The sensitive timing necessary for the realization of this ideal in a practical switch is as yet out of the question because of mechanical difficulties.

In general the most severe duty imposed on a circuit breaker is that when it is called upon to interrupt short circuit currents. It is well known that one of the characteristics of these currents is their low lagging power factor. The condition is encountered that when the current is passing a zero value the voltage is near its maximum. If the current were interrupted at its zero value a high voltage would be impressed across the switch immediately and tend to strike an arc. If the contacts were separated at a time when the current value was not zero the voltage of induction would act to maintain the flow of current and an arc would necessarily result. Two examples will illustrate these points: Assume an inductive circuit draws a 50-cycle current of 100 amperes at 0.3 power factor lag from a 10,000 volt source. If the contacts are opened at the time of zero current the voltage immediately impressed across the switch would be [...] volts. Assume the contacts to be separated 0.001 inch. The stress on the material interposed would be at least [...] volts per inch. It is evident that such stress in the case of an oil circuit breaker would cause a break down the oil barrier and a formation of an arc. On the other hand if the contacts were opened at zero voltage a condition represented by the following calculation would result. From the above date – Inductance = 0.303 henry. Assuming the electric strength of 0.001 inch of oil to be 2500 volts, the permissible decay of the current would be 8200 amperes per second. For a condition of arcless switching the current should drop instantly to zero and this is of course inconsistent with the permissible decay of 8200 amperes per second. Many oscillograms of switching operation taken on various types of switches confirm the above and furthermore establish the fact that the arc once initiated continues until the current passes a zero value on some succeeding half cycle. It may be concluded that switching alternating currents is normally accompanied by an arc across the switch contacts with the possible exception of interrupting a unity power factor circuit when the current passes a zero value.

In view of the fact that this phenomenon does enter into the switching problem it was considered pertinent to investigate the characteristics and especially to measure the voltages identified with it. The arc voltage is of considerable theoretical importance in the vacuum switch investigation and has never been accurately determined. It was also of interest to compare the arc characteristics of the oil switch with those of the vacuum switch and with those reported for oil switches by other investigators. This work has been undertaken with the purpose in mind of developing a method for accurate arc voltage measurement on a high voltage breaker and applying it to a vacuum switch and an oil switch.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Electrical Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Unknown, Unknown
Thesis Committee:
  • Unknown, Unknown
Defense Date:1 January 1928
Record Number:CaltechETD:etd-02182005-095331
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-02182005-095331
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:671
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:18 Feb 2005
Last Modified:26 Dec 2012 02:31

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