at a time of zero current. With no current flowing, the new cathode has no melting copper to provide an ion source and the current will not flow during the following half-cycle. By the time the original electrode becomes a cathode again, (1/2 cycle), the molten puddle has frozen and the current remains off. With this characteristic, the contact always interrupts at a current zero, so that no voltage spikes are generated. Since interruption at current zero eliminates any “inductive kick”, the major shortcoming of the vacuum contact was solved.

All of the advancements produced a load interrupter contact that is almost totally transient free, with a long high vacuum life, and far superior to any other type of circuit interrupting contact.

Oscillograms of old style oil or gas interrupters show saw tooth waveforms, which take the place of the normal sine wave currents and voltages during the act of actual load interruption. An analysis of the source of the saw toothed voltage and current wave shapes reveals that during the time of contact parting, and until the arc is extinguished, the gap between the contacts is filled with a turbulent mixture of ionized and un-ionized gas or fluid. When the contacts first part, the contacts “see” a purely ionized path for the electrons. When the interruption is complete, the contacts “see” a totally unionized path for the electrons. In between these two extremes, the contacts “see” a turbulent mixture that during one microsecond looks like a good conductor, and during the next microsecond looks like an insulator. Fluctuation between the two extremes occurs rapidly as the turbulence mixes the ionized and un-ionized materials. When these impedance surges occur in an active circuit, the

“inductive kick” phenomena results in spike shaped high peak voltage surges, as the inductance tries to maintain a normal current. These voltage “spikes” are commonly known as “Switching surges”. As switching surges travel down a distribution circuit, they will reflect with double magnitude from open switches or dead ends. When they meet other surges traveling in the line they will either subtract from or add to

between live parts is reduced creating a very compact switch. Since the contacts only need to travel a short distance, the mechanical life is much longer. It is not uncommon to get a rating of ten thousand full loadbreak operations with an oil-insulated mechanism.

Since all load interruption is done in a vacuum “bottle”, no by-products are produced in either gas or liquid insulated switches. With proper mechanism and tank design, all maintenance requirements are eliminated.

Today, vacuum load interrupter contacts have an extremely long life. They can interrupt 600 Amps 50,000 times with only 1/8” contact erosion. In applications requiring this long life, metal bellows are available which have an equally long life. In normal load switching however, a bellows having a life of 10,000 operations is used since it is appreciably more economical and requires less space. The amount of contact erosion is not an issue due to the initial 1/8" of overtravel built into the Trayer Operating Mechanism.

Presently, vacuum switches will interrupt many times their continuous load current rating and they are not critical with regard to load power factor. They can be applied for switching capacitors or can be used to switch highly inductive loads, or any type of load in between. With utilities moving toward remote switching, the number of switching operations will be more important and vacuum switching more desirable.

This makes vacuum switching the obvious choice for distribution load switching applications.

the surge magnitude. It is not unusual for switching surges to operate lightning arrestors.

In contrast, the Trayer vacuum load interrupter produces pure sine wave voltages and currents of normal magnitude during the complete process of load interruption.

Summary

A totally underground distribution system is shielded from lightning surges. If all switching is done with vacuum switches, the other major source of surges on the system is eliminated.

By immersing the vacuum switching mechanism in insulating liquid or SF6 the necessary clearance