DC and AC Switching

DC and AC Switching DC Switching Arcing occurs during switching and contact bounce in a "hot" circuit. The differences between vacuum and air relay performance can be seen in Figure 4. In an air or hermetically sealed relay, arc voltage is high and lasts for a considerable period. In a vacuum relay, the contact arc maintains a constant 18-23 volts regardless of the voltage or current being switched. This arc has a random sawtooth waveform of 1-10 MHz. Arc time is also appreciably less in a vacuum relay because of the rapid arc quenching nature of a vacuum. Vacuum relays have small contact gaps because of the high dielectric strength, and this allows fast operation while minimizing contact bounce and arcing. The combination of constant arc voltage acting as a current limiter and short arc time, means vacuum relays generally wear less than other types and give stable performance over the life of the relay. The refractory metals used for contacts in relays intended for "hot" DC switching are selected to withstand arcing by their high melting temperatures and hardness. Vacuum relays have higher switching capabilities than most relays, but above 1 kV, they are generally limited to a maximum of 15 Amps. When higher current levels are to be switched, suppression or bucking circuits (Fig. 8) should be used to develop artificial current zeroes. This circuit keeps contact differential voltage at a minimum until the contacts fully open.


AC Switching

AC circuits are inherently much easier to switch at high current levels with vacuum relays. Current zeroes occur twice per AC cycle, and the high recovery voltage of the vacuum dielectric assures extinction at the first current zero, reducing arcing and subsequent contact erosion. (See Fig. 9) A relay that can "hot" switch 5 Amps DC can switch higher AC currents.



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