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.