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The Physics of Vacuum & Gas-filled Relays

Vacuum as a Dielectric

A vacuum is, in many ways, the ideal dielectric. Since most high voltage arcs are initiated by the ionization of the insulating medium, a hard vacuum, which, by definition, is the absence of any such media, produces the greatest possible isolation between contact electrodes. It is possible to obtain dielectric strengths of 2000 volts per thousandth of an inch contact gap in a vacuum relay. A vacuum dielectric has the additional advantage of providing an inert atmo-sphere in which high voltage contacts can operate completely oxide-free. Thus, vacuum relays typically have contact resistance which is lower and more stable over life than other types of relays.

Voltage breakdown can occur even within an absolute vacuum but the contact materials themselves must become the source of ionized materials to support an arc. Therefore, high strength and high work function materials like refractory metals are commonly used for contacts in order to raise the electrostatic field strength necessary to cause a voltage break-down. In addition, refractory metals have high melting temperatures which reduce contact damage from arcs and result in longer life.

Comparison of Arcs and Vacuum
Figure 2. Comparison of Arcs and Vacuum

In load switching, an arc will always be created at the point when the contacts are close enough together to allow a voltage breakdown. Vacuum arcs are sustained at the relatively low voltage of 18 to 23 volts compared to arcs in air which are more erratic and range over a wider voltage (See Figure 2). Vacuum arcs tend to be more easily controlled and extin-guished than arcs in air. The high pressure region formed around a vacuum arc has a strong tendency to dissipate or “blow out” into the surrounding low pressure vacuum. This phe-nomena, along with the ability to use contact materials like pure tungsten and molybdenum which have very high melting points, means that vacuum relays and contactors typically experience much less contact erosion and have longer life than comparable air-break devices.

SF-6 Gas as a Dielectric

Some Kilovac relays contain a proprietary gas mixture consisting of, primarily, sulfur hexaflouride at several atmospheres of pressure. The pressurized sulfur hexaflouride has good insulating qualities and can come close to achieving the same standoff voltage as a vacuum. These gas-filled relays are almost always recommended when an application in-volves changing or discharging of a capacitor especially when the voltage is >1 kV. When the relay is switched, ionization of the gas causes electrical continuity to occur before me-chanical continuity is achieved. Thus, contact bounce may occur without electrical interrup-tion. Since the gas ionizes fairly easily once an arc is formed, contact erosion is reduced, which contributes to the very long life that these relays will exhibit in capacitive make & break applications, and reduces electrical noise during switching. However, this tendency to ionize makes gas-filled relays unsuitable for applications which require interruption of a load.

SF-6 gas-filled relays offer a highly reliable alternative to vacuum and other gasses when circuit conditions permit their use. There are two significant shortcomings to SF-6 gas-filled relays:
  • Due to a film that forms on the relay contacts, SF-6 gas-filled relays have a higher and less stable contact resistance at low voltages than vacuum relays. Contact resistance is typically in the range of .5 to 1.5 ohms when measured at 28Vdc on new relays. However, even in applications up to 100 volts, when the current is very low, higher contact resistance may occur. When low contact resistance is important, and the voltage and/or current is low, a vacuum or other type of gas filled relays should be used instead of a SF-6 gas-filled relay.
  • Due to the ease with which sulfur hexaflouride ionizes, gas-filled relays cannot normally be used to interrupt loads.
Hydrogen as a dielectric

Some other Kilovac relays use a proprietary gas mixture consisting of, primarily hydrogen at various atmospheres of pressure. These mixtures do not have the same high dielectric and low leakage current as vacuum or SF-6, so are not normally used for high voltage applica-tions above 3 kV, but are ideal for dc "make & break" load switching applications. The gas mixture, combined with the use of external magnets to control the direction of the arc, cools and extinguishes the arc in a very predictable manner. Because there is no oxygen in the mixture, more conductive contact materials such as copper can be used that provide the lowest possible contact resistance. This results in very small package designs. These gas mixtures & magnets combined with Kilovac's patented armature & contact design provide high current interrupt capabilities up to 3,500 A at 320 Vdc with switching capabilities as high as 1800 Vdc. They also provide the capability to handle highly inductive dc load switching.