This is the maximum voltage the switch can reliably open without drawing an arc.

In the first few microseconds of the switch opening an arc will draw. If the voltage is within the rating of the switch, as the gap increases the arc will extinguish and when the switch is fully open the voltage will be isolated. If the voltage is beyond the rating of the switch, the arc will continue, even with the switch fully open and breakdown will occur. Also, it should be noted that arcs can introduce noise into control / low voltage circuitry.

The voltage that can be reliably and repeatedly switched (as well as the standoff voltage) is determined by the size of the gap between the contacts, which in turn is determined by the sensitivity of the switch and depth of the vacuum in which it is sealed.

For a non-vacuum switch, the gas atoms and molecules will act as a medium for the voltage to continue to arc. The highest switching voltage rating of a non-vacuum-sealed switch tends to be less than 500V.

When the switch is in a vacuum, it is the vacuum that helps to extinguish the arc. However, the depth of the vacuum is important. As the switch opens, and the initial arc begins, any gas in the tube will be electrostatically attracted to the arc and help it to continue. Note: some gas might be a result of outgassing from the contact plating.

For a high-voltage reed relay, the switching voltage will be significantly lower than the standoff voltage (rating). Also, it is important whether the voltage being switched is AC or DC.

Closing a High Voltage

As mentioned, the above relates to the reed relay’s switch opening a high voltage. When closing a high voltage, things are somewhat different.

As the contact surfaces approach each other, there will be a point when the voltage will arc between them, but this will be for only a few microseconds before the switch blades physically touch. When they do, the energy from the power being switched will melt some of the plating material on the surface of the blades. The quantity that melts will depend on the voltage and current switched.

Also, it is important to consider switch bounce, something all dry high voltage switches do when closing, i.e. the contacts will open a few times before settling. Although the duration of these bounces is extremely short, each bounce will produce an arc. As noted above, arcs can introduce noise in control / low voltage circuitry.

On a general note, the voltage a high-voltage reed relay can close is significantly higher than the voltage it can open, even factoring in the brief openings that take place during switch bounce.

For this reason, if the application only requires the switch to close a high voltage but to never open it, a working voltage higher than the rating can be used. This is commonly employed where capacitance discharge is required, always ensuring the peak current will not exceed the overall switching power rating of the device.

If you would like advice on this or any of the other issues discussed above, please contact us.