Power Switching

The ability of a vacuum relay to switch both resistive and inductive loads greatly simplifies the problems of the systems engineer. In power switching applications non-isolated relays (which includes all relays not identified as ground isolated) must be used with caution when the relay mounting is at ground potential and the circuit to be switched at high potential. Fault conditions may cause internal arc over to the grounded housing.

Ground isolated relays can be used within their voltage ratings without concern for ground faults because the switching part of the relay is completely isolated from ground.

Vacuum relays are made in models designed to be switched "hot" or "cold". "Hot" switching often entails contact arcing upon opening and during contact bounce. AC and DC circuits have special considerations when switched "hot" and are discussed under their specific headings.

For "cold" switching, where the circuit is switched with no load across the relay terminals, a relay performs either as an insulator or a conductor.

In the make mode the contacts conduct the full current of the load, and contact current handling capacity is limited by heating caused by contact resistance. Special low resistance copper alloys are used for most cold switching relays to assure high current handling capabilities.

In the break mode, the relay must perform as a high voltage insulator. Stand-off voltages are highest at DC and low AC frequencies, and decline at higher frequencies due to RF heating of the insulator. Ceramic insulators provide the best withstand capabilities for high RF applications.

Circuit loads can generally be considered as capacitive, inductive, or reactive, even though they may be comprised of both active (tubes & solid state devices) and passive elements (capacitors, resistors, inductors, etc.). Circuits with significant capacitive or inductive elements are more difficult to switch due to the stored energy. Switching these different kinds of loads has a specific effect on relay voltage as shown in the general diagram (Fig. 2). Circuits made up primarily of resistive components have little effect upon voltage across HV terminals. With inductive elements present, a high momentary voltage transient occurs when the circuit is broken, which decays rapidly to open line voltage. When circuits with large capacitive elements break, a negative bias voltage appears equal to the stored energy of the capacitor.

This stored energy can cause a momentary high current surge upon make. This position of a relay in a circuit can well determine its maximum capabilities. For example, a relay with a ground plane inside the vacuum envelope will break much more power when one contact is at ground than when both contacts are between the power supply and load (Fig. 3). When a "hot" circuit is switched, an arc usually occurs. This can transfer to ground when a relay with internal grounding is placed between the load and the power supply. This ground fault or breakdown results from ionized gas and vaporized metal from the contacts that bypass the load and conducts between the high voltage lead and the ground plane of the actuator. The only limit to this current surge is the inherent current limitation of the power supply itself. When one contact is at ground potential, however, the load limits the surge current.

Relays with internal grounds that are to be "hot" switched should be placed on the ground side of the load to prevent breakdown damage. To eliminate this type of breakdown problem, a number of relays are available with ground isolation from the vacuum enclosures. This includes the RF 1, RF 3, RF 4, RF 10, as well as all RF 40, 50 and 60 series relays. Vacuum relays operated "cold" may be installed in any circuit location, as the relay does not interrupt power, but acts either as a low loss conductor or as a high voltage insulator.


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