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Power Switching Guidelines

Misapplication of high voltage relays occurs most commonly in power switching applications. Since most Kilovac relays are used only to isolate the load, do not assume that the rated carry current is the power switching rating. Here are a few tips to keep in mind when selecting a relay for power switching:

High Voltage Applications (typically above 1,800 V)

Make only applications - select a SF-6 gas filled relay whenever possible. These relays are designed specifically for these applications. See page 66 for typical "make" current capabilities, or contact the factory for assistance in selecting the proper relay. Never use these relays to "break" the load without approval from the factory.

Make & Break applications - select a vacuum relay that has contacts made of a material with a high melting point, such as tungsten or molybdenum. These relays are indicated as "make & break load switching" next to the part number on the enclosed data sheets. Some Kilovac vacuum relays have copper contacts for high current carry applications, and are not suited for power switching. These relays are indicated as "no load switching" next to the part number on the enclosed data sheets.

In a vacuum relay, part of the contact material vaporizes during power switching and depos-its itself on the inside walls of the relay. When this occurs, the dielectric stand off voltage (leakage current) will decrease with the number of power switching cycles, possibly making it unsuitable for your application. The power switching rating of a vacuum relay is therefore dependent on the power to be switched, the number of cycles, the dielectric stand off volt-age, and the maximum leakage current allowed in your circuit. The relay data sheet and page 66 gives power switching ratings. If you do not find what you need, contact the factory with your specific application requirements.

Medium Voltage Applications (typically < 1,800 V)

Kilovac offers both vacuum & hydrogen filled relays rated for power switching applications at 1,800 Vdc and below. These relays are listed as the first products in this Product Guide. The vacuum relays typically have a longer life cycle rating than the hydrogen filled relays, but do not carry or interrupt as much current. The technology is not as important in selecting the best relay for your application as an understanding of the application. The biggest "tip" to remember is what the relay will be required to switch during "abnormal" switching condi-tions. In many applications, the relay may have to interrupt the entire current capacity of the system before a circuit breaker or fuse has time to perform it's function. If the wrong relay is selected, the relay could be vaporized during opening resulting in electrical shorts within the system.

Whenever a relay is power switched, an arc is generated. The duration of this arc and the level of current and voltage are critical factors in determining relay life and reliability. Whenever a relay is required to switch a load, there are several precautions that should be taken to ensure a satisfactory result. These are:

1. Characterize the circuit elements; are they primarily resistive, inductive, or capacitive? (which, for the purpose of this discussion, include motor and lamp loads).

Resistive Loads

In resistive loads (See Figure 3), the duration of the arc is primarily determined by the speed at which the contacts separate. The interruption of an AC load is easier on the contacts than a DC load since the AC interrupts itself each half cycle as the current goes through zero. Resistive loads are considered the standard against which all other loads are measured. All relay load switch ratings, unless otherwise specified, assume a resistive load.

Figure 3. Typical Resistive Load Profile

Inductive Loads

Inductive loads (See Figure 4) in high voltage circuits can be extremely destructive. The release of stored energy when the load is interrupted serves to maintain the current and cause voltage spikes which can damage many components in the circuit, including the relay. Inductive loads in AC circuits are less of a problem than in DC circuits. However, in both cases, we recommend that the inductive load be suppressed with the appropriate protective device at its source. If the inductive load is properly clamped, it becomes, in effect, a resistive load.

Figure 4. Typical Inductive Load Profile

Capacitive, Lamp, and Other High In-Rush Loads

When switching on a lamp, charging, or discharging a capacitor, the inrush current may be 15 to 100 times the steady state current (See Figures 5, 6, and 7). The primary concern with high inrush loads is that contact bounce and associated arcing can cause the relay contacts to weld when making the load. For this reason, you should select power switching type relays for lamp or capacitor charge or discharge applications. Normally the maximum interrupt rating should be used to determine the proper relay for lamp applications. SF-6 gas-filled relays are usually the best choice for capacitor discharge applications.

Figure 5. Typical Capacitive Load Profile

Figure 6. Typical Lamp Load Profile

Figure 7. Typical Motor Load Profile

2. Are voltage spikes present in the circuit? Small amounts of inductance can generate extremely high voltage spikes which can damage many components in the circuit. For this reason, steps should be taken to clamp any inductance at its source.

3. Select a ground isolated relay for high voltage load switching, whenever possible. Relays which do not have an internal ground plane are known as ground isolated and when a ground-isolated relay can be used for load switching, the potential for ground faults is nearly eliminated. If a ground-isolated relay is not available for your application, try to locate the relay on the ground side of the load. If this is done, the load will limit the fault currents in the event of an internal arc-over to ground in the relay. (See Figure 8).

Non-Ground Isolated Relay Connections in High Voltage Power Switching Circuits

Figure 8. In A, if arcing transfers to ground, the load limits the current. In B, if arcing transfers to ground, current bypasses the relay. Note: Power control applications often must utilize B.