Contact Performance in Relays

5.7 Contact Protection

Dc circuits: When the circuit to an inductive load is opened, much of the energy stored in the load must be dissipated as arcing at the contacts unless some alternative means of energy absorption is provided. Some of the load energy is dissipated as heat in the load resistance, in eddy current losses in its magnetic circuit, and in the distributed capacitance of the coil winding. For dc circuits, a number of simple solutions are available to lessen or inhibit contact arcing:


1. A semiconductor diode may be connected across the inductive load (see Fig. 5.1) so that it blocks the applied voltage at contact closure but allows the stored energy in the load to recirculate through it at contact opening. The time for the load current to decay to 37 percent of its steady state value equals L/R. The blocking diode will prevent any inductive transients from appearing across the contact during the switching operation.

For loads below the minimum arcing current, the time required for load de-energization can be materially reduced by adding a Zener diode, resistor, or a varistor in series with the blocking diode, thus increasing R. The rate of energy dissipation after the contacts are opened is thereby increased as the load current circulates back through this additional voltage drop. The instantaneous voltage plus the source voltage should not exceed 320 volts.
2. The load or the contacts may be shunted with a resistor-capacitor combination (see Fig. 5.2). For load currents in the stable arc range, the resistor, R_c, can be selected to match the load resistance, or it may be 1/2 or 1 ohm per volt of the power source. For smaller load currents than can cause a stable arc, the resistor can be higher in value. A reasonable value is one resulting in a voltage transient of less than 300 volts for the sum of the source voltage and the instantaneous voltage generated by the load current in the resistor, calculated thus:


The resistor is essential and must be large enough to limit the current transient from the capacitor discharge (or charge) on the contact closure to prevent contact welding. The capacitor should be large enough to accept the stored energy of the load without permitting an electric breakdown of the contact gap, normally at greater than 320 volts. An oscillograph is the best way to determine when these transients are adequately suppressed. In Fig. 5.2 the capacitor can be connected either at C or C'. Connections at C is preferred since it protects against source and line, as well as load, inductance.
Fig. 5.2 Use of capacitor-resistor combination to suppress current surge from inductive load. Capacitor may be at either C or C'.


3. A varistor (voltage-sensitive resistor) or "Thyrite" may be used to shunt the load. If such a device carries 10 percent as much current as the load, the maximum switching transient will be about twice the source voltage. This method is also suitable for ac circuits.
4. For extremely inductive loads, for the longest possible life, or for load power and contact gap length above the minimums for a stable arc, the circuit of Fig. 5.3 may be used. In this circuit, the capacitor is charged through the diode but can discharge only through the resistor. This arrangement gives essentially zero contact voltage drop at the instant of contact opening. The capacitor value should be such that when the energy transfer from the load is complete, the peak voltage to which it charges will not cause a breakdown of the diode, the contact gap, or itself. Usually the peak voltage should not exceed 200 to 350. For dc inductive loads for which the conditions for a stable arc may be satisfied by the partially opened contacts, the circuit of Fig. 5.3 permits the contact gap to be established without drawing an arc, and the stored energy transfer is accomplished more quickly than would have been the case if the contacts had been allowed to arc. The reason for this is that the integrated inverse voltage to which the capacitor charges is greater than voltage drop in an arc, were arcing permitted.
5. Where the inductive load of a relay coil presents a hazard to transistor drive circuits, coils with dual windings wound together on the coil bobbin (called bifilar coils) may be used with one winding shorted. This arrangement provides a pronounced damping effect on the rate of change of magnetic flux in the iron and hence provides a significant moderating effect on the induced voltage.
Fig. 5.3 Use of capacitor-resistor diode combination for arc suppression with a highly inductive load.


Switching ac-inductive loads
Such loads are most commonly treated in a different manner from dc loads because of the fact that a stable arc will normally be terminated when the current passes through zero and reverses at the end of the first half cycle following contacts separation. Fairly common practice is to use arc-resistant contact material, preferably in a relay in which the contacts separate slowly, and let arcing be terminated by the reversal of the current rather than by the continuing separation of the contacts. When load currents get too heavy for safe interruption by small relays (greater than 10 to 25 A), the current reversal effect can be supplemented by magnetic or air blowout, multiple break contacts, arc gap cooling labyrinths, or by evacuating the contact chamber.
Under moderate arcing conditions, contact life may be greatly increased by shunting the load with a resistor-capacitor-diode combination whose time constant is equal to that of the load:


This network makes the load characteristics essentially resistive. When the maximum possible contact life is required, either of the capacitor-diode combinations shown in Fig. 5.4 may be justified. For 115-volt ac service, the diode should have a peak inverse voltage rating of 400, the capacitor should have a dc working voltage of 200 Vdc, and there should be a 100K-ohm resistor, which will dissipate nearly 1 watt. The capacitor discharge time after a switch closure may be as long as second.
The transient voltage developed when the contacts open the load circuit may exceed the dielectric withstanding voltage between contacts and another part of the relay. In some circuits, these voltages may be high enough to cause breakdown of another circuit component. These transient often cause interference in adjacent or associated circuits. Usually a resistor-capacitor network, applied in accordance with the rules outlined in Sec. 5, will reduce the voltage to a level that suitably protects the contacts and avoids dielectric breakdown. However, it is sometimes necessary to use diodes to eliminate radio interference from arcing. For these latter cases, no general rules can be formulated because the interference is closely associated with the particular circuits.
In general, careful attention to contact protection can increase life expectancy as much as three orders of magnitude. System reliability may be greatly improved by elimination of high voltage transients, and the speed of response and its consistency is often substantially improved.
Fig. 5.4 Resistor-capacitor-diode combinations for suppressing contact arc on ac inductive load.