Priniciples of Electromechanical Relay Operation


3.15 Effects of Load Currents

Physical conditions at the contact interface. The chemical and catalytic actions of these various contact materials are accelerated because, for an instant of time at each switching operation, the metal at the actual point of contact may be exceedingly hot, molten, and often vaporized. Arcing temperatures at the instant of circuit closure or interruption will be on the order of 3,000 to 6,000° K. In this temperature range, all metals melt and probably boil, and all common chemical compounds tend to decompose.

Softening and melting voltage. For each metal in the form of a short electrical conductor-such as a point of contact at a contact interface-there is a current density above which heat will be evolved faster than it can be conducted away until temperature equilibrium is reached at the melting point of the contact metal. The voltage drop across the contact interface is the electrical characteristic necessary for such a current density to maintain this metal softening temperature. This is commonly referred to as the "softening voltage". As the contact interface softens, the area in contact increases and the interface resistance decreases. This contact resistance decrease is referred to as the "softening drop". Increasing the contact current well beyond that necessary to produce softening will raise the contact interface temperature to such as extent that melting occurs. The melting temperature is the upper limit for temperature in solid contacts and the corresponding maximum contact voltage drop is called the "melting voltage". Any further increase in current results in additional contact melting causing the contacts to sink together and resolidify as they cool, increasing the contact area and decreasing the contact resistance.
Typical softening and melting voltages at 23° C are as follows:

Material Softening Voltage Melting Voltage
Silver 0.09 0.37
Gold 0.08 0.43
Palladium - 0.57

Contact arcing. Conditions for voltage breakdown of a contact gap depend on various geometric and environment factors. For a static gap, initiation of an arc discharge can be related to the type of gas (air, nitrogen, hydrogen, argon, helium), gas pressure, and gap length. Secondary variables that influence this relationship are contact shape, material and surface texture, gaseous contaminants such as water content, and the degree to which the gas in the gap is being subjected to ionization. Discharge is initiated when ions that may normally exist in the voltage charged gap are accelerated sufficiently to generate more ions and have a number of "mean free path lengths" in which to accomplish this. The minimum voltage for this kind of breakdown is approximately 320 volts at any pressure for air, and at normal atmospheric pressure is the gap length is approximately 0.0003 in. (0.0076 mm) For both shorter an longer gaps, the breakdown voltage is greater unless the gap becomes exceedingly small (in the Angstrom range). For materially shorter gaps, the voltage breakdown follows the rules for vacuum discharge or direct molecular ionization.

Arcs will form in a voltage stress of approximately one half million volts per centimeter when the gap is small compared to the mean free path length of the gas molecules (or ions) in the gap. Therefore, when the gap is sufficiently small, much lower voltages are capable of initiating breakdown. Typical values are 100 volts for a gap length of 10,000 angstroms(0.001mm), 10 volts for 1,000 angstroms, or 1 volt for 100 amgstroms. At these extremely small gaps, the contact damage caused by the breakdown is accentuated by the discharge of the air capacitor formed by the very closely spaced contacts(and the nearby parts of the circuit). This discharge takes place as the closing contacts are about to meet. This arc is normally invisible and not to be confused with visible arcing (frequently seen on the contact closure), which is actually caused by contact reopening during contact bounce.

Metal transfer. Since the high current density in contact A-spots at the first instant of closure can cause metal melting and probably boiling, even at fractional ampere loads, there tends to be some metal transfer at contact closure. These phenomena become matters of practical concern when contact life must extend over many millions of operations, especially when significant circuit or line capacitance must be discharged in addition to the contact gap capacitance.

When contact separation takes place, the last instant metallic conduction is subject to the conditions of small contact area and very light force. This results in molten or boiling metal (sometimes called a bridge) that becomes a copious source of electron emission and of the ionized, positively charged vapor of the contact metal immediately thereafter. These effects meet the requirements of generating the plasma (electrified particles) for an arc discharge when the circuit power can supply the energy needed at a rate sufficient to maintain the plasma. The amount of energy dissipated in the arc is the principal factor in governing the amount of metal transfer during arcing.

Inrush transients and contact closure. Contact damage is frequently caused by current surges at the instant of closure when the contact forces are light. Contact sliding and bouncing probably take place, and the load current is often many times greater than the steady state value. A microscopic weld or "bridge" will frequently form at the point of contact closure. In dc circuits this bridge usually ruptures asymmetrically at the next contact opening, resulting in metal transfer. In ac circuits, a net loss of contact material usually occurs. The metal vapor that condenses in the vicinity of the actual contact area is normally black and is frequently mistaken for carbon.
Loads that produce transients at contact closure are as follows:

1) Tungsten lamps whose cold resistance is 7-10% of their hot resistance.
2) Transformers and ballasts that may cause transients 5-20 times their normal currents when switched in their inputs.
3) AC solenoids and some kinds of motors.
4) Capacitors places across contacts or loads with inadequate series connected, current limiting resistance.

To meet these circuit conditions, the relay designer may elect to employ:
1) Heavy duty contacts and high contact force to minimize contact bounce on closure.
2) Contact materials with the highest possible electrical and thermal conduction, usually silver or silver alloys.
3) Contact material additives to inhibit welding, such as cadmium or cadmium oxide.
4) Relay actuator mechanisms to enhance weld-breaking ability of contacts.
The circuit designer can add small values or series resistance to the circuit to reduce current surges.

Contact activation. In atmospheres containing certain hydrocarbons, contact arcing can cause carbon deposits on contact surfaces increasing the duration of subsequent arcs with a resultant rapid increase in contact erosion. This phenomenon is referred to as "contact activation". Gold, silver, platinum, palladium and copper are subject to activation in varying degrees; however, it does not occur on surfaces of tungsten, nickel, or molybdenum. Activation lowers the electrical field at which an arc will start and lowers the minimum arc current thus increasing the duration of the arc causing a significant increase in the erosion rate. Arcing on activated contacts produces a relatively uniform erosion over the contact surface rather than the pit and crater normally observed in non-activated arcing contacts along with a black powder (carbon particles) over the contact surface and adjacent parts. Active arcs can result in a contact life of 1/4 to 1/8 that of non-active arcing.

Sources of the hydrocarbon vapors that cause contact activation are certain contact cleaners, paint and exhaust fumes, lotions and certain aerosol sprays, certain plasticizers, phenolic plastics, some magnet wire insulations, organic lubricants, etc. Erosion due to activation is a greater problem in sealed relays with internal hydrocarbon sources. The presence of air (as is the case with open or dust covered relays) inhibits contact activation by reducing the number of hydrocarbon molecules absorbed by the contact surfaces. Low duty cycles (long "off" times) allowing the absorption and formation of hydrocarbon based films, and intermediate current switching (0.1 to 0.5 amperes) providing enough energy to turn the film into carbon but not enough to burn off the carbon are the conditions most favorable to contact activation.