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Application Note

Relay Contact Life

Relay contacts are available in a variety of metals and alloys, sizes andstyles. There is no such thing as a universal contact. The relay usershould select contact m aterials, ratings, and styles to meet , as preciselyas possible, the requirements of a particular application. Failure to do socan result in contact problems and even early contact failure.

For example, some contact m aterials require an arc to keep t hem free ofsulfidation, oxidation, and contaminates. Such materials on contactsused in a dry or low-level circuit can result in t he contacts failing electricallyto close the circuit, even though they m ake physically. The contacts maylook clean, but this is deceiving. In reality, there is a very thin film ofinsulating sulfidation, oxidation or contaminates on the surface of thecontacts. This film m ust be removed for circuit continuity to beestablished, and arcing can accomplish t his. (For dry and low -level circuits,bifurcated contacts should be used.)

Applications ConsiderationsIn some applications, the contacts may be subjected t o punishing currentsurges w hich can drastically reduce their life. Consider an incandescentlamp. A 40 watt , 120V AC lamp has a current rating of .33 ampere. Theresistance of t he filament w hen cold, however, is so low that initial inrushcurrent may be as much as 6 amps! To attempt to sw itch the 40 wattlamp w ith, say 2 amp contacts w ill result in early contact failure.

The same situation exists in m otor and transformer applications, and inapplications w here significant dist ributed line capacitance exists. Duringstart-up, a motor can pull 600% or more of its running current. Thus, a 3amp m otor may actually pull 18 amps or m ore during start-up. A contactrated at least 20 amps should be used. Addit ionally, when disconnect ed,a motor acts as a voltage generator as it slow s to a stop. Depending onthe motor, it can feed back into the circuit voltages well in excess ofrated line voltage. These voltages appearing across the separatingcontacts can cause a destructive arc to exist between the contacts, w hichcan lead to early failure of the contacts. Because of t his, it is desirable tosuppress t he arc. (Techniques for arc suppression are discussed later inthis application note.)

Transformers can present an unusual trap for an unsuspect ing relay user.When power is removed from a transformer, its core may containremanent magnetism . If power is reapplied when voltage is of the samepolarity as that of the remanent magnetism, the core may go intosaturation during the f irst half-cycle of reapplied power. As a result,inductance w ill be m inimal and an inrush current of perhaps 1,000%may exist for a few cycles until the core comes out of saturation. Worse,if reapplied pow er occurs at or near zero voltage and the increasing voltageaids remanent magnetism, the core and the air gap may saturate. Aninrush of perhaps 4,000% or more may result! Also, as w ith motorloads, w hen power is removed from a transformer, the transformer willdevelop a counter voltage w hich can cause a destructive arc to existbetw een separating contacts.

Distributed line capacitance presents particular problems for relays andtheir contacts. This occurs w hen a relay is located a considerable distancefrom the load to be swit ched. The instant the contacts close, distributedline capacitance charges before load current f low s. This capacitance canappear as an initial short-circuit to the contacts, and can pull a currentw ell in excess of load current. Prior to selecting a relay to sw itch a circuitw hen distributed line capacitance may be significant, instantaneous inrushcurrent should be m easured, and contacts selected accordingly.

Contact Materials

Fine SilverFine silver has the highest elect rical and thermal properties of all metals.It is the best general purpose material available. How ever, it is affectedby sulfidation. The rate of sulfidation indoors in a metropolitan area isapproximately 70 mic rograms per square centim eter per day. Thissulfidation forms a film on the surface of t he silver which increases contactinterface resistance.

Because silver and silver alloys sulfidate, cont act pressures must be greaenough to break through this film . (Controlled arcing w ill also be helpfuin that it burns off t he sulfidation, and contact overtravel wipes away theresidue.) While such pressures have no appreciable effect on silvercadmium contacts, they do result in increased material w ear of fine silvecontacts. Also, an interface voltage of several tenths of a volt can resulw ith fine silver contacts because of the sulfide film . This film has beenknown t o capture and imbed airborne dirt. Breaking through this filmgenerates elect rical noise. Because of this, fine silver contacts are noused for low-level sw itching, such as audio circuits. Rather, fine silveand silver alloy contacts are for use in circuits of 12 volts, 0.4 ampere, omore.

Gold-Flashed SilverFor relays w hich must sit idle for long periods of t ime before initiaoperation, sulfidation of silver contacts can result in an impregnablecontact interface resistance. Instead of specifying silver contacts fosuch applications, gold-flashed silver contacts should be specif ied. Goldflashing on each contact result s in minimal sulfidation, and provides goodelectrical make upon contact. Because gold has a low boiling tem peraturethe f lashing w ill burn off after just a few sw itch cycles if arc voltage andcurrent is exceeded. The silver underlayment is then exposed, and maydevelop a sulfide film . Unless this situation can be tolerated, gold-flashedcontacts should not be subjected to arcing.

Gold OverlayA common contact for use in dry-and low-level circuits is gold overlayThe overlay is of suff icient thickness that it should not wear through tothe base met al unless subjected to arcing conditions.

Silver NickelDepending on t he application, m aterial transfer may be quite prevalenw ith fine silver contacts. Typically, material tends to accumulate in t hecenter of one contact, while the loss of material on the other contacleaves a hole, or pit. This pitting may cause premature contact f ailureIn such an application, it may be desirable to use fine grain silver contactsThese contacts are alloyed w ith 0.15% nickel, w hich gives the contactsa fine grain structure. As a result, material transfer is evenly distributedacross the entire surface of t he contact and t he contacts last longer.

Silver Cadmium OxideSilver cadmium oxide contacts have long been used for sw itching loadsthat produce a high energy arc. Silver cadmium oxide contacts are lesselectrically conductive than fine silver contacts, but have superioresistance to material transfer and material loss due to arcing. They doexhibit greater interface resistance betw een mated contacts, and also aslightly greater contact assembly heat rise. The minimum arc voltageratingof silver cadmium oxide is 10 volts and, like fine silver contacts, the silvein this alloy will oxidize and sulfidate. Therefore, an arc is necessary tokeep these contacts clean.

Silver Tin Indium OxideSilver tin indium oxide contacts, although not readily available, exhibibetter resistance to arc erosion and welding t han silver cadmium oxidecontacts. They are even less electrically conductive, though, and areharder than silver cadmium oxide contacts. They have greater interfaceresistance between mating contacts and, therefore, a greater voltagedrop and heat rise. At t he present time, silver tin indium oxide is moreexpensive than silver cadmium oxide, and m any relay users limit its useto applications such as incandescent lamp loads and capacitors w herethere is a massive inrush current during contact bounce. For low andmedium power resistive and inductive loads, silver cadmium oxide isstill the most commonly used and is recommended by SiemensElectrom echanical Comp onents (SEC). For applications where it isbelieved that silver tin indium oxide should be used, contact SECapplications engineering.

Silver Copper NickelSilver copper nickel contacts are for use in high inrush DC applications

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Application Note

such as incandescent lamps and capacitive loads. These contacts exhibitgood resistance to welding.

Gold Silver Nickel AlloyGold silver nickel alloy contacts are for use in sw itching loads generallyof less than one ampere, and are characterized by less electrical noiseon make and break than fine silver contacts. Gold diffused silver contactsoffer characteristics similar to gold silver nickel alloy, but are lessexpensive.

Palladium

Palladium contacts do not sulfidate or oxidize, and so offer extremelylow electr ical noise levels. They have an elect rical life expectancy ofapproximately 10 times that of fine silver contacts. However, becauseof relatively poor conductivity properties, load currents are limited toabout 5 amperes.

Palladium contacts require .006 to .012 overtravel to insure good w ipingaction. Because of this, they are used primarily on telephone-type relaysthat is, relays on w hich the contact arms are parallel to the length of thecoil, and on w hich such overtravel is easy to obtain. Also, palladiumcontacts should be bifurcated t o help insure circuit continuity on contactclosure.

TungstenTungsten contacts are for use in high voltage applications , usually w herehighly repetitive sw itching is required. Tungsten has a melt ingtemperature of 3,380oC which gives it excellent arc-erosion resistance.

Tungsten m ay develop troublesome oxide films, especially w hen usedas the anode contact in some DC applications. Therefore, tungsten isoften used as the cathode contact, and a palladium alloy used as theanode contact. Such a combination also minimizes contact interfaceresistance and material transfer.

MercuryM ercury has a melting tem perature of -38.87oC. Thus, as used in relays,it is in a liquid state. M ercury will cling to the surface of any clean metal,and is used as the contacts in mercury-wetted reed relays. It has goodelectrical conductivit y and, being liquid, there is no material transfer build-up from contact to contact. Any such material transfer is negated by thefact that w hen the contacts open and the mercury returns to the pool inthe bottom of the relay, fresh mercury takes its place at the very nextsw itch operation. M ercury has a boiling temperature of 357oC. Becauseof this, mercury contacts cannot switch currents of more than a few

amperes.

Contact Life

The electrical life expectancy of general purpose and power relays isgenerally rated to be 100,000 operations minimum, while mechanicallife expectancy may be one million, 10, or even 100 million operations.

The reason electrical life is rated so low compared w ith m echanical lifeis because contact life is application dependent . The electr ical ratingapplies to contacts sw itching their rated loads. When a set of contactssw itches a load of less t han rated value, contact life m ay be significantlygreater. For examp le, 25 amp, 240V AC, 80% P.F. contacts may beexpected to swit ch such a 25 amp load in excess of 100,000 operations.If these contact s are used to sw itch, say, a 5 amp, 120V AC resistiveload, however, life may be in excess of a million operations.Rated electrical life also t akes into consideration arc destruction of t he

contacts. By use of appropriate arc suppression, contact life may belengthened. Table 1 lists some arc voltage and current values for severaldifferent met als. An arc will ignite if both minimum arc voltage and current

are exceeded. However, there w ill be no arc if load current at a givenminim um arc voltage is less than the m inimum arc current for that voltage.Likewise, there w ill be no arc if load voltage (or counter emf ) is less thanthe minimum arc voltage of the contact metal.As stated, an arc may be necessary in order to burn off the contacts anysulfidation, oxidation or contaminates . How ever, by its very nature, anarc is destructive. For maximum contact life, the arc should be suppressedas quickly as possible as soon as it ignites. Such arc suppression m aybe accomplished using techniques presented in t his application note.

At voltage and current values of less than those required to ignite an arc,

a spark may occur betw een separating contacts. This spark is a capacitivedischarge, and is weak compared w ith an arc. Even so, the spark maybe sufficient t o keep sulfidation, oxidation, and contaminates from buildingup on the contacts. (Note: arc suppression has little, if any, effect oncontact sparking.)

Contact life is terminated when the contacts stick or weld, or whenexcessive material is lost from one or both cont acts and a good electricalmake is not possible. These conditions are the result of cum ulativematerial transfer during successive sw itching operations, and of materialloss due to splattering.

Material Transfer and Material LossM aterial transfer occurs as a result of I 2R heat. As switch contacts beginto separate, the area of contact dim inishes. Load current flow ing throughthis increasingly constricted area generates heat w hich causes the contactmaterial to melt, then boil. The liquified metal tends to collect on the

cathode contact because that contact is cooler than the anode contact.M aterial transfer also occurs during arcing. How ever, under this condition,material transfer is from cathode to anodethe amount of t ransfer beingdependent on the severity and duration of the arc, and the type of cont actmaterial used. Because contact material migrates first one w ay, thenthe other, the ideal arc suppression technique w ould be to quench thearc just as t he anode recovers all of the m aterial lost t o the cathode justprior to the arc striking. However, this is impractical even to attempt .

M aterial loss is due primarily to splattering of the molten and boilingmetal as contacts bounce on make. Such loss can be significant overthe course of tens of thousands of operations, and the only practicalw ay to minim ize it is by arc suppression. Arc suppression quicklyquenches the arc, thereby holding contact tem peratures low er.

In DC applications, met al migration is predictable in that one contact isalways negative, and the other, positive. In AC applications whe re

sw itching is at random, either contact m ay be negative or positive whenarcing occurs. M igration w ill not be in the same direction each time theswitch opens, and material loss from either contact should not besignificantunless load conditions cause splattering.

Not all AC applications incorporate random sw itching, however. In someapplications, the relay is operated at a set rate or frequency. In suchinstances, the contacts break load current at the same approximate pointon the sine w ave. That is, the same contact is alw ays positive, and theother negative at the instant of contact separation. M aterial transferduring arcing w ill alw ays be in the same direction. In such applications,contact arc suppression may be necessary.

This is not to say that arc suppression is not needed on random-sw itchingAC applications. On the cont rary, arc suppression can help control theloss of contact material due to splattering of the m olten met al. That is,w hen the arc is suppressed, contact temperature is held to a minimum .

Controlled arcing of short duration can be beneficial in actually achievingthe rated life of the cont acts. As stated, this is because such arcingburns off of the contacts any deposits that m ight prevent electrical make.Such control is achieved by arc suppression. Unless arcing and/or contactovertravel cleans the contacts, films may develop on the contact surfaces,or foreign matter may collect. For this reason, it is best to apply generalpurpose and pow er relays only in applications w here load voltage (orcounter emf) and current is in excess of the arc voltage and currentratings of those contacts.

Contact Protection

Perhaps the most popular method of quenching an arc betweenseparating contacts is w ith an R-C netw ork placed directly across thecontacts. As the contacts just begin to separate and an arc ignites, load

Table 1 -Characteristics of Various Contact Materials

ElectricalConductivity Melt Arc Arc

Material %IACS Voltage Voltage Current

Cadmium 24 10 0.5Copper 100 0.43 13 0.43Gold 77 0.43 15 0.38Nickel 25 0.65 14 0.5Palladium 16 0.57 15 0.5Silver, f ine 105 0.37 12 0.4Tungsten 31 0.75 15 1.0

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Application Note

current feeding the arc will be shunted into the capacitor through theseries resistance, depriving the arc of some of its energy. As a result,arc duration w ill be shortened and material loss will be m inimized.

Contact Protection Diagram

Theoretically, the ideal arc suppression method would simply be acapacitor placed directly across the contacts. However, with no resistorin the circuit, when the contacts make, there is nothing to limitcapacitordischarge current. This nearly instantaneous discharge currentcan generate a brief, but severe arc that may cause welded contacts,depending on contact material and characteristics. Thus, the resistor isnecessary to limit capacitor discharge current . How ever, there is onedrawback. That is, the resistor t ends to isolate the capacitor from thevery contacts the capacitor is supposed to protect. Because of this, theamount of resistance should be kept as small as possible.

M any relay users are unfamiliar with the selection of a capacitor for arcquenching service. To begin wit h, AC differs from DC in that AC crosseszero 120 tim es per second for 60 Hert z service while DC, of course, iscont inuous current . In AC service, the capacitor need not be as large as

in DC service because the AC arc w ill extinguish at a zero crossoverpoint. In DC service, the capacitor must continue to shunt load currentsway from the contacts unt il the contacts separate far enough apart forthe arc to extinguish.

Capacitor Select ionAssume a DC application of 28 volts, 5 amperes. Further assume anR-C netw ork is needed that w ill result in contact voltage of perhaps 15volts 1 msec. after the contacts have separated. Since the value ofresistance should be as small as possible, a 2 ohm resistor might bechosen. At 2 ohms, peak capacitor discharge current w ill be 14 amperesat time zero. Depending on contact material and size, this 14 amperesmay be quite acceptable for such a short period of time.

Conact voltagethat is, arc voltageat any given instant of t ime is simplythe sum of the voltage drop of the resistor and the capacitor voltage.Select a capacitor voltage of, say, 10 volts. The remaining 18 volts must

appear across the 5.6 ohm load and the 2 ohm resist or. Thus,instantaneous capacitor current is:

I =E

=18V

= 2.4 ampere,R 7.6

and the voltage drop of the 2 ohm resistor is 4.8 volts. Arc voltage,therefore, one m icrosecond after contact separation is 4.8V + 10V =14.8V, or about 53% of supp ly voltage.

To determine the size of capacitance needed, the basic equation forcapacitor voltage may be used:

ec

= E(1 - -t/RC)

Rearranging the equation to solve for capacitance gives 1.1 mfd .

c =-t

= 1.1 x 10-6 farad

[In (1-ec)]RE

W here: t = 1 msec.e

c= 10 volts = capacitor voltage at time t .

E = 28 volts (for AC, use peak value).R = 2.0 ohms.

The next question concerns capacitor construction. Can the capacitorw ithstand discharge surge currents? When t he contacts close, thecapacitor will discharge through the resist or. For a 1 mfd. capacitor anda 2 ohm resistance, the time constant is: R x C = 2 x 1 mfd. = 2.0msec.

To det ermine discharge di/dt:

i = Cdv

= 1 x 10-628 x .63

= 8.8Aavg/msec.dt 2.0 x 10-6

w here; .63 is the capacitor voltage loss during one time constantof 2.0 msec.

This di/dt isnt very severe and a w ide variety of capacitors should beable to w ithstand it. However, the di/dt of a 5 ampere 240 volt ACapplication w ould be 107A/msec. at p eak of the AC linethat is, 340volts; and capacitor selection* should be m ade accordingly.

Of course, di/dt m ay be lowered by a larger value of resistance to limicapacitor discharge current even more. But , the greater the value oresistance, the less effect the capacitor has on the arc.

Other Arc Suppression Methods

For quenching DC arcs in certain applications, relays are available thathave a permanent magnet located in close proximity to the contactsThe magnet repels t he DC arc, thereby stret ching the arc and causing ito extinguish quickly.

Some relay users connect a diode across the inductive load to preventcountervoltage from reaching the contacts. When t he relay contactsopen, the storedenergy of t he inductance recirculates through t he diodenot through the arc. While this is an acceptable method of protecting thecontacts, it does result in lengthened hold-up time of the inductive load

For those applications that cannot tolerate lengthened hold-up time, aresistor may be placed in series with t he diode. The resistor doeshowever, lessen the effect iveness of the diode and, usually, a comprom isemust be reached by trial and error.

By using a zener diode in place of the resistor, hold-up time is greatlyreduced. This is because the diodes cannot t urn on until the voltageacross them equals the sum of their voltage drops.

In some circuits, space is at a premium and there may not be sufficienroom fo r a zener and a regular diode. In such circuits, some designersuse a metal oxide varistor. The M OV performs in a manner similar toback-to-back zener diodes. And, since the M OV is a bidirectional deviceit can be used in both AC and DC circuits.

An added benefit of arc suppression is the minim ization of EMI. Anunsuppressed arc betw een contacts is an excellent noise generator. Such

noise can be troublesome t o sensitive components in a circuit, or w ithinthe RFI field. In worst -case conditions, EM I can cause unwanted turnon of IC logic gates, SCRs, and triacs, and can cause damage to othersemiconductor devices.

* Suggested capacitor atypes are metalized foil and film foil. Check capacitor specifications fo

dv/dt and di/dt ratings.

Application Notes M easure the current inrush of the circuit to be switched before

specifying the relay. Never para lle l re lay contacts to double the contact rat ing

Unless the relays are specially adjusted, they will not pick upand drop out simult aneously. Even if they were to be specialladjusted, they w ould not hold this adjustment over life.

Paralleling Form C contacts may result in an unwanted makebefore-break arrangement.

Contacts rated low level to 2 amps may be used to switch a 2amp load. Once having done so, however, they cannot beused to sw itch reliably a low level load.

In a circuit comprising a series of open relay contacts (ANDlogic), al l but the last set of contacts to close wil l be dryLikew ise, in a circuit comprising a series of closed contactsall but t he first to open w ill be dry.

The use of many relay contacts in series may be l imited btotal circuit contact resistance.

A low level circuit that pulls a capacit ive inrush current odevelops an inductive counter emf is not low level. Worscase circuit conditions dictate contact rating, not steady stateconditions.

Specifications and availability subject to change w ithout notice13C3236 Printed U.S.A. IH/12-00

Tyco Electronics Corporation P&B, W inston-Salem, NC 27102Technical Support Center: 1-800-522-6752, w w w .pandbrelays.com