Alloy For Electrical Contacts

Of those three, brass would be the best choice.

This sounds an awful lot like a homework problem. The materials chosen in an electrical contact will vary depending on a whole variety of concerns. A few of these concerns are cost, signal voltage level, signal power level, contact operation. Depending on what your concerns are you might want any of the three metals you've suggested, a different metal all together, or a bimetallic construction of a base metal with a cladding.

Usually the best metal is the metal piece already certified to be used in your application. Most firms do not want to go through all of the work of re-certifying a design on a whim. Those manufacturers that don't certify their redesigns are fabricating counterfeits.

Good answer Redfred. It advances the understanding of the issues and the discussion. For low current applications beryllium copper BeCu is often used. Potent hazard during fabrication, however.

Milo

Sintered silver would be my choice, but it's not in your list.

As previosly stated it all depends on the intended use.

High duty cycle (like mechanical relays) you will be interested in contact wear and bounce.

DC you will be interested in resistance, wetting and arc response of the metal. (And you might actually use different metals for anode and cathode.)

Rare operation (like a domestic main circuit breaker used once per year) you might consider issues of oxide formation and separation caused by that.

For your specific application, you need to talk to your contact material supplier METALURGIST (Not the salesman) and discuss the operational expectations of your product.

By the way, there are hundreds of variations on brass (mixtures, hardness, additives etc.) and also bronzes. Your selecgtion list will not be as simple as you might hope.

Silver is the best choice, the reason is that silver has the highest conductivity of all metals and the silver oxide that forms over time is also a conductor. Copper is next, but it slowly oxidizes in dry air, and any heating accelerates the oxidation, making the connection worsen as it heats and the added oxidation creates a feedback look that leads to a fire in the box. This happens only in over rated boxes (using a 20 amp box for 50 amps etc.) or ones with years of thick corrosion from moist or salt air.

Admiralty bronze is good in moist air, it does not conduct as well, but resists corrosion far better. Then there are silver plated copper, silver plated bronze, sintered silver inserts etc.

Regarding silver, some years ago I would have agreed and argued the case, but some "hard lessons" have now chnaged my mind.

The material choice will depend on the application. Silver has its place, but can also be the "worst" selection. High current density DC loads with high inrush current (halogen headlight circuits for instance) lead to the silver "welding" itself together on the "make" portion of the cycle giving high erosion rates, and the thing about silver oxide being conductive can mean that debris from switching creates bridges for sliding contacts.

The answer depends not only on the load, but also the intended design of the contact mechanism. We don't yet know whether these contacts are expected to work 5 times in their life (like a wiring harness in a computer) or 5,000,000 times (like a relay contact). We don't know what the intended load is. (Inductive like a relay coil, high inrush like a lamp filament or resistive like a stove element) We don't know whether the contact process is a sliding movement (where self cleaning can be incorporated) or like relay contacts touching against each other. We do not know the current intended (from micro-Amps for computer hardware to huge currents in wleders and power stations.) We don't know the voltages involved and whether it's DC or AC.

All these things will influence what is the most appropriate selection.

Yes, Silver is good in the right place, but contact is not a "one solution fits all" world.

For some reason I thought gold was the best conductor

The issue here is not merely conduction, it also includes mechanical integrity, resitance to deformation, creep, bridging etc. Gold is indeed a fine conductor, And malleable as heck... For high number of duty cycles, it will easily deform and the loss of dimensional integrity could affect performance. Engineering is seldom about the one "perfect choice." It is almost always about the least troublesome alternatives from a variety of different points of view. When the determining point of view is strictly accounting or economic criteria, often people die.

Milo

Yes, that is a common, instinctive mistake most people (me included) make, perhaps because gold is so expensive. Its conductivity is well below that of copper, and its use is restricted to dry circuits and where corrosion is rampant.

Small signal contacts are much different to large current contacts

(three good examples in post 3).

Small signal: down to below millivolts, stable contact resistance of a few milliohms, very limited allowable inductance and capacitance to be switched to ensure long life!

Gold with a few percent Cobalt for hardening, this coated above a nickel layer to prevent diffusion and copper-zirconium or copper-titanium as base is a very good combination.

Copper-Cobalt-Beryllium or Copper-Nickel-Beryllium often used if additional flexibility is needed. (These need a totally different heat treatment than CuBe2!). Collector brushes in small high quality motors use these. Bigger motors (above typically 10W DC) rely on copper or silver infiltrated graphite.

Primitive and home made: Brass or Cu-Zr (the material from cars starter-motors windings) or CuSn (the most popular of the very many bronzes) against jewelry silver (800 purity, so AgCu20) for small sliding contacts (oil lubricated) have proven to be pretty good in my anodising equipment to transmit up to 10 A and up to 120V to the rotating part.

RHABE

Good answer kvsridhar. Hey, how about some blow out magnets, to push those nasty arcs off the pit zone just a skosh?

Thanks.

Yes, i have used magnets for DC relays mainly, but the device sometimes becomes polarised and puts a restriction on the user. It is possible to design such that the relay doesn't get polarised, but there is a cost.

i prefer a blow-out coil for higher powered devices, no problem of polarity.

Regular bronze, or red brass as it often is supplied and yellow brass do not really have that good of a conductivity. CDA 83300 or Contact Metal has about twice the conductivity of the leaded red brass CDA 83600 or any of the CDA 85000 series yellow brass.

If strength is needed the 3C or 10C BeCu alloys have about a 45% conductivity compared to the 35% on Contact Metal or the 15% of red or yellow brass.

Pardon me, but i have been thinking that the query was about contacts, which make and break current, and therefore are subject to arcing. Not just carrying current, where only conductivity is the key feature. i could be wrong...but...

The materials I referred to are used for contacts that make or break and are not used as common current carrying materials in a system. Since a system is only as good as its weakest link one doesn't want to install a resistance point where the conductivity goes from 98% with copper wires down to 15% roughly 1/6 if one can only go down to 34% or 1/3.

The contacts in essence become an orifice so one needs a much larger surface area to pass the same amount of current when the conductivity of the contacts is low.

Oh excuse me. i forgot that my area of expertise (if any) is low voltage switchgear, where silver alloys are almost mandatory, and i forgot about hundreds of other applications. Sorry.

Seems to me that it's time to hear from phillip.

Hey guys I've interpreted a much advance understanding by your comments and I say thanks for sharing that knowledge with me. I was amazed to see all these comments though. Its a practical experiment assignment that I"m doing but we are gonna use aluminum to compare differences also, I'm personally gonna try silver though..

Is a,"practical experiment assignment" anything like homework?

It would be nice to know what you are doing, because there is a firm prohibition against homework assignments being done by CR4 members for students.

So, if you start out by explaining why you are looking for the answer, we can determine how much and what kind of help we can provide and still not break the rules.

I, for one, do not like to do homework for people, but others may. Helping find a path to the answer is usually OK.

LYN, I do not need to know more , these guys have fill me in on more than enough info that I need I even took note of it.Probably you do not understand the concept of my question and I do not know what you are getting at but so far you have not helped me in anyway, in case you did not know I"m looking for the best material out of these three to modify/alter the terminals of four separate 30 amp contactors each that supplies a circuit of 12 fluorescent 220V light fittings, due to the fact that when switched off the collapsing magnetic field and sparking would not affect my contacts as much as it is now,for longer lasting purposes. Your comments are considered off topic to me LYN.

Thank you for clearing that up for me. I obviously do not understand the concept and you obviously misunderstood my answer to your original question. You gave three material choices; I gave my opinion of the best of the three. That concept I do understand.

Many other materials, unmentioned in your OP have been suggested here by others. After much speculation by all, you now add significant information that would have been useful in the beginning.

Never mind.

What?!

NOW you decide to reveal the real question

That you didn't first up, is no reason to criticize a contributor.

_{I"m looking for the best material out of these three to modify/alter the terminals of four separate 30 amp contactors each that supplies a circuit of 12 fluorescent 220V light fittings, due to the fact that when switched off the collapsing magnetic field and sparking would not affect my contacts as much as it is now,for longer lasting purposes.}

Excuse me perhaps i am a little thick. Are you looking for the best material for terminals or for the contacts ? Why would terminals arc ? And given your load level, you will definitely need power contactors which will have silver alloy tips (most likely AgNi), which you cannot change anyway. Can you please elaborate a little ?

Maybe you want to change the terminal material to improve life of the AgNi contacts inside? Not likely i think.

Exactly, but the present contactor is not of silver alloy tips it carry aluminum ,that is why I put aluminum in my question to make sure I am doing the right thing in changing it.But it seems to me that silver alloy is of best choice so silver alloy it will be then.

Wow, never heard of aluminium contacts ! Are you sure ? Ag alloy may look like aluminium....

Contact tip joining to the base material (usually copper or brass at this current level) is a specialised process, and is best left to the manufacturer. The tips are also made specially by their manufacturers, who usually supply by the thousands....are you sure you can get them ?

Do you think you can give more details of the contactors : Manufacturer, model number and major name plate details ? Thanks in advance...