For more formal explanation you can ask the manufacfurer.

But one of the manufacturers (in the USA) indicated that silver plating is the standard offering and tin plating is added cost option.  Tin plating is recommended for protection against corrosive environment. Unless the application has corrosive environment (for the copper), for all "normal" applications sivler plating is as good as any.

Agree with rbulsara.  Tin-plating is superior for corrosion resistant to things like hydrogen sulfide, and maybe chlorine.  

The NEMA standards are somewhat inconsistent on this.  For some categories silver-plating is standard and for others, tin-plating is standard.  I can never remember which is which.  

Before using anything in tin-plated copper, I recommend doing a google on "tin whiskers". It seems that tin-plated copper items are very prone to forming whiskers of tin out of the plated surface. Under good conditions, a forest of tin whiskers several mm deep can grow! This can lead to short-circuit failures and flashovers between adjacent busbars.

I am told that this phonomenon was well known back in the 1940's and was the reason that lead was put into solder - lead suppresses the growth os the whiskers.

From what I have seen Tin plating is standard for most bus unless it is a critical point, such as where a breaker or cell racks in - then silver plating is offered.  From what I can remember silver plating is better than tin, not the other way around...

Silver plating is more expensive in the manufacturing process,...

Regards,
TULUM

Silver and tin can both "grow whiskers", it depends upon the environment. Silver grows whiskers in the presence of H₂S, as is sometimes found in wastewater treatment plants and geothermal power plants. This is a picture of silver whiskers from H₂S. From this you can see that they can be very dangerous.


Link to photo site

Tin whiskers as well, but the available research on tin whiskering indicates that the whiskers are rarely over a few mm long, so if proper bus spacing is maintained, they don't tend to cause troubles. For this reason, many facilities engineers who know that sulpher may exist in their enviroinment, even in very low concentrations, will specify tin plating over silver.

Silver has better bus-to-bus conductivity however, so it is preferrable wherever it can be used without problems. But because of the whiskering issue, some manufacturers have started offering tin as standard and charging a premium for silver.

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jraef

It is amazing how you dig up these pictures. Definitely, this one looks scary.

I have never seen something like this. How the hell does this happen ?

edison123,
I had to go through an exercise in H₂S "certification" two years ago and got some help in this forum (thread238-103502), which included this link to a paper by a Schneider engineer who did a study on it for NASA in 2002. 2002 Chudnovsky paper on silver whiskering. She used some of the photos taken from the site I listed above.

I had known of the phenomenon without knowing exactly what it was because of a rather catastrophic failure I was involved in at a geothermal power plant in Northern California in the mid 1980's. The company I worked for sent out some MCCs with silver bus plating, even though the spec called for tin. An upper manager rejected my original engineering package because he didn't want to have special bus bar ordered (our standard was silver plated). His reasoning was that "silver was better, so give them the silver and tell them that we won't charge them extra", thinking it was strictly an economic issue. They missed that tidbit in the submittal review. When they called us a year after the installation and said our bus bars were "hairy", we thought maybe they were inhaling the local fumes from the pot growers that were prolific in that area. By the time we got the pictures back to start investigating (no digital pics in those days of course), there was a flash-over. Very very expensive lesson for everyone.

JRaef.com
"Engineers like to solve problems.  If there are no problems handily available, they will create their own problems."   Scott Adams  
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I have heard of whiskers growing under floor tiles. So I am not surprised at jraef's pictures, but I should admit I have not seen or heard of it in electrical equipment until now.

It must be a rare exception and an utterly ignored for maintenance and unattended installation.

tultum:
Cost of tin vs silver plating may differ depending upon location, but in the USA silver plating is cheaper than tin plating for whatever reason.

jraef

You sure have lived a life of surprises (& learning) and thanks for sharing it.

I have learnt in the past days that you never stop learning.

(2004 ??? Man, how could you remember that far ? Me, I live day to day.)

Thanks for sharing this info jraef...a big bold star is on the way for you!!

Jraef, Thank you for sharing this interesting information. A star for you !I have  seen specifying silverplating when the contact temperature rise is higher.May be  due to oxidation of tin coating.Also when the coating is over aluminum, silver plating always gives problems.

I thought plating was meant only for copper busses not aluminum.

Also be aware that tin plating is susceptible to galling on sliding connections.  It does not work well where breaker primary stabs connect on draw out units.  In one particular case I am familiar with, a manufacturer lost several 2000A breakers to severe overheating on the breaker connections.  The failures occurred in an area where corrosion was suspected but after several years of frustration trying to correct the problem the switchgear manufacturer finally revealed that a supplier had furnished tin plated components mixed in with silver plated ones.  All the failures were on the tin plated parts.

Please keep in mind that the whisker issue on silver plating is specific to a narrow batch of environmental contaminants, mainly hydrogen sulfide. In the average industrial plant where sulfur compounds are not in use, this shouldn't be a reason to not specify silver. I had never heard of this problem until I did that geothermal plant and it has only come up a few times since.

edison123,
Until my most recent stint of 11 years in one place, I had a resume that practically needed a forklift to move it from place to place. I can't remember names to save my soul, but being a "kinetic learner", I remember experiences with (sometimes annoying) clarity.

My wife calls me "a fountain of useless information".

JRaef.com
"Engineers like to solve problems.  If there are no problems handily available, they will create their own problems."   Scott Adams  
For the best use of Eng-Tips, please click here -> FAQ731-376

Any thoughts on unplated copper in MCC's, switchgear, etc?

Should be OK if the environment is clean, exceptionally dry and free from airborne contaminants as discussed above. I have seen a lot of older unplated busbar systems in use, some for a very long time. One I remember in particular was at a water pumping station that was built in 1906 in Spokane Washington. They had 2300V wound rotor motors and all of the power was controlled by open slate board switches with a room behind them filled to capacity with unplated copper busbars. They calle it the "suicide room".

Some day I have to find my photo essay on that and publish it here. It was from the pre-digital camera days so I need to dig up the photos and scan them, but I can't find the envelope that I put them in.

JRaef.com
"Engineers like to solve problems.  If there are no problems handily available, they will create their own problems."   Scott Adams  
For the best use of Eng-Tips, please click here -> FAQ731-376

GE told us a several years ago that they recommend a thin layer of Mobil Red Grease between any unplated copper busbar connections. The grease, even though it is not conductive, is pushed out by the pressure of the bolt connection and seals the connection. Red Grease should be used because it does not dry out like regular grease. We have been using this method for several years without a failure.

Burndy has a basic writeup on making electrical connnection at: https://portal.fciconnect.com/res/en/pdffiles/brochures/O-References.pdf

In a nutshell:
Unless the copper is badly oxidized, good
contact can be obtained with very little or no
cleaning.
Silver oxide is even more easily broken down
by the contact pressure;
On the other hand, aluminum oxide is a hard,
tenacious, high resistance film that forms
very rapidly on the surface of aluminum
exposed to air. The article also talks about the advantages and disadvantages of plating aluminum.

This is a good source of information on busbars in general.  The CDA recommends petroleum jelly on bare copper for most applications.

http://www.cda.org.uk/megab2/elecapps/pub22/index.htm