hodges - 23-7-2004 at 14:48
I was reading a financial book recently that described industrial uses of metals. It said silver is used increasingly in switches because "it is
the only metal that does not arc when the contact is broken". It then went on to say that companies use silver contacts where possible to avoid
lawsuits over fires. I don't understand the statement about silver "not arcing" at all. Is this to say if I make two electrodes out
of silver and apply a high voltage they won't arc? Doesn't sound reasonable to me. I do know silver is the best conductor of electricity,
but I think the "not arcing" statement is BS. Anyone care to comment?
The_Davster - 23-7-2004 at 17:48
I am not really sure but I think it means that if a silver switch is "on" and then turned "off" that there will be no spark.
Kinda like how if you short a 9v battery that there will be a tiny spark, I think silver contact points simply dont spark. Unlike other metals do.
[Edited on 24-7-2004 by rogue chemist]
chemoleo - 23-7-2004 at 17:55
Is it possibly because, unlike other metals, silver is rather reluctant to form ionised species (i.e. react to form oxides etc)?
I.e. carbon forms CO/CO2, and arcs very nicely.
Same for most metals.
This of course would be easy to test once an inert atmosphere is employed - then silver should behave like Fe, or any other metal - except that the
boiling poing may play a role.
Thats just a thought, might be completely wrong on that though.
Marvin - 23-7-2004 at 23:07
I think you are on the right track chemleo.
When I used to play with low voltage charged capacitors the sparks produced were a bit different. Touch the wires together and there would be a
"crack" and tiny golden sparks would explode outwards from the point of contact. I assume that these sparks are fragements of metal burning
in air. Since silver does not burn in air it should be immune from this. Silver does corrode though and I was under the impession important contacts
were plated with gold to stop it.
[Edited on 24-7-2004 by Marvin]
Nevermore - 25-7-2004 at 01:35
i don't completely agree with that, arcing depends mainly on the current and voltage of the two electrodes.
Of couse a substance with an high ionization potencial will raise the current and voltage needed for arcing, but can't prevent it. Also a very
good conductor like silver is less likely to heaten up for joule effect so doing will be less likely to start arcing, but when the A and V goes up,
there is nothing that can prevent arcing.
most high quality conductors are gold plated since gold is an excellent conductor plus doesn't make oxids...
personally i've opened many high power circuit breaker and i found out that in most of them the electrodes have two tiny plates of a dull silvery
material soldered on, also are covered with a silvery paint like substance, i think is an high melting point material, to avoid sticking of the
contact, but no silver have ever been found. when u deal with high currents is always better to avoid sticking of the contacts, you can still easily
turn off the arcing by some other ways.
unionised - 25-7-2004 at 14:32
I have recovered silver from plenty of switches and relays, they do use the stuff.
It will arc, but presumably it must be less of a problem than other metals or they wouldn't spend the money on it.
Once you start an arc the hot gas is the thing that matters, but the fact that silver doesn't form an oxide layer and is a good thermal conductor
may help to prevent arcing.
Twospoons - 27-7-2004 at 19:59
Sounds like BS to me. Silver is often used because it has the rather nice property that its oxide is conductive, and its cheap enough to use bulk
metal, rather than plating as used with gold. End application is critical to contact material choice - e.g. high current breakers use different
materials to telecom signal relays. Breakers have to avoid welding, signal relays have to avoid oxidation increasing contact resistance.
Cyrus - 28-7-2004 at 09:42
If silver won't arc because it won't oxidize, wouldn't gold arc even less?
jimwig - 28-7-2004 at 11:35
I recover silver contacts from circuit breakers and the like. fair amounts in the larger, industrial ones. looks like pure silver or at least a high
percentage
JohnWW - 14-8-2004 at 02:58
There is no such thing as electrical switch contats being made of a metal that "does not arc". Whether or not arcing occurs is a function of
the separation of the electrodes, and the dielectric field gradient limit of air (or other dielectric medium) in volts/distance, which together
determine the applied potential difference at or above which arcing occurs.
In connection with this, see a post that I have just made about electrical arcing in air on another thread/topic, in which I quoted the book
"Electrostatics And Its Applications" by A.D. Moore (Wiley-1973) in order to work out the potential difference required for arcing across a
1-inch gap.
The only advantages in using Ag instead of Cu in electrical switch contacts are (a) its slightly higher conductivity, (b) the lesser degree of
oxidation that Ag undergoes as the result of arcing in the presence of moist air (buit it still tarnishes), and (c) the Ag20 formed in the surfaces
being much more conducting than the CuO formed on Cu (although the Cu2O initially formed is also a good conductor).
The problem of oxidation due to arcing, and its potential to cause broken circuits or excessive resistance to currents, has been addressed on board
spacecraft using Au contacts. However, besides cost, the problem is that pure Au or Au plating wears out too easily, although Au-Ag alloys containing
at least 33% Au are sufficiently hard and do not tarnish. I have heard of Ni and Ni-Cr alloys being used as contacts to solve the problem, as a
substitute for Au.
John W.
Experiment, experiment and experiment!:
Tacho - 28-8-2004 at 14:04
I grabbed two pieces of rather pure silver and a power supply with high current capacity.
Big fat sparks!
Silver does spark.
Mr. Wizard - 29-8-2004 at 12:09
Any conductor will arc under the right circumstances. The advantage to silver is the contacts don't corrode as much and will have a lower
resistance contact, and thus carry a bigger load than say a tungsten or copper contact. Relay contacts are designed to 'wipe' each other as
they make connection, thus cleaning themselves. Watch a set of contacts as you close a relay by hand. Contacts rarely make 100% surface contact, so
you want the area that does contact to make a good low resistance connection. Maybe it even microscopically welds itself, as the old coherer radio
detectors did? This represents a huge use for the silver suppliers.