As stated in an earlier post, high current contacts need to be of a harder material. One problem high current contacts must overcome is the propensity to weld themselves together at higher current densities when they are opening. This is done in several ways. One, the chemical makeup is one that doesn't weld to itself when arcs occur. Tungsten is good for this. Two, the contacts are made large enough so that the current (upon breaking) is spread out over a larger surface area. And three, the relay design causes the contacts to have a wiping action which helps them break and also aids in wiping them clean (there is usually some carbon deposits). If you take a relay and slowly close the contacts until they just barely touch, then pull the armature down into the fully seated position, you might be able to observe this wiping motion. When I worked for Westinghouse building circuit breakers for power plants, we used an MG-6 model relay for the breaker control circuits. Not only could all 6 sets of contacts be configured individuall as Normally Open (N.O.), but, any or all of them could be configured as Normally Closed (N.C.). They even had one other interesting feature. By changing the geometry of the stationary contacts (bending them) and the springs on the moving contact arm, as the relay would energize and pull in, you could make any set of open contacts complete their respective circuit BEFORE the closed contacts would open (Make Before Break or M.B.B.). You could do this for when the relay de-energizes as well. The moving arms were set up so they had a fair amount of free play in both the opening and closing position which gave lots of wipe. Additionally there were two springs on each moving arm assembly. One was slightly stronger than the other so if you reversed them it wold place more tension on the contacts when closed, or help give the contact a little boost when opening. And THAT, my dear friends, is about all I know about relays.
The circuit breakers we made were all three phase with the largest current interrupting capacity up to 6,000 amps. The largest current breakers usually had a several sets of main contacts and one set of arcing contacts for each phase. When the breaker closed, the arcing contacts would make up first, followed by the main contacts. This would help limit inrush current. Main contacts had a very heavy silver plating and the individual fingers had to be adjusted to make solid contact. Arcing contacts were much smaller and had a tungsten based tip which would take the brunt of the arc as they were opening. The breakers were typically filled with oil which would make a small explosion when opening to help extinguish the arc. The extinguishing medium would then rush back in to insulate everything electrically. In later years, many of the higher voltage breakers (up to 765,000 volts) used Sulfur Hexafluoride gas as the extinguishing medium. Upon opening, the arc would alter the molecular structure of the gas and then it would return back to its original state after a short time. It was also an excellent insulator. Some of the circuit breakers were so large, they required 3 or 4 railroad flat cars to ship out to the customer where they had to be reassembled on site. Sorry for the rant, I hope it is not too boring.
