I disassembled (tore apart?) an old 812H-1C-C relay to see what's inside, and found an expected piece of metal which moves between two other pieces of metal when activated by an electromagnet.
I was however surprised that there is nothing special to prevent arcing, and the contacts were rather small, compared to the contacts of a 10A 250VAC wall socket, for instance. Despite the fact that the relay was used a lot, there was no traces of contact wear.
What's the trick? Why arcing is such an issue with wall sockets or any other connector, but is not an issue inside a relay rated 16A 277VAC? Is it because the relay is sealed, and filled with non-conductive gas which somehow prevents an electrical arc?
Best Answer
Relays at full capacity will spark a bit and generally wear out after a relatively small number of operations, perhaps 100,000 though some are rated for only 10,000-50,000 if the load is heavy and/or somewhat inductive or has a surge. They will, however, generally be extremely reliable during that lifetime- they tend to run quite cool and are robust against surges in voltage and current, unlike semiconductor switches.
If no current is flowing through the contacts (or minimal) they can easily last for millions of operations.
The relay contact material and the speed of opening/closing is what minimizes the arcing. Different metals and alloys are used for different purposes, optimized for the best possible life at reasonable cost.