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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?

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    \$\begingroup\$ Who says it's not an issue, because you found a single relay with no wear? I'm an industrial engineer, I can't tell you how many relays I've found that the contacts fused shut or blew completely off. It's possible the relay you found was used way below it's specs, or didn't switch frequently. \$\endgroup\$
    – Ron Beyer
    May 23 '20 at 16:15
  • \$\begingroup\$ You don't say what the relay was switching and in what kind of circuit it was in. There might have been external components to help the relay contacts to last longer. A snubber. \$\endgroup\$
    – Justme
    May 23 '20 at 16:26
  • \$\begingroup\$ @RonBeyer: Interesting. Maybe you can post it as an answer. Regarding your own question, I supposed that it's not an issue for two reasons: (1) the specification for most relays I've seen are pretty impressive when it comes to the number of times it can switch, that is tens or hundreds of thousands of operations. (2) I have seen discussions about arc prevention in wall sockets, but nothing about relays, making me think that the problem is somehow solved for the relays (especially since I can hardly imagine someone plugging/unplugging something in a wall socket hundreds of thousands of times). \$\endgroup\$ May 23 '20 at 16:27
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    \$\begingroup\$ In practice, it is common to bypass relay terminals with a capacitor and/or a small RC network as a "snubber". Doing so can reduce arcing pretty significantly. (Example: 40 years ago, gasoline engines had "points" to switch the current in the primary of the ignition coil -- basically a mechanical switch. These were bypassed by a capacitor (usually called the "condenser") for exactly this reason, to help suppress arcing.) \$\endgroup\$
    – Atomique
    May 23 '20 at 17:01
  • \$\begingroup\$ @ArseniMourzenko Relays have both mechanical life and electrical life. Mechanical life is measured with unpowered contacts. Electrical life is measured with powered contacts. Electrical life is significantly lower than mechanical life. And relays with vacuum or inert gas do exist. \$\endgroup\$
    – Justme
    May 23 '20 at 17:04
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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.

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Why arcing is such an issue with wall sockets or any other connector, but is not an issue inside a relay rated 16A 277VAC?

Arcing is such an issue with wall sockets, because plugs can not be very quickly inserted or removed. The speed of opening and closing a relay is known to the designer because that is an important part of the design. The speed is fast and consistent. It is generally not affected very much by the way the relay is energized, if used as recommended.

Wall sockets are also expected to be used for either resistive or inductive loads.

The rating I found is less than "16A 277VAC." I found life rating curves for resistive loads up to 100,000 hours at 7 A and 240 VAC.

I found a 1/4 Hp motor rating that appears to pertain only to the single-throw, normally-open contact. The 1C part of the relay number designated a single-pole double-throw contact. The life rating was not given for that.

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