[londons_explore]

There are 2 ways to design these. They could use a regular relay, or they could use a solid state relay.

Solid state relays have widespread fraud. Like 60% of the ones on amazon will catch fire or fail before they hit the rated current. Trade suppliers generally don't sell them at >30 amps.

Regular relays up to 10 amps are cheap and reliable. Beyond that, they get expensive surprisingly fast, and the reliability is hit or miss. They fail in numerous ways, but the most concerning one is the plastic case melting and catching fire. The chance of failure depends on the nature of the load (capacitive or inductive loads will dramatically shorten a relays lifespan).

In my professional career, I have witnessed ~20 of the above devices failing, with melted bits or burn marks, but of that sample none has burned down a building, yet. But I'd say that was more down to luck than good design.

In general, I would trust a china-device for monitoring power, but not for switching anything more than ~10 amps (1 outlet).

[jjeaff]

I've been down this road quite a bit on Amazon, eBay, and AliExpress/Alibaba. Are there any good options for relays that work for 100-200amps? I've found 30 amp relays of the type that are built into PCBs and I have tried a few 100 amp contractors that can be triggered with a separate line but I'm not sure I trust them. Most tend to top out at 65a rating and I get the feeling the 100a ones are just the same but with a more belligerent seller willing to advertise a false rating. What serious options are available for switching larger loads remotely.

[bityard]

Relays and contactors are the same thing. The main difference is that they tend to be called contactors more often when they are carrying mains voltage and current. (Or higher.)

It's very common to use a relay to control a contactor.

[baq]

Is it the switching that kills the relay or is it enough to start/stop the load? E.g. if I have a 10A smart plug monitoring power on a small waist-tall freezer in the garage will it eventually damage the switching function?

[londons_explore]

Its generally the switching, although I wouldn't professionally sign off an undersized relay on the basis of 'it never switches, so its fine'.

The switching under load causes the contacts to get worn, then the large load causes the now-worn contacts to get hot, and the plastic supports melt and catch fire.

[squarefoot]

A mains connected circuit designed to minimum costs could have all sorts of corner cuts: bad capacitors, no overvoltage protection, no back EMF from load protection, bad isolation to the driving circuit, etc, so that it could work flawlessly for decades, but also stop working after a while because of a spark that welded the contacts together, or mains voltage to enter the driving circuit because of moisture and insufficient creepage between high and low voltage tracks, etc. I use a lot of cheap Chinese products in non critical contexts, but I'd avoid them with mains voltage or in any safety critical use.