Be aware that thyristors may work OK on low voltage or when cold but may leak and latch up when hot or under rated voltage. A judicious application of another Yihua unit (non melted down variety) with the triac in series with a mains bulb (no gate drive) might determine if this was the problem.
Of course if the second Yihua melts down while you're doing this... (Just kidding)
I have the Atten 858D+ and a quick examination shows that:
It has a fused IEC socket, and the fuse is in the live. (good)
There is a proper mains switch, in the live. (good)
The earth wire to the wand is not very substantial, and I wonder if it can pass enough current to blow the fuse. (bad)
There might be a thermal cutout in the wand, but it is hard to tell without pulling the insulation to pieces. (?)
I'm thinking I will look into improving its safety by fitting a thermal cutout close to the element.
The lightweight earth is hard to upgrade without changing the whole flex, but I was wondering about fitting an RCD in the unit. (My bench has one anyway but you have to allow for someone using it elsewhere)
Or I could just fork out for a better brand of unit.
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif)