Why is my smart switch tripping the breaker?
You don't have neutral in that box!

Yellow/green is the one wire whose color code is is standardized worldwide: yellow/green, green or bare.c   It always means safety earthing.  

Anywhere you might have used those for neutral, you must correct that.  Whoever taught you that is wrong, and full of shortcuts which are unsafe. 

Misusing ground for neutral is what's causing the breaker trip.  Even the ones that "seem to work" are still a problem and will still tend to cause trips.  

The black wire is also not neutral. Since there are only 2 wires besides earth, the two wires can only be supply always-hot, and switched-hot to the lamp.  

In this box, there is no neutral because it is a switch loop.  However it also looks like in surface conduit, so it may be feasible to open up the conduit and change the cable to 3-core+earth, which can include neutral.  

Regardless, those switches are wrong here

Since you don't have neutral, you need smart switches which support lack of neutral.  I recommend going down to the local Wickes and seeing what they have. 

The switch in your picture will not do.  It is a cheap Chinese no-brand thing that you mail-ordered or bought from a dodgy shop who smuggled them in.  Note the complete lack of certifications by any reputable testing lab such as TUV or CSA.    See the CE and RoHS marks?  That is not any promise by the importer that they self-certify, and are taking legal responsibility for, the item being compliant with EU regs.  And guess what. You're the importer. 

This thing will cheerfully kill you or burn your house down.  I wouldn't want to be answering inquiry why I fit this particular thing instead of a listed unit. 

It's not an anti-mail-order conspiracy.  The conspiracy is Alibaba's, because their stuff is such complete junk and they don't even try to meet safety standards.   The Alibaba junkstream feeds AliExpress, eBay and Amazon Marketplace (anything that doesn't say "ships from and sold by amazon.com"). Feel free to buy from, say, Real Amazon.