Safety critical interfaces

While the blog will focus on software, I will occasionally highlight good and bad interfaces elsewhere. Take a look at this picture of my current tester (you hold it near a wire and it beeps and flashes if the wire is live). The end of the tester turns to switch it on. Do you think it's on?

It isn't - it's actually rotated to the orientation where you can pop the cap off and change the batteries. But the cap doesn't pop off - it just sticks in this position until you wiggle it. To switch it on, you have to rotate it the other way, but this way it looks on, so if you weren't paying attention you could test a wire and assume that it's not live.

How should it be designed? Perhaps the cap should pop off as soon as you get it near the stop point - but that would be annoying as I'd be constantly picking up the batteries. Maybe the stop for the remove batteries should be beyond the 'on' setting, so at least you have to turn it past to screw up. Even better, there should be a completely different mechanism for changing the batteries (which you only have to do every few years...)

You'd be foolish to not check it's working by testing it next to a known live wire but people cut corners, work when they're tired, etc. This design is a safety hazard, but neither the manufacturers, insurers, or whoever tested it for its UL listing picked up on this.