I too carry one. Useful, not only on your own electrical, but on what it is been supplied to you.

One of the extension cable given to me had the positive and negative reversed.

Another time, I had checked an outlet, which was ok for a short time. Then I felt a little tingling sensation while operating the mixer, I quickly re checked the socket and the earth was missing. Something to do with a loose wire. That got fixed before I reconnected the equipment.

It's a good tool to have around and for less than £10 on Amazon, it's a no-brainer.