Back in the happy days when England still manufactured medical equipment, Cambridge Instruments made the famous Transrite series of ECG recorders (certainly up as far as the Transrite 5).
They also made the Camsafe. It was a nice looking little thing, in a blue and black box. It featured a 13-Amp socket with a meter alongside. I can't recall what the meter scale read (it may have been uA), but it would have been a milli-volt meter across whatever "measurement model" was "the one" at the time. Across a 1 k resistor in parallel with a 0.15 uF capacitor, I should imagine.
It was used for measuring earth leakage current etc.
back in the 1970's. You plugged the equipment into the Camsafe to bring the meter into circuit. There was probably a switch to open the protective earth (but my memory may be open circuit as well). We're talking pre-Rigel 233 here, and probably even pre-Liverpool Safety Tester. Not to mention pre-digital, pre-PC and certainly pre-Google!
I have spotted them hidden away at the back of certain biomed department store rooms, so they are still around, although probably never used these days.
By the way, does anyone know if there was a Rigel 211 and 222? Does anyone have either of them?