Edgcumbe Peebles Metrohm Earth Loop Impedance Tester teardown

Another skip-rescued tester means another teardown of some vintage gear. Let's take a look inside a loop impedance tester from 1979 and hopefully figure out how it works.
Schematic at drive.google.com/file/d/1P_No... (download the file to view in high resolution)
INDEX
0:00 Index
0:10 Introduction: Metrohm Loop Impedance Tester
1:19 The test leads
2:40 Cracking open the tester
7:57 Schematic and (my) theory of operation
16:52 Putting it back together
17:36 Testing (on a non-RCD supply)

[@richarddunkling2807]

Hi - Just been given a metrohm so your hard work regarding the circuit and operation is really appreciated. The circuit is more complex than I was expecting. I have yet to test it as I first need to source a suitable mini Bulgin mains plug. Well done you must have spend many hours working it out.

[@sparkyprojects]

We had one of those in the last company i worked for, it was already there in 196 when i joined And yes, i was one of those tthat cut the duraplugs, but i just did the one cut, however you could pull out the pins and thread them through the hole, if you could be bothered to undo the clamp Seems like duraplug have taken the hint these days :D