Left is 220 volt three phase power line input. Top is line A, Middle is line B, and bottom is line C, perhaps going to three phase motor on right.
The four 220 volt, 10 watt light bulbs with mounting sockets from left are #1, #2, #3, and #4. These bulbs are water pipe grounded. Under normal operating conditions all four lamps burn dimly.
If a ground in line A, light #1 goes out and light #2 burns brighter.
If a ground in line B, lights #2 and #3 go out and lights #1 and #4 burns brighter.
If a ground in line C, light #4 goes out and light #3 burns brighter.
