Kikusui TOS9213S Operation Manual page 32

When Connecting to an Ungrounded Outlet
2-8
If you have to connect the tester to an ungrounded outlet, connect the protective con-
ductor terminal on the rear panel of the tester to an earth ground.
Have specialized engineers select, manufacture, and install cables. To ensure secure
connection, use proper tools.
Fig.2-5 Grounding by using the protective conductor terminal
Electrical ground
(safety ground)
TOS9213S/ TOS9213AS