If the guard lead is grounded, and since the negative terminal
is at the same voltage as the guard terminal, the leakage into
the negative terminal will be considerably reduced. This will
improve accuracy because the current flowing into the
negative terminal is measured by the instrument and used to
calculate resistance. This technique is only permissible if the
item under test is isolated from ground. In this context isolated
means insulated by a resistance of at least 5 MΩ for the
positive terminal, or at least 10 kΩ for the negative terminal.