Chauvin Arnoux C.A 6505 Notice De Fonctionnement page 45

The insulation resistance is outside the
measurement range.
V
Check your connections; one of the terminals
of the instrument may be disconnected, or else
the value measured is in fact > 4 T
The spurious voltage present on the terminals
is greater than 25 VAC or 35 Vpeak.
V
AC
The instrument alerts you but does not prevent
making the measurements.
The spurious voltage present on the terminals is
too high for a measurement to be made:
V
peak spurious V > 0.4 Un
AC
The test voltage, Un, is indicated by the setting
of the switch.
Eliminate the spurious voltage and restart the
measurement.
Indicates that the protective fuse of the G
terminal is defective.
Replace the fuse as indicated in § 6.1.2.

Ω
.