HT instruments SPEED418 - Manual (Page 74)

400 Series
EN - 73
ALLOWABLE VALUES
Rated circuit voltage
(V)
Test voltage
(V)
Insulation resistance
(M
)
SELV and PELV* 250 0.250
Up to 500 V included, except for the above
circuits. 500 1.000
Over 500 V 1000 1.000
* In the new standards the terms SELV and PELV replace the old definitions "safety
low voltage" or "functional".
Tab. 4: Test voltage values and relative limit values
NOTE:
 If the circuit is quite large the conductors running side by side make up a capacity
which is to be charged by the instrument in order to carry out a correct
measurement; in this case it is recommended to keep the GO key pressed (in case
a test is effected under manual mode) until the result gets stable.

The indication "> 1999M " or "o.r." (out of range) warns that the insulation
resistance measured by the instrument is higher than the maximum resistance limit
(see technical specifications); this result is obviously far higher than the minimum
limits of the above table therefore if during a test this symbol is displayed the
insulation of that point is to be considered in compliance with standards.
CAUTION
When you effect measurements among active conductors it is essential to
disconnect all the users (alarm lamps, intercom transformers, boilers etc)
otherwise the instrument will measure their resistance instead of the
installation insulation. Moreover any insulation resistance test among active
conductors could damage them.