“A Stitch in Time...”

Whether you test monthly, twice a year, or once a year depends upon the
type, location, and importance of the equipment. For example, a small
pump motor or a short control cable may be vital to a process in your plant.
Experience is the best teacher in setting up the scheduled periods for your
equipment.
You should make these periodic tests in the same way each time. That is,
with the same test connections and with the same test voltage applied for
the same length of time. Also you should make tests at about the same
temperature, or correct them to the same temperature. A record of the
relative humidity near the equipment at the time of the test is also helpful
in evaluating the reading and trend. Later sections cover temperature
correction and humidity effects.
In summary, here are some general observations about how you can interpret
periodic insulation resistance tests, and what you should do with the result:
Condition
(a) Fair to high values
and well maintained.
(b) Fair to high values,
but showing a
constant tendency
towards lower values.
(c) Low but well maintained.
What To Do
no cause for concern.
locate and remedy the cause and
check the downward trend.
Condition is probably all right, but
cause of low values should be
checked.
(d) So low as to be unsafe.
(e) Fair or high values,
previously well
maintained but showing
sudden lowering.
Clean, dry out, or otherwise raise the
values before placing equipment in
service. (Test wet equipment while
drying out.)
Make tests at frequent intervals until
the cause of low values is located
and remedied; or until the values
have become steady at a lower
level but safe for operation; or until
values become so low that it is unsafe
to keep the equipment in operation.
A STITCH IN TIME