salts that have the property of absorbing moisture
(chemists call them "hygroscopic" or "deliquescent"
materials). Their presence could unpredictably affect
your readings; they should be removed before tests are
In electrical equipment we are concerned primarily
with the conditions on the exposed surfaces where
moisture condenses and affects the overall resistance of
the insulation. Studies show, however, that dew will
form in the cracks and crevices of insulation before it is
visibly evident on the surface. Dew-point
measurements will give you a clue as to whether such
invisible conditions may exist, altering the test results.
As a part of your maintenance records, it is a good
idea to make note at least of whether the surrounding air
is dry or humid when the test is made and whether the
temperature is above or below the ambient. When you
test vital equipment, record the ambient wet- and dry-
bulb temperatures from which dew point and percent
relative or absolute humidity can be obtained
Preparation of Apparatus for Test
NOTE: Before interrupting any power, be certain
to check with your seniors (crew leader, project chief, or
engineering officer, as appropriate) so that any
necessary notification of the power outage may be
made. Critical circuits and systems may require several
days or even weeks advance notice before authorization
for a power outage may be granted.
TAKE OUT OF SERVICE.Shut down the
apparatus you intend to work on. Open the switches to
de-energize the apparatus. Disconnect it from other
equipment and circuits, including neutral and protective
(workmens temporary) ground connections. See the
safety precautions that follow in this section.
MAKE SURE OF WHAT IS INCLUDED IN
THE TEST.Inspect the installation carefully to
determine just what equipment is connected and will be
included in the test, especially if it is difficult or
expensive to disconnect associated apparatus and
circuits. Pay particular attention to conductors that lead
away from the installation. That is important, because
the more equipment that is included in a test, the lower
the reading will be, and the true insulation resistance of
the apparatus in question may be masked by that of the
Care should be taken in making electrical
insulation tests to avoid the danger of electric
shock. Read and understand the manufacturers
safety precautions before using any
megohmmeter. As with the ohmmeter, never
connect a megger to energized lines or
apparatus. Never use a megger or its leads or
accessories for any purpose not described in the
manufacturers literature. If in doubt about any
safety aspects of testing, ask for help. Other
safety precautions will follow in this section.
Observe all safety rules when taking equipment
out of service.
Block out disconnect switches.
Be sure equipment is not live.
Test for foreign or induced voltages.
Ensure that all equipment is and remains
groundedboth equipment that you are
working on and other related equipment.
Use rubber gloves when required.
Discharge capacitance fully.
Do not use the megger insulation tester in an
When taking equipment out of service, be sure to
observe all rules for safety. Block out the disconnect
switches. Test for foreign or induced voltages. Apply
workmens grounds. (Workmens grounds are grounds
you or others use to ground equipment while you are
working on it;)
When you are working around high-voltage
equipment, remember that because of proximity to
energized high-voltage equipment, there is always a
possibility of voltages being induced in apparatus under
test or lines to which it is connected; therefore, rather
than removing a workmens ground to make a test, you
should disconnect the apparatus, such as a transformer
or circuit breaker, from the exposed bus or line, leaving
the latter grounded. USE RUBBER GLOVES WHEN
CONNECTING THE TEST LEADS TO THE
APPARATUS AND WHEN OPERATING THE
APPARATUS UNDER TEST MUST NOT BE
LIVE.If neutral or other ground connections have to
be disconnected, make sure that they are not carrying
current at the time and that when they are disconnected,