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 made.
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
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 (workmen's 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 associated equipment.
Care should be taken in making electrical insulation tests to avoid the danger of electric shock. Read and understand the manufacturer's 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 manufacturer's 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 grounded - both 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 explosive atmosphere.
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 workmen's grounds. (Workmen's 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 workmen's 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 MEGGER.
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,Continue Reading