no other equipment will lack protection normally provided by the ground.
Pay particular attention to conductors that lead away from the circuit being tested and make sure that they have been properly disconnected from any source of voltage.
SHOCK HAZARD FROM TEST VOLTAGE. - Observe the voltage rating of the megger and regard it with appropriate caution. Large electrical equipment and cables usually have sufficient capacitance to store up a dangerous amount of energy from the test current. Make sure this capacitance is discharged after the test and before you handle the test leads.
DISCHARGE OF CAPACITANCE. - It is very important that capacitance be discharged, both before and after an insulation resistance test. It should be discharged for a period about four times as long as test voltage was applied in a previous test.
Megger instruments are frequently equipped with discharge switches for this purpose. If a discharge position is not provided, a discharge stick should be used. Leave high capacitive apparatus (for instance, capacitors, large windings, etc.) short circuited until you are ready to re-energize it.
EXPLOSION AND FIRE HAZARD. - So far as is known, there is no fire hazard in the normal use of a megger insulation tester. There is, however, a hazard when your test equipment is located in a flammable or explosive atmosphere. Slight sparking may be encountered (1) when you are attaching the test leads to equipment in which the capacitance has not been completely discharged, (2) through the occurrence of arcing through or over faulty insulation during a test, and (3) during the discharge of capacitance following a test. Therefore:
WARNING Do NOT use the megger insulation tester in an explosive atmosphere.
Suggestions: For (1) and (3) in the above paragraph, arrange permanently installed grounding facilities and test leads to a point where instrument connections can be made in a safe atmosphere.
For (3): To allow time for capacitance discharge, do not disconnect the test leads for at least 30 to 60 seconds following a test.
As a Construction Electrician, you must understand the principles of operation and construction of electrical motors and controllers. This knowledge is necessary so you can perform troubleshooting, maintenance, and repair of this equipment. You must be able to determine why the motor or controller is inoperative, if it can be repaired without removing it from service, or if it must be replaced. You must know what equipment substitutions or replacements to make and how to make the proper lead connections. The various types of motors and controllers have many elements in common; therefore, maintenance is fairly uniform. Once a motor or controller has been installed and the proper maintenance performed, you will have very little trouble. However, if something should go wrong, you must understand motors and controllers and how they operate to determine what troubleshooting steps to take and repairs to make. Remember, YOU are the repairman.
Motors operate on the principle that two magnetic fields within certain prescribed areas react upon each other. Pole pieces, frame, and field coils form one field; and as current is sent through the armature windings, another magnetic field is set up. The units of a motor, then, are the poles and the armature. The poles are ordinarily the static part, and the armature is the rotating Part
The poles are formed by placing magnetized bars so that the north pole of one is placed directly opposite of the south pole of the other. The air gap between these poles contains the magnetic field Just as a conductor must be insulated to prevent its electrical charge from being grounded, so the magnetic field must be shielded from the earth's magnetic field, or from the field of nearby generators or motors. This shielding is usually accomplished by surrounding the field with a shell of soft iron. The armature carries the coils which cut the lines of force in the field.
Direct-current motors and controls are seldom installed, maintained, or serviced by CEs unless they are assigned to special units, such as the State Department, where they will receive special training on this type of equipment. Therefore, we will not go into the depth on dc motors and controls as we will with ac. For information on direct-current motors and controls refer to the Navy Electricity and Electronics TrainingContinue Reading