Sunday, April 10, 2011

Basic Electrical Safety

Instrument Transformer and Power Management (P1) Course
Chapter (2)  :Safety basic
2.1 Introduction :
       - Every one must be able to identify the source of hazards at every place he works. so the worker must look around him and identify the source of hazards at every position. 
      -  Every worker must protect himself from these hazards.
      - Every one that works near high voltage equipment should be aware of some of the hazards that are present.
These include :
  1. Pressurized system. As found in the operating mechanism of some circuit breakers. Air blast circuit breakers are special concern as the air used to extinguish the arc is at a very high pressure.
  2. Load noise. An air blast circuit breaker, for instance, gives off a blast of air when it operates that can harm your hearing.
  3. Chemicals. Most station batteries contain liquid acid, when testing or replacing this acid one should be especially careful not to splash yourself or one some else. This acid is so strong that, it could cause permanent skin damage or cause blindness if it gets in your eyes.
  4. Mechanical system. Most station equipment operates by some mechanical means and generally are very powerful. Breaker operating mechanism, switches linkage, and transformer cooling fans are all examples.
  5. Heights. When working on top of a transformer station structure,...etc. There is always possibiltiy that you might fan. Special care should be taken when working in these locations.
  6. Electrical hazards, in the substation can generally be broken into areas. These are the hazards of watching on electrical arc and the hazards presented by the high voltages present.

2.2  Safety basics :
The following basic safety steps should be used a maximize worker safety
  1. Identify the hazards.
  2. Eliminate the hazards wherever practical.
  3. Control the hazards when they cannot be eliminated.
  4. Protect against injuries in case a hazards gets out of control.
  5.  Minimize the severity of injury if an accident occurs.
2.3 Basic result from electrical hazards to beings :
       The basic hazards to human beings as a result of an electrical shock and flash over are :
  • The sensation of the shock alone can cause an involuntary movement that can cause physical harm due to slips, falls, ..etc.
  • Physical damage to the body caused by heat generated by the current passing through the body.
  • Cessation of the functioning of vital body organs caused by the passing of current.
  • Physical damage to the body in the form of external burns and even blindness caused by the ultraviolet light produced by an electrical arc.
2.4. Basic concept :
       There are three components of impedance that can exist in an A.C. circuit, Resistance, Capacitance, and Inductance which effect the behavior of the A.C. circuit.
2.4.1 Resistance (R) :
       It is the element which opposes the flow of current in the circuit determined by the voltage and the rate of electrical flow and measured in ohms.( )

2.4.2 Capacitance (C) :
       A capacitor is defined as any tow conductors separated by an insulator known as the dielectric across which a potential may be applied. A charge can exist at any time that there is a voltage applied.
       The value of any capacitor is governed by the type of dielectric, the area of the conductors ( plates) and the distance between them.
Fig. (1)

       In many other cases the charge exists unintentionally such as the case when an ungrounded object is placed on the proximity of live equipment. In this case there are toe charges associated with it, one to live component and one to ground.
       The voltage induced in the isolated object depends upon the ratio of the mutual capacitances between the isolated object and the energized line, and between the isolated object and ground.
       This ratio remains constant for any given separation distance and is independent of the conductor length or object size. Thus the electrostatically  induced voltage on the object is independent of conductor length or object size.
       But the current that flows is determined by total series capacitance in the circuit between live conductor and ground, and this is dependent upon conductor length and object size.

Fig. 2

       When the isolated object is grounded the current flow becomes proportional to capacitance C1. Since C2 has been shorted as shown in Fig.(2-a)
       This conditions also apply to an isolated conductor instead of isolated object as in Fig.(2-b).
       The voltage across C2 is replaced by a steady state current to ground, on long parallel lines this current can be dangerous to human life.
       The most important reason for having this knowledge is to make it possible for the worker to inticipate all hazardous situations and protect himself from electrical energy.

2.4.3. Inductance (L) :
       Inductance in a circuit is the direct result of the magnetic field produced by current flowing through a wire.
       Inductance connects from a single element in a circuit, however when or more coils are brought close enough together so that magnetic field will link the other an effect known as mutual inductance.
       This same phenomena occurs between tow adjacent conductors that parallel energized current carrying equipment. The isolator conductor can have voltage  induced in it by electromagnetic induction and current will flow if there is a closed path.
Fig.  3
      Usually the open end voltage that are electromagnetically induced are not as great as those that are electrostatically  induced, but the current are usually much greater.
       The electromagnetically induced voltage is proportional to the separation distance, the parallel line length and the current in the energized conductor.
        In doesn't matter where the object is placed between live conductor and ground.
       It will assume a voltage proportional to ratio of the distance.

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1 التعليقات:

  1. It is imperative to put electrical safety a priority in any home or office to prevent hazardous such as fire and electrocution.

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