Internal and External Causes for Failure of Equipment
Q. List internal and external causes for failure of equipment’s.
Internal Causes :
- In some unavoidable situation the machine may have to be run “over loaded” for a longer period which causes temperature rise beyond the permissible limit.
- During normal working shot-circuit may occur due to some reasons.
- Some live terminal touches the metallic part developing phase to ground fault.
- Rotating part and stationary part shall have uniform air-gap. If not, then the behavior of machine becomes abnormal.
- Single phasing means one of the three phases become open circuited and the load is shared by only two phases instead of 3-phases. Hence over loading on phases and behavior becomes abnormal.
- Due to facing of stator and rotor teeth sometimes the teeth become magnetically locked and motion of motor does that possible.
- Phase winding may electrically touch each other may create problem.
- Some turns of phase windings get shorted.
- Worn out bearings, oiling / lubrication may not have been done frequently involving more mechanical / frictional losses.
- Unbalance rotating weights and fault in rotor may also the cause of abnormal behavior of the machine.
These are the causes creating problems of smooth working of machine and the machine show abnormal behaviour.
External Causes :
These causes are external to the machine from supply to the terminal box of the machine.
- A 3-phase machine is supplied with 3-line wires L.L.L. These are called as lines. If the fault is developed in the feeder / Distributor line only two lines will supply the equipment. The two lines supply the power to 3 phases of machine and two phases become excessive of load and over heating take place and machine condition becomes abnormal (single phasing on distributor side).
- Sometimes due to wind, rains, birds etc. short circuit occurs in the supply system which affects working of machine instantly.
- If supply system generator working in generating station is subjected change in the speed of prime mover, then supply frequency changes which affects the working of machine connected to this supply system.
- Sometimes in the generating station receiving station / substation there occurs over voltage or under voltage which affect the normal working of the machine.
- Some times negative sequence currents are developed due to unbalance supply system which affects the normal working of the machine.
- If external load on the machine is sustained for a length of period, the insulation may start burning due to excessive temperature rise.