Types of Electrical Faults

Types of Faults :

Q. State the different types of faults.
Q. Classify faults.
Q. List types of faults.
Q. What are the common faults likely to occur? Suggest protections against such faults.
Q. List out any four types of faults that can occur in a system and give causes for the same.
Q. State the different faults in power system.
Q. List the various types of faults that occur in a power system.
Q. Explain the following terms and write down one example each type :
      (1) Symmetrical fault
      (2) Unsymmetrical fault.
Q. List normal and abnormal conditions of power system.

A.C faults can be classified as :

• Single line to ground fault
• Line to line fault
• Double line to ground fault
• Simultaneous fault
• Open circuit fault
• Three phase fault

             Other abnormal conditions in A.C. system includes :

• Voltage and current unbalance.
• Over voltages
• Reversal of power
• Power swings
• Under frequency
• Temperature rise
• Instability etc.

But mainly faults can be classified into two category :
   (a) Symmetrical faults and
   (b) Unsymmetrical faults.

Symmetrical faults :

• That fault which gives rise to symmetrical fault currents (i.e. equal magnitude of fault currents with 120° phase displacement is called as symmetrical fault.
• The most common example of symmetrical fault is when all the three conductors of a 3 phase line are brought together simultaneously into short circuit condition with each other including or excluding the ground.

Unsymmetrical faults :

• Those faults which gives rise to unsymmetrical currents (i.e. with unequal magnitude of fault currents with unequal phase displacement) are called unsymmetrical faults.
• The unsymmetrical faults may take one of the following forms :

(a) Single phase to ground (L-G) :

• A short circuit between any one of the phase conductors of 3 phase system and earth is called a single phase to ground fault.
• It may be due to the failure of insulation between a phase conductor and earth or due to a phase conductor breaking and falling to the ground.

(b) Two phase to ground fault (L-L-G) : 

• A short circuit between any two phases and earth is called a double line to ground or a two phase to ground fault.

(c) Phase to phase faults (L L) :

• A short circuit between any two lines (phases) is called as line to line or phase to phase fault.

(d) Open circuited phases :

• This type of fault is caused by a break in the conducting path. Such faults occur when one or more phase conductors break or a cable joint or joint on the overhead lines fails.
• Such situation may also arise when circuit breakers or isolators open but fail to close one or more phases.
• Due to the opening of one or two phases, unbalanced currents flows in the system, thereby heating rotating machines.

(e) Simultaneous faults :

• Two or more faults occurring simultaneously on a system 'are known as multiple or simultaneous faults.
• In simultaneous faults, the same or different types of faults may occur at the same or different points of the system.
• An example of two different types of faults occurring at the same point is a single line to ground fault on one phase, due to insulation failure and breaking of the conductor of another phase, both simultaneously present at the same time.
• If the two L-G faults are on the same section of the line, they are treated as a double line to ground fault. If they occur in different line sections, is known as cross country earth fault.
• Cross country faults are common on systems grounded through high impedance or Peterson coil but they are rare on solidly grounded systems.