General Interview Questions on Short Circuit Currents and Overcurrents.

Question Bank # 2:

1. 3 things affect the selection and application of Overcurrent protective devices:
   a. Current path
   b. Current magnitude
   c. Current type
2. Types of current:
   a. Normal current
   b. Overload current
   c. Short-circuit current
   d. Ground-fault current
3. Normal Current:
   a. Normal Current or Load is defined as current drawn by a load under normal, operating conditions. Depending upon the nature of the load, current may vary from low values to full load values (also known as full load ampere FLA values or maximum load current).
   b. Normal current flows only in the normal circuit path. The normal circuit path includes the phase and neutral conductors. It does not include equipment grounding conductors.
   c. e.g. Motor. Normal motor current varies from low values (under light loading) to medium values (under medium loading) to high values (under maximum loading).
4. Overload Current:
   a. Greater in magnitude then normal current but less then LRA (locked rotor amperes / current) and flows in normal circuit path.
   b. Commonly caused by overloaded equipment, single-phasing, or low line voltages
   c. E.g. Motor starting current.
5. Locked Rotor Current:
   a. Motor starting current is called locked-rotor current and is included on the motor nameplate as LRA (Locked-Rotor-Amperes).
   b. Motor starting current is a function of the motor design and can be as high as 20 times FLA.
6. Short Circuit Current:
   a. Greater than Locked Rotor Current and may range upwards of thousands of amperes.
   b. Maximum value is limited by the maximum short-circuit current available on the system at the fault point.
   c. Includes two or more phase conductors; line-to-line faults of two-phase conductors (A-B, B-C, C-A) or 3 phase faults (A-B-C).
   d. Classified further as:
   -Bolted Short circuit current
   -Arcing Short circuit current
7. Bolted Short circuit current:
   Phase to phase faults, phase conductors becoming solidly connected together.
8. Arcing Short circuit current
   a. Results from loose connections or insulation failure.
   b. Current flows through loose connections
   c. Arc presents impedance to the flow of current, smaller current will flow into an arcing fault compared to bolted fault.
9. Ground Fault Current:
   a. any current which flows outside the normal circuit path.
   b. In low voltages: current flow in the equipment grounding conductor. Detecting is a problem because of low level arcing ground faults.
   c. In medium / high voltages: ground fault current may return to the source through the earth. Ground current relays are used for detection.
   d. Classified further as:
   -Leakage Ground Fault Current
   -Bolted Ground Fault Current
   - Arcing Ground Fault Current
10. Leakage Ground Fault Current:
    a. Low magnitude current (mA range) associated with portable tools and appliances.
    b. Caused by insulation failure and is a serious shock hazard.
    c. Protection by using ground-fault circuit interrupter (GFCI).
11. Bolted Ground Fault Current:
    a. When phase conductors become solidly connected to ground i.e. Ph-G, 2Ph-G, 2Ph-G.
    b. If solidly grounded system, bolted ground fault current exceeds 3Ph bolted short circuit current.
    c. Protection by using standard phase and ground Overcurrent devices.
12. Arcing Ground Fault Current
    a. Depending upon the arc impedance, arcing ground-fault current may be as low as several amperes or as high as 20-38% of three phase, bolted short circuit current on a 480V system.
    b. Due to ionization of the air, arcing ground faults may escalate into Phase-to-phase or three-phase faults.
    c. Protection by using ground-fault protective (GFP) devices.
13. Sources of Short Circuit Current:
    a. Synchronous generator
    b. Synchronous motor
    c. Induction motor
    d. Supply transformers
14. Synchronous generator
    a. When a Short Circuit occurs downstream of a synchronous generator, the generator may continue to produce output voltage and current if the field excitation is maintained and the prime mover continues turning the generator at synchronous speed.
    b. The flow of SCC from the generator into the fault is limited only by the generator impedance and downstream circuit impedances.
    c. The magnitude of generator fault current depends on the armature and field characteristics, the time duration of the fault and the load on the generator.
    d. The ability of a generator to supply current during a fault is a function of the excitation system.
15. Synchronous motor (SM)
    a. When a Short Circuit occurs upstream of a synchronous motor, the system voltage goes to zero, and motor begins losing speed.
    b. As the motor slows down, the inertia of the load is actually turning the motor and causing it to act like a generator.
    c. The synchronous motor has a dc field winding and delivers a SCC into the fault until the motor completely stops.
    d. SCC is limited by SM impedance and the circuit impedance between the motor and the fault.
16. Induction motor (IM):
    a. When a Short Circuit occurs upstream of an induction motor, produces same effect as SM.
    b. Since the IM has no dc field winding, there is no sustained field current in the rotor to provide flux as is the case with a SM.
    c. SCC decays very quickly.
17. Supply Transformers:
    a. Not sources of SCC
    b. Trafo impedances limit the amount of SCC.
18. Standard tolerance on impedance is ±7.5% for two winding trafo and ±10% for 3 winding trafo.
    a. Minus tolerance is used for SCC studies
    b. Plus tolerance is used for load flow and voltage regulation studies.
19. 3 values of reactances are assigned to rotating machines for the purpose of calculating short-circuit current at three specified times following the occurrence of fault. Because SCC from rotating machines varies with time.
    These three values are:
    a. Subtransient reactance
    b. Transient reactance
    c. Synchronous reactance
20. Subtransient reactance (Xd’’)
    a. Is a value used to determine the SCC during the first few cycles after a SCC occurs.
    b. This is the SCC value to be used in all Short Circuit Studies.
21. Transient reactance(Xd’)
    a. Is a value used to determine the SCC from the first few cycles up to about 30 cycles after the SCC occurs.
    b. This value is often used in voltage regulation studies.
22. Synchronous reactance(Xd)
    a. Is a valued used to determine the SCC when the steady state condition has been reached and may take several seconds.
    b. This value is often used to determine the setting of generator backup Overcurrent relays.
23. Totally Symmetrical SCC:
    If short-circuit occurs in an inductive reactive circuit at the peak of the voltage wave-form, the resulting SCC will be totally symmetrical.
24. Totally Asymmetrical SCC:
    If short-circuit occurs in an inductive reactive circuit occurs at the zero of the voltage waveform, the resulting SCC will be totally asymmetrical.
25. Partially Asymmetrical SCC:
    If short-circuit occurs in an inductive reactive circuit occurs at some time between the zero and peak of the voltage waveform, the resulting SCC will be partially asymmetrical.
26. The rate of decay of the DC component:
    - The X/R ratio (ratio of inductive reactance to resistance looking upstream from the fault point) will determine the rate of decay of the DC component.
    - As X/R increases, the rate of decay decreases.
27. Momentary rating is the Short Circuit Duty during the first cycle after a fault and defines the equipment’s ability to close and latch against worst-case mechanical stresses.
28. Interrupting rating is the Short Circuit duty as the equipment contacts part and is expressed in symmetrical amperes or MVA.
29. Draw the curve for Assymetrical / Symmetrical / SCC.