Electric Power Distribution Handbook, T. A. Short

11. Voltage Sags and Momentary Interruptions


  1. An overhead circuit that uses fuse saving has 5 miles of mains and 20 miles of total exposure. Annual line interruption rates are 0.25/mile for permanent faults and 0.25/mile for temporary faults. The average annual momentary interruptions will be closest to which value?

    1. 2
    2. 5
    3. 10
  2. Repeat for the same circuit with fuse blowing.

    1. 2
    2. 5
    3. 10
  3. Which of the following are likely to ride through a 0.4-sec momentary interruption. Select all that apply.

    1. Computer workstation
    2. PLC controller
    3. Digital clock
    4. Adjustable-speed drive
  4. For sags of 6 cycles or more, how much more frequent are sags to 90% than are sags to 70%?

    1. 2X
    2. 1.5X
    3. 1.3X
  5. For sags of 6 cycles or more, how much more frequent are sags to 70% than are sags to 50%?

    1. 2X
    2. 1.5X
    3. 1.3X
  6. Which PQ devices will help with sags? Mark all that apply.
    1. Dynamic voltage restorer (DVR)
    2. Static var compensator (SVC)
    3. Filter
    4. UPS
    5. Ferroresonant transformer
    6. Static transfer switch
    7. Mechanical transfer switch
    8. MOV
  7. If you close a substation bus tie that is normally open, what will happen to the average frequency of voltage sags for customers fed by the substation?

    1. Voltage sag frequency will stay the same
    2. Voltage sag frequency will increase
    3. Voltage sag frequency will decrease


  1. For a line-to-ground fault 2 miles from the substation, what does the bus voltage drop to? 12.47-kV system, Ifault=7 kA at the substation

    • Repeat for a fault 4 miles from the substation.
    • Repeat for a three-phase fault.
    • Repeat for a 25-kV system (line-to-ground fault).
  2. A PQ monitor shows a voltage sag that dropped to 50% of nominal for a line-to-ground fault. About how far was the fault from the substation? 34.5-kV system, Ifault=10 kA at the substation

  3. Repeat for a 12.47-kV system with the same fault availability at the substation.

  4. A critical customer is 4 miles from the substation. The customer is sensitive to sags that drop below 0.7 per unit. Draw an area of vulnerability marking the area of the circuit that can affect the customer. Estimate the number of events per year that could affect the customer. Assume the following:

    • 12.47-kV
    • Ifault=7 kA at the substation
    • assume 1 ohm/mile of line impedance (line-to-ground faults)
    • 3 feeders fed from the station bus
    • fault rate = 0.5 faults/mile/year

    Here is the circuit showing three distribution circuits coming from one substation:


    Then, show the area of vulnerability for momentary interruptions. Estimate the number of momentary interruptions for this customer.


  1. Using OpenDSS or another short-circuit tool that can be scripted, write a script to implement the fault positions method of evaluating voltage sags. (Challenging)

To the extent possible under law, Tom Short has waived all copyright and related or neighboring rights to these study questions. This work is published from the United States. Please use this material however you want.