Electrical Knowledge Quiz: Faults and Protection

A fault is an abnormal condition that disrupts the intended operation of a power system. Depending on the circumstances, it can cause abnormal currents or voltages, equipment damage or interruption of supply. Read more about faults in power systems.

A line-to-line fault involves two phases; a single line-to-ground fault involves one phase and earth. Both are examples of unbalanced faults in a three-phase system. The fault category alone does not specify the current or the extent of damage.

An open-circuit fault interrupts a normal current path, as can happen when a conductor breaks. A short circuit involves an unintended conducting path. These are different fault conditions and can affect the system differently.

A transient fault exists briefly and then clears. NIMI gives lightning and contact with vegetation as examples associated with such faults. The word “transient” describes the duration; it does not establish whether equipment was damaged. NIMI, PDF page 261.

The described biased differential function uses currents from transformer windings 1 and 2 to calculate its operating and restraint quantities. This is an example from the 7SR242 manual, not a complete description of every protection scheme. See the introduction to power-system protection. Siemens, PDF page 34.

In the example described by NIMI, SF₆ is part of the arc-interruption system. When the contacts separate, the gas helps the medium regain insulating strength as the arc is extinguished. Designs differ; read the SF₆ circuit-breaker overview for the scope of that example.