Notes

Voltage sag: A decrease of the normal voltage level between 10 and 90% of the nominal rms voltage at the power frequency, for durations of 0,5 cycle to 1 minute.
Causes: The faults on the transmission or distribution network (most of the times on parallel feeders), connection of heavy loads and start-up of large motors.
Consequences: Malfunction of information technology equipment, microprocessor-based control systems (PCs, PLCs, ASDs, etc) that may lead to a process stoppage. Tripping of contactors and electromechanical relays. Disconnection and loss of efficiency in electric rotating machines.

Voltage Interruption: Total interruption of electrical supply for duration from few milliseconds to one or two seconds.
Causes: Mainly due to the opening and automatic reclosure of protection devices to decommission a faulty section of the network. The main fault causes are insulation failure, lightning and insulator flashover.
Consequences: Tripping of protection devices, loss of information and malfunction of data processing equipment. Stoppage of sensitive equipment, such as ASDs, PCs, PLCs, if they’re not prepared to deal with this situation.

Voltage spike: Very fast variation of the voltage value for durations from a several microseconds to few milliseconds. These variations may reach thousands of volts, even in low voltage.
Causes: Lightning, switching of lines or power factor correction capacitors, disconnection of heavy loads. Consequences: Destruction of components (particularly electronic components) and of insulation materials, data processing errors or data loss, electromagnetic interference.

Voltage swell: Momentary increase of the voltage, at the power frequency, outside the normal tolerances, with duration of more than one cycle and typically less than a few seconds.
Causes: Start/stop of heavy loads, badly dimensioned power sources, badly regulated transformers (mainly during off-peak hours).
Consequences: Data loss, flickering of lighting and screens, stoppage or damage of sensitive equipment, if the voltage values are too high.

Harmonic distortion: Voltage or current waveforms assume non-sinusoidal shape. The waveform corresponds to the sum of different sine-waves with different magnitude and phase, having frequencies that are multiples of power-system frequency.
Causes: Classic sources: electric machines working above the knee of the magnetization curve (magnetic saturation), arc furnaces, welding machines, rectifiers, and DC brush motors. Modern sources: all non-linear loads, such as power electronics equipment including ASDs, switched mode power supplies, data processing equipment, high efficiency lighting.
Consequences: Increased probability in occurrence of resonance, neutral overload in 3-phase systems, overheating of all cables and equipment, loss of efficiency in electric machines, electromagnetic interference with communication systems, errors in measures when using average reading meters, nuisance tripping of thermal protections.

Flicker: Oscillation of voltage value, amplitude modulated by a signal with frequency of 0 to 30 Hz. Causes: Arc furnaces, frequent start/stop of electric motors (for instance elevators), oscillating loads. Consequences: Most consequences are common to undervoltages. The most perceptible consequence is the flickering of lighting and screens, giving the impression of unsteadiness of visual perception.

Oscillatory transients: Transients are brief but powerful over-voltages and over-currents lasting up to a few hundred microseconds (as defined by ANSI/IEEE C62), reaching in excess of 100,000 volts. Transient sources vary from external sources such as lightning, power system faults and utility grid switching, to internal sources which are generated by load switching (turning equipment on and off) and normal equipment operation, including electronic equipment. While much lower in voltage and current as compared to external sources, internal switching transients can occur over 1,000,000 times per hour in active industrial environments.
Causes: Lightning Strikes, switching activities(Opening and closing of disconnects on energized lines, Capacitor bank switching, Reclosing operations, Tap changing on transformers, Loose connections in the distribution system that results to arcing.

Notch: Voltage Notching is described by IEEE as a recurring power quality disturbance due to the normal operation of power electronic devices (i.e. rectifier), when current is commutated from one phase to another. Conventionally, the current waveform is used as the starting point for harmonic analysis, and voltage notching is simply derived from the IZ drops of the harmonic currents.
Causes: Three-phase rectifiers or converters, which generate continuous DC current.
Consequences: overloading of electromagnetic interference filters, and other similar high-frequency sensitive capacitive circuits.