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Power Problems



Power Line Disturbances


Category 1 Voltage Spikes - from lightning, power network switching and equipment operations such as elevators, spot welders, etc.
Category 2 Decaying Oscillatory Transients - from switching power factor correction capacitors and other network or load switching.
Category 3 Under-voltage and Over-voltage - caused from faults and the action of fault clearing devices, power cutbacks, and sharp load changes.
Category 4 Outages - total voltage loss for longer than one half cycle.

A power line study was conducted by two engineers from a major data processing company. They picked 29 different locations to give a geographical cross section. The results recorded summarize their findings. The threshold for data recording were set such that most computers would be affected by the voltage recorded.

Summary of Power Line Disturbances


Category Disturbance Types of Disturbance Rate of Disturbance/Month Fraction of Total Disturbance
1 Voltage Spikes (25% threshold) 50.7 39.5%
2 Oscillatory Decaying (15% threshold) 62.6 49%
3 Under-voltage/Ove-rvoltage (10% threshold) 14.4 11%
4 Voltage Outage 0.6 0.5%

While individual voltage spikes are typically brief (10 to 100 microseconds), bursts of such pulses sometimes last for several milliseconds. Also nanosecond spikes occurred.

Oscillatory transients span the frequency range from 400 Hertz to over 5000 Hertz. Their initial amplitudes are often 100% of nominal peak voltage. Usually they decay to zero within one cycle.

Under-voltages and over-voltages are usually caused by large changes of load. The utilities normally respond slowly to these large changes.

The table above recorded only the variations outside the thresholds notes. VOLTAGES OUTSIDE THE THRESHOLD LIMITS ARE CONSIDERED CAPABLE OF CAUSING PROBLEMS IN COMPUTERS. These problems take the form of errors, memory loss, program loss, and even complete shutdown.