American Power Conversion 250, 400, 600 user manual Noise and surge suppression

4.0Principles of operation

4.1Noise and surge suppression

The UPS contains high performance EMI/RFI (Electro-Magnetic and Radio Frequency Interference) noise and surge suppression circuitry to protect your computer equipment. The UPS provides this suppression continuously, whether or not it is turned on. Normally, the UPS suppresses noise and surges without your notice; that is, the UPS doesn’t transfer your load to its internal power source. Instead, the suppression circuitry reduces the amplitude of noise and surges to a level well below that which can be tolerated by your relatively delicate computing equipment.

The illustration above shows what a typical “medium” amplitude surge or spike looks like when present on the utility voltage. Surges up to 15 times larger than this are easily suppressed by the UPS. Surges are commonly caused by nearby lightning activity and motor load switching created in air conditioners, elevators and heat pumps.

The illustration above shows what EMI/RFI noise looks like when present on the utility voltage. The UPS “filters” out this noise with components whose electrical resistance is very high at radio frequencies. EMI/RFI noise is commonly created by the same activity which produces surges but can also be caused by nearby radio transmitters and blinking neon bulbs and signs.

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