Electrical Noise

Electrical Noise

The term “electrical noise” is very broad and is used in several contexts. What is normally meant is that there are electrical signals getting into circuits where they are unwanted.

Noise occurs on both power as well as signal and data circuits, but generally speaking it is more often a problem in signal or data circuits. Signal and data circuits are particularly vulnerable to noise because they operate at fast speeds and at very low voltages.

Noise does however occur on power circuits and this is the type of noise that we are interested in under the general heading of power quality. Frequency of electrical noise signals on power conductors can be in the range of 2.5 kHz up to many megahertz. In the range of 2.5 kHz through to 150 kHz the allowable limits are not well defined by regulation, but where there is a problem reactors and low frequency filters can often by employed to reduce the noise, it is a tricky area though as low frequency filters tend to be bulky and quite costly. In the range of 150 kHz to 1 GHz the allowable limits are well regulated and many quality appliances have line filters on the input to ensure adequate immunity to this range of noise. Filters in this range tend to be quite compact and economic and standard stock units can normally be fitted to the power supply if there are issues.

Typically such interference is generated by equipment on the customer’s site or on a nearby site (although spikes can come from a long way away and sometimes these are also included in the broad category of noise.) Persistent noise on the power supply is not a wide spread problem but the reader needs to know of its existence. Due to the high frequencies involved specialist equipment such as a high speed digital oscilloscope is needed to investigate thoroughly and would not ordinarily be included in a power quality survey unless high frequency interference was suspected to be a problem. If it is suspected to be a problem a practical option is often to straight away install a good EMI filter on the input mains, as this is typically cheaper than employing very specialist equipment and investigators.

A word of warning though; EMI filters are very good at filtering out noise in the range of about 100 kHz up into many megahertz, but they do nothing to correct normal harmonic distortion and do little if any correction in the frequency range of tenss of kHz. Some EMI filters have a degree of surge protection built in, but if lightening surges are likely to be an issue then a dedicated surge protection device would likely be warranted also.

As a last point, the reader should also to be aware that, particularly in the upper frequency range, that frequencies are also transmitted through open space in the same way as radio waves and that an appliance experiencing problems could be due to electro magnetic frequencies transmitted in this way, and the interference may not be present on the power supply at all. (Remember the old days of how the portable battery operated AM radio would sound when a hair-drier or vacuum cleaner was being used).

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