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This is the group of disturbances ranging from sub-microsecond through to about 2 seconds in length. I want to concentrate on the faster end of the scale first. Impulses i.e. disturbances ranging in length from a half-cycle and less, continuously assaults the sensitive equipment found in the process control and automation industry as well as the technology used in business in general. Unstopped, impulses eventually cause the failure of sensitive components. Even if failure is not in the form of component breakdown, it can still exist in the form of disruption of normal operation. In this microprocessor dependent world we now live in, these electronic components which operate on fast on/off code sequences are exceedingly sensitive to impulses on power or signal lines - otherwise known as 'corruption', 'glitch', 'lockup', or 'crash'. In the section on "Arcing", I covered the primary causes of impulses also indicating that up to 80% of such impulses emanated from within a site, not from the power feed (the latter only carrying about 20% of the total impulse onslaught a site need withstand). But, when mentioning impulses (more specifically 'spikes' and 'transients') the tendency is to think along the lines of lightning strikes. This is probably only accountable for up to 5% of the total impulse pollution for a site in relatively high lightning occuring areas. The rest of the externally emitted pulses will come from network switching with the internal 80% coming from switch and contact make/break operations, especially during the switching of inductive loads. But this flawed thinking has a built-in "good fault". By immunising a site against lightning strikes, which are nothing more than high-energy impulses, one automatically immunises against impulses from other sources. Doing it the other way round would end in gnashing teeth! So this is where I will start this journey, dealing with installing lightning protection first as this way we will ensure the site is "impulse resistant", regardless of the source! By the way, even if you don't live in a lightning prone area, still consider installing such protection as it will, in the long run, save valuable electronic equipment. I then deal with momentary raised voltage, the disturbance between a half cycle and 2 seconds in length.
© 23.07.04 |
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