Preparing for Installation 2-13
Plant Wiring
The following are guidelines for setting up the plant wi ring and c abling a t you r site. Whe n plannin g
the location of the new system, consider the distance limitations for signaling, EMI, and connector
compatibility, as described in the following sections.
Interference Considerations
When wires are run for any significant distance in an electromagnetic field, interference can occur.
This fact has two implications for the construction of plant wiring:
•Bad wiring practice can result in radio interference emanating from the plant wiring.
•Strong EMI, especially when caused by lightning or radio transmitters , can destroy signal dri vers
and receivers and can even create an electrical hazard by conducting power surg es through lines
and into equipment. (Review the safety warnings in the section “Maintaining Safety with
Electricity,” earlier in this chapter.)
Note To predict and remedy strong EMI, you might need to consult experts in radio frequency
interference (RFI).
If you use twisted-pair cable in your plant wiring with a good distribution of grounding conducto rs,
the plant wiring is unlikely to emit radio interference. If you e xceed th e recommended dist ances, use
a high-quality twisted-pair cable with one ground conductor for each data signal, when applicable.
If wires exceed recommended distances, or if wires pass between buildings, give special
consideration to the effect of a light ning stri ke i n your vicini ty. The electromagnetic pulse c aused b y
lightning or other high-energy phenomena can easily couple enough ene rgy into unshielded
conductors to destroy electronic devices. If s uch problems have occurred in the past, you may want
to consult experts in electrical surge suppression and shielding.
Distance Limitations and Interface Specifications
The size of your network and the distances between connections depend on signal type an d spe ed
and on transmission media (the type of cabling used to transmit the signals). For example, standard
coaxial cable has a greater channel capacity than twisted-pair cable. The distance and rate limits in
the following descriptions are the IEEE recommended maxi mum speeds and di stances f or signaling;
however, you can usually get good results at speeds and dis tances far greater than these. For exa mple,
the recommended maximum rate for V.35 is 2Mbps, but it is commonly used at 4Mbps without any
problems. If you understand the electrical problems that might arise and can compens ate for them,
you should get good results with rates and distances greater than those shown here; however, do so
at your own risk.
Note We recommend that you do not exceed specified tra nsmission rate and distance limits.