User's Manual Part 2
Release8InstallationandConfigurationGuide
Issue2,November2007 Draft5forRegulatoryReview 174
Based on the electrical/thermal analysis of these wires, Last Mile Gear recommends 10-
AWG copper wire for all grounding conductors. Although roughly double the cost of 16-
AWG copper clad steel wire, 10-AWG copper wire handles six times the surge current
from lightning.
Shielding is not Grounding
In part, NEC 810-21 states:
A lightning arrester is not required if the lead-in conductors are enclosed in a
continuous metal shield, such as rigid or intermediate metal conduit, electrical
metallic tubing, or any metal raceway or metal-shielded cable that is effectively
grounded. A lightning discharge will take the path of lower impedance and jump
from the lead-in conductors to the metal raceway or shield rather than take the
path through the antenna coil of the receiver.
However, Last Mile Gear does not recommend relying on shielded twisted pair cable for
lightning protection for the following reasons:
◦ Braid-shielded 10Base-T cable is uncommon, if existent, and may be unsuitable
anyway.
◦ At a cost of about two-thirds more than 10-AWG copper UTP, CAT 5
100Base-TX foil-shielded twisted pair (FTP) cable provides a 24-AWG drain wire.
If this wire melts open during a lightning surge, then the current may follow the
twisted pair into the building.
More than 80 percent of all direct lightning strikes have current that exceeds
8,500 amps (see Table 39 on Page 172). A 24-AWG copper wire melts open at
8,500 amps from a surge that has a 1-microsecond by 70-microsecond
waveform. Hence, reliance on 24-AWG drain wire to comply with the intent of
NEC 810-21 is questionable.
Shielded twisted pair cable may be useful for mitigation of interference in some
circumstances, but installing surge suppressors and implementing the ground
recommendations constitute the most effective mitigation against lightning damage.
NEC Reference
NEC Article 810, Radio and Television Equipment, and associated documents and
discussions are available from http://www.neccode.com/index.php?id=homegeneral
,
http://www.constructionbook.com/xq/ASP/national-electrical-code-2005/id.370/subID.746/qx/default2.htm,
and other sources.
15.3 CONFORMINGTOREGULATIONS
For all electrical purposes, ensure that your network conforms to applicable country and
local codes, such as the NEC (National Electrical Code) in the US. If you are uncertain of
code requirements, engage the services of a licensed electrician.
15.4 PROTECTINGCABLESANDCONNECTIONS
Cables that move in the wind can be damaged, impart vibrations to the connected device,
or both. At installation time, prevent these problems by securing all cables with cable ties,
cleats, or PVC tape.