User's Manual Part 2
Release8InstallationandConfigurationGuide
Issue2,November2007 Draft5forRegulatoryReview 172
• Install lightning arrestors to transport lightning strikes away from equipment.
For example, install a lightning rod on a tower leg other than the leg to which
you mount your module.
• Connect your lightning rod to ground.
• Use a Cyclone 600SS Surge Suppressor on the Ethernet cable where the
cable enters any structure. (Instructions for installing a Cyclone 600SS Surge
Suppressor are provided in Procedure 28 on Page 349.)
• Install your modules at least 2 feet (0.6 meters) below the tallest point on the
tower, pole, or roof.
15.2.2 GroundingSMs
This section provides lightning protection guidelines for SMs to satisfy the National
Electrical Code (NEC) of the United States. The requirements of the NEC focus on the
safety aspects of electrical shock to personnel and on minimizing the risk of fire at a
dwelling. The NEC does not address the survivability of electronic products that are
exposed to lightning surges.
The statistical incidence of current levels from lightning strikes is summarized in
Table 39.
Table 39: Statistical incidence of current from lightning strikes
Percentage
of all strikes
Peak Current
(amps)
<2 >140,000
25 >35,000
>50 >20,000
>80 >8,500
At peak, more than one-half of all surges due to direct lightning strikes exceed 20,000
amps. However, only one-quarter exceed 35,000 amps, and less than two percent
exceed 140,000 amps. Thus, the recommended Surge Suppressor (300SS) provides
a degree of lightning protection to electronic devices inside a dwelling.
Summary of Grounding Recommendations
Last Mile Gear recommends that you ground each SM as follows:
◦ Extend the SM mounting bracket extend to the top of the SM or higher.
◦ Ground the SM mounting bracket via a 10-AWG (6 mm
2
) copper wire connected
by the most direct path either to an eight foot-deep ground rod or to the ground
bonding point of the AC power service utility entry. This provides the best
assurance that
• lightning takes the ground wire route
• the ground wire does not fuse open
• your grounding system complies with NEC 810-15.
◦ Ground the Cyclone Surge Suppressor 300SS or 600SS ground lug to the same
ground bonding point as above, using at least a 10-AWG (6 mm
2
) copper wire.
This provides the best assurance that your grounding system complies with
NEC 810-21.