User guide

Grounding the Equipment Avoiding Hazards
178
pmp-0229 (Mar 2013)
Grounding SMs
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 45.
Table 45: 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 provides a degree of lightning protection to electronic
devices inside a dwelling.
Summary of Grounding Recommendations
Cambium 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
o lightning takes the ground wire route
o the ground wire does not fuse open
o your grounding system complies with NEC 810-15.
• Ground the surge suppressor 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.