User guide

Grounding the Equipment Avoiding Hazards
180
pmp-0229 (Mar 2013)
Based on the electrical/thermal analysis of these wires, Cambium 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, Cambium 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 45 on Page 178). 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.
Grounding PMP 400 SMs
PMP 54400 APs and SMs and PTP 54200 BHs use a nominal 30-V DC power system with power
on Pins 7 and 8 and return on Pins 4 and 5. PMP 54400 APs and PTP 54200 BHs can be powered
from either a CMMmicro with a 30-V DC power supply or a CMM4 with a 30-V DC power
supply. A 29.5-V DC power supply is available for PMP 54400 SMs.
In contrast, PMP 49400 APs and SMs and PTP 49200 BHs use a nominal 56-V DC power system
with power on Pins 5 and 8 and return on Pins 4 and 7. PMP 49400 APs and PTP 49200 BHs must
use a CMM4 with a 56-V DC power supply. A CMMmicro will not power these units, because it
provides the wrong voltage on the wrong pins. A 56-V DC power supply is available for PMP
49400 SMs.
IMPORTANT!
When working on sites with both power systems, use care to not wrongly mix
power supplies and radios, because the two power systems use different pinout
schemes and require different voltages.