Grounding and Lightning protection

02

General guidelines

Logical setup

The typical logical configuration of the Infinet Wireless units is presented in Figure 1 below:

  • left picture: standard Infinet Wireless solution when connecting to an AC power supply. The Outdoor Unit is the radio device itself and the Indoor Unit, called IDU is the PoE power supply unit for the ODU. As their names suggest, the ODU can be installed outdoors, it has ingress protection against water and dust, while the IDU is installed indoor.
  • right picture: standard Infinet Wireless solution when connecting to a DC power supply. Typically, the power injector has to be installed outdoors in such situations. The auxiliary device AUX-ODU-INJ-G must be used as outdoor power injector in this scheme and IDU-LA-G(V.01)/IDU-CPE-DC as indoor power injectors. These devices are described in the sections below

Figure 1. Logical setup for the Infinet Wireless units

The general recommendations when implementing this scheme are the following:

  • The IDU/ODU power injector must be connected to the ODU radio unit using a CAT5e FTP cable. This cable is not included in the package of the unit, so it will have to be prepared on site for each device. The cable must be a straight cable, so the pinout is the same at both ends and the color code for the wiring is T568B. The maximum cable diameter must not exceed 7mm, while the length must not exceed 100m. 
  • The FTP Cat5e cable is an outdoor cable and it’s usually black because it must have UV protection, so that the solar radiation doesn’t damage it. But most importantly, besides the 4 copper twisted pairs, the cable must also have a metal shielding and a drain wire inside (presented in Figure 2 below), to provide a more effective grounding. 

  • Two RJ-45 connectors for the FTP cable are included in the package and it is very important to mention that they are different: one is shielded and the other one is unshielded (standard). 
  • The end of the FTP cable that will be connected to the power injector should be assembled with the shielded RJ-45 connector. The other end of the FTP cable that will be connected to the wireless device should be assembled with the unshielded (standard) RJ-45 connector.
  • The metal shielding and the drain wire of the cable must make good contact with the shielded connector. This is very important to remember, because the shielding, along with the drain wire, helps to quickly remove the unwanted electrical noise and allows for a very effective grounding.
  • Besides the power source, your local network (LAN) must be connected as well, so the FTP cable between the ODU and the IDU will carry both data and power. 

Figure 2. Shielded RJ-45 connector attached to an outdoor FTP CAT5e cable in the rght side, and metal shielding and drain wire of the FTP cable in the left side

Physical setup

The figure below shows the physical setup for the installation of the Infinet Wireless units using the standard IDU units for connecting to an AC power supply: IDU-BS-G or IDU-BS-G(60W). The grounding connections when using a DC power supply are described in the sections below, but the setup is similar to the one presented in the figure below. The grounding circuit is explicitly shown using the green wired connections.

Figure 3. Grounding when using IDU-BS-G or IDU-BS-G(60W)

The general recommendations that should be taken into account for ensuring a proper grounding are the following:

  • The wireless device should be placed on the pole at a height that is at least 1 meter below the top of the pole. In this case, there is a significant probability that the lightning strikes the pole and not the wireless device. 
  • The pole should be properly grounded: connected to the building's grounding circuit according to your local regulations.
  • All devices must use separate grounding cables that should be connected to the grounding circuit and not to the mast. The best scenario is when the grounding cables are lined in parallel with the Ethernet cable.
  • All equipment must be connected to stabilized and surge protected power sources which must be properly grounded, too.
  • The IDU is grounded via a three-conductor power cord and a grounded socket.
  • The antenna pole and the wireless device should be connected to a common ground ring. The grounding cables should be no less than 10AWG thick and must use corrosion-resistant connectors.
  • Special attention should be paid if the external antenna used is not DC-shorted. In this case, an additional lightning protection device should be used between the antenna and the ODU for each of the 2 RF cables.
  • The recommendations mentioned in the previous section concerning the cabling and the connectors should be carefully taken into account

Circuit flow in case of lightning

Figure 4. Circuit flow in case of lightning strike on different elements

This section describes the flow of the electrical current in case of lightning strike on different elements/ parts of the radio communication system:

  1. Lightning strike on the pole (scheme A. in Figure 4 ): the  electrical current goes through the pole directly to the ground without impacting the radio communication system
  2. Lightning strike on the antenna (scheme B. in Figure 4 ): the electrical current goes through the coaxial cable to the ODU case, which is connected through the ODU grounding clamp to the grounding ring, so the radio system is safe. 
  3. Lightning strike on the ODU (scheme C. in Figure 4 ): the same as the previous point. The ODU is grounded using the ODU grounding clamp that connects it to grounding ring (the section below describes how is the grounding of the ODU performed)
  4. Lightning strike on the FTP cable (scheme D. in Figure 4 ):The direct lightning strike on the FTP cable (ODU-IDU) is partially terminated on the grounded IDU case. Partial termination means that the direct lightning strike will probably destroy the FTP cable. The service cable pickups from the electromagnetic impulses are terminated on the IDU case by the winding shield, and further - on the IDU grounding.
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