Guide for Lightning Protection of Power over Ethernet (PoE

Copyright © Daniel G Ashton 2021

Guide for Lightning Protection of Power over Ethernet (PoE) Devices Installed

Outdoors


Abstract: Devices such as security cameras, when installed outside of a building, may be exposed to direct or indirect lightning strikes. These devices utilize a twisted pair ethernet cable to carry the signals and a DC feed to power the device. Devices placed on poles separate of a building are more likely to be struck by lightning. Some form of protection is necessary to prevent damage to the DC power feed and connected equipment. Several examples of protection methods are given.

The document content is of a general nature only and is not intended to address the specific circumstances of any particular individual or entity; nor be necessarily comprehensive, complete, accurate or up to date; nor represent professional or legal advice.


Contents

1.0 Introduction ...........................................................................................................4 2.0 Definitions and Abbreviations ................................................................................4 2.1 Definitions .............................................................................................................4 2.2 Acronyms ..............................................................................................................5 3.0 Overview ...............................................................................................................6 3.1 PoE Devices Installed Outdoors ............................................................................6 3.2 Lightning Protection System (LPS) ........................................................................7 4.0 Exposed Versus Unexposed ...............................................................................10 5.0 Power over Ethernet (PoE) Versus Power over Data Line (PoDL) .......................13 6.0 Basic Installation Overview ..................................................................................14 7.0 Device Grounding................................................................................................16 8.0 Surge Protective Devices (SPD’s) .......................................................................17 9.0 Recommended Lightning Protection for Externally Mounted Devices ..................18 9.1 Security Cameras ................................................................................................18 9.2 Ethernet (Fixed Wireless) Radio Systems ...........................................................22 10.0 References ..........................................................................................................25


Guide for Lightning Protection of Power over Ethernet (PoE) Devices Installed Outdoors

1.0 Introduction

Devices such as security cameras, when installed outside of a building, may be exposed to direct or indirect lightning strikes. These devices utilize a twisted pair Ethernet cable to carry the signals and a DC feed to power the device. Devices placed on poles separate of a building are more likely to be struck by lightning. Some form of protection is necessary to prevent damage to the DC power feed and connected equipment. Several examples of protection methods are given.

2.0 Definitions and Abbreviations

2.1 Definitions

For the purposes of this document, the following terms and definitions apply.

Ground Potential Rise (GPR) is a phenomenon that occurs when large amounts of current enters the earth at a given point, such as in the case of a lightning strike. The closer to the lightning strike point, the greater the voltage and current.

Lightning Protection System (LPS) a system designed to protect a structure from damage due to lightning strikes by intercepting such strikes and safely passing their high currents to ground. See NFPA 780® (9)

Midspan PSE A device that adds voltage and current to an Ethernet circuit for the connected powered device (PD)

Physical (layer) (PHY) The layer responsible for interfacing with the transmission medium. This includes conditioning signals received from the media access control (MAC) for transmitting to the medium and processing signals received from the medium for sending to the MAC.

Power Over Ethernet (PoE) technology that provides power over LAN cables to connected powered devices without the need for batteries or AC outlets.


2.2 Acronyms

AC Alternating current ACEG Alternating current equipment ground AWG American wire gauge DC Direct current EMI Electromagnetic interference GPR Ground potential rise IEEE Institute of Electrical and Electronics Engineers LAN Local area network LEMP Lightning Electromagnetic Pulse LPS Lightning protection system NEC® National Electrical Code NFPA® National Fire Protection Association NRTL Nationally recognized test laboratory PD Powered device PHY Physical layer (usually Ethernet transceiver) PoDL Power over data line PoE Power over Ethernet PSE Power source equipment RRH Remote radio heads SPD Surge protective device


3.0 Overview

3.1 PoE Devices Installed Outdoors

Traditionally security and communications equipment mounted on the exterior of buildings has been powered either by 120V AC or from a separate DC power source. In recent years, PoE technology has been used to power these types of devices (see Figure 1 for examples of PoE devices). These systems incorporate a PoE router or a Midspan power source equipment (PSE) to provide power to the device. These devices may be mounted under the eaves of the roof, over the eaves of the roof, on top of the roof or on a separate mounting structure such as a pole. This document will provide information to assist in protecting PoE devices installed where they may be exposed to lightning strikes, induction, and surges.

Figure 1

PoE Powering and Powered Device Examples


3.2 Lightning Protection System (LPS)

When Lightning strikes, it can damage unprotected equipment by either a direct strike or through induction from the lightning electromagnetic pulse (LEMP) as illustrated in Figure 2. A well-engineered lightning protection system will help minimize this damage.

Figure 2

Lightning Strike Examples


A lightning protection system includes a network of air terminals, bonding conductors, and grounding electrodes designed to provide a low impedance path to ground for potential strikes. See NFPA 780® (9) and examples shown in Figures 3 and 4.

Down conductorsized perNFPA 780 ®.

Air Terminal(lightning rod)

Lightning Protection System

LPS ground ring

Main Conductor

FIGURE 3

Pitched Roof Lightning Protection System


Flat Roof Lightning Protection System

Air Terminal (lightning Rod)Main Conductor/Roof Top Ring

Down Conductors

LPS Ground Ring Figure 4

Flat Roof Lightning Protection System


4.0 Exposed Versus Unexposed

When metallic devices are installed outside of a building, they are exposed to the effects of lightning. When they are placed within the “zone of protection” of an LPS, the risk of a direct strike is greatly reduced. Two examples of a zone of protection are shown in Figure 5, where anything below the solid line would be considered protected.

Figure 5

Figures from NFPA 780®(9) Section 4.8 depicting a 45-degree and a 63-degree angle from the vertical

Per NFPA 780® (9) section 4.8.2.2, the zone of protection is delineated as a cone, with the apex located at the highest point of the strike termination device and its surface formed by a 45-degree or 63-degree angle from the vertical, based on the height of the strike termination device above the ground. See NFPA 780® section 4.8 for additional details on the zone of protection. For a structure with an LPS on the roof, any device installed under the LPS should be considered within the “zone of protection”.


Any PoE powered device mounted above the roof line of a structure that does not have an LPS, would be considered fully exposed to a direct lightning strike and may need additional protection measures.

Lightning caused ground potential rise (GPR) is another source of damage from lightning. Grounding and bonding, as laid out in section 7 of this document, will help minimize damage from lightning GPR.

Ground potential rise [GPR] is a result of lightning striking the ground. Current flows away from the point where the lightning hits, creating potential gradients. The voltage at each point on the potential gradient can be calculated; but in general, the calculation is very complicated. In the simple case of uniform earth resistivity, the ground potential GP of the earth at a distance “r” from the point where a lightning strike enters the ground is given by equation (1).

𝐺𝐺𝐺𝐺 = 𝜌𝜌𝜌𝜌

2𝜋𝜋𝜋𝜋 (1)

Where ρ is the resistivity of the earth generally being a function of distance, angle, and depth], and I is the lightning current. The important feature of GP is that it falls off as 1/r. In the more general cases, it falls off as a power series in (1/r)n. For the simple case just discussed the potential gradient looks like the equipotential circles shown in Figure 6. Depending on the earth resistivity ρ and the current I, the potential difference between the lightning strike point and a remote ground can reach 10 kV or more, as Figure 6 shows. For a more extensive discussion of GPR, see references (3) and (9)


Figure 6 Lightning GPR


5.0 Power over Ethernet (PoE) Versus Power over Data Line (PoDL) (1) PoE uses multiple paired conductors for power and data transmission (see (1) for

more details). PoDL uses a single twisted pair of conductors for both power and data transmission (see IEEE P802.3bu (2) and “A Quick Walk Around the Block with PoDL” by Dwelley, D. (2015) (1) for more detail). Long Reach (up to 1000 m) PoDL is commonly used in industrial applications. Where a PoDL powered device (PD) is installed in a manner that exposes it to lightning strikes and surges, the contents of this document would apply.

• In order to work, PoE requires (at least) two pairs connected between the pairs and the center tap.

• PoDL requires only one pair which is connected with a lowpass/high pass band splitting network. It works with single-pair Ethernet.

Figure 7 PoE and PoDL basic design after IEEE P8023bu (6)


6.0 Basic Installation Overview

A well-established practice is to install video surveillance cameras on building exteriors. Originally this required a coaxial cable and an AC power source for each camera. (See Figure 8) The AC electrical ground (ACEG) and the shield of the coaxial cable helped to mitigate the effects of lightning induced voltages.

Figure 8

Traditional Surveillance Camera Installation

With the advent of PoE powered devices, the need for a well shielded cable and an ACEG where eliminated. Lightning induced voltages may increase since a 24 AWG, non-shielded cable will have greater impedance than the combined coaxial cable shield and ACEG conductor. It should also be noted that the metallic shield on a twisted pair ethernet cable is an EMI shield and not designed to carry fault current. See Figures 9 through 11.

Coax to monitorAC Power


Figure 9

PoE Surveillance Camera

Figure 10

PoE Ethernet Radio


Figure 11

Pole Mounted PoE Security Cameras

7.0 Device Grounding

Electronic devices, powered using a PoE powering device, mounted in a manner that exposes them to a lightning strike or the effects of a nearby lightning strike, should be installed with proper grounding to help mitigate the effects of a direct or near by lightning strike. These devices should be grounded to the site grounding system per chapter 8 of the NFPA 70® (8). In most cases this should be the local AC service ground or the intersystem bonding termination device.

Equipment mounted on a pole separately from a building should have at least one grounding electrode placed at the base of the pole. This electrode should be bonded to the building grounding system with a bonding conductor sized per NFPA 70® (8) Article 250. See Figure 17 in this document for details.

Where the SPD is placed at a point greater than 20 conductor feet from the existing building ground, an additional grounding electrode should be placed at the SPD. The additional grounding electrode should be bonded to the existing ground with a minimum 6 AWG copper conductor. See Article 800 of the 2017 edition of the NFPA 70® (8) for additional information. Grounding conductors should be as short and straight as possible. All bend radii should be a minimum of 8 inches with an inclusive angle of more than 90 degrees.


8.0 Surge Protective Devices (SPD’s)

SPDs should be installed at both ends of any PoE circuit exposed to lightning. As explained in (7), if an SPD is only installed at one end of the exposed circuit during a lightning strike, there will be a voltage rise that is nearly equal to the maximum induced voltage, applied to the unprotected end of the circuit, once the SPD clamps. (See Figures 12 and 13) For this reason, SPDs should be installed as described in this document and as laid out in chapter 8 of NFPA 70® (8) and chapter 4 of NFPA 780® (9)

Cabling such as a twisted pair ethernet cable, should have an SPD installed at the external equipment and as close as practical to the point where the cable penetrates the structure’s exterior wall. The SPD should be listed by a nationally recognized test laboratory (NRTL) and rated for the appropriate data speed.

Figure 12 Results of a Single SDP

Figure 13

Results of Two SPD’s


9.0 Recommended Lightning Protection for Externally Mounted Devices

9.1 Security Cameras

Cameras, installed in an outdoor environment, should be grounded to the site grounding electrode(s) or bonded to the intersystem bonding termination device. Twisted pair ethernet cables should have a listed SPD, rated for the appropriate data speed, installed at the camera (see Figure 19) and as close as practical to the point the cable enters the structure. See Figures 14 through 17

For cameras mounted on a pole separate from a building, a single air terminal may be mounted on top of the pole above the camera(s) and grounded to the pole grounding electrode (per NFPA 780® (9) Annex J). See Figure 18

Figure 14

Long Ethernet Cable Exposure


Figure 15

Short Ethernet Cable Exposure/ Pitched Roof

Figure 16

Short Ethernet Cable Exposure/ Flat Roof


Figure 17 Pole Mounted Cameras

Figure 18 Pole Mounted Cameras With an LPS


Figure 19 Pole mounted Camera with SPD


9.2 Ethernet (Fixed Wireless) Radio Systems

PoE radio systems for Ethernet services have a remote radio head (RRH) placed on the exterior of the structure being served. These radios, when placed above the roof line of a structure without an LPS or placed above an LPS, are susceptible to damage from direct strikes. Proper grounding and the placement of SPDs should help limit the damage to these systems. See Figure 20

The Ethernet cables may have long exposures on the structure’s exterior depending on where the RRH’s are installed. Under certain conditions, adding a single air terminal (lightning rod) above the RRH may add additional protection. See Figure 21

See Figure 22 for buildings with an existing LPS. For information regarding the installation of a lightning protection system, refer to NFPA 780® (9).

Figure 20

Ethernet Radio Grounding and SPD Installation


FIGURE 21

Add Single Lightning Rod


Figure 22

Existing or Added Lightning Protection System

For structures with an LPS, the same rules for the zone of protection apply. Any PoE powered device should be installed in a manner that places it within the zone of protection. The LPS should be bonded to the local building ground and any device installed within 6 feet of an LPS conductor or air terminal should be bonded to the LPS system.


10.0 References

(2) Dwelley, D. (2015), “A Quick Walk Around the Block with PoDL” (3) IEEE 802.3TM series on techniques for transmitting power over Ethernet cabling. (4) Martin, A. R. (2011a). “A new TIA standard for equipment installations with two

or more separate grounds” ATIS PEG Conference 2011 (This Document is available from the Alliance for Telecommunications Industry Solutions (ATIS) <www.atis.org>).

(5) Martin, A. R. (2011b). “Lightning Damage to Equipment without a Metallic

Connection to an External Communications Service” In Compliance Magazine, September 2011, pp 40-47.

(6) Martin, A.R. (2016A), “Effects of Lightning on ICT Circuits: Induction and

GCR”, In Compliance Magazine, April 2016, pp36-46. (7) Martin, A.R. (2016B), “Loop Currents Caused by Lightning-Induced

Induction and GCR”, ATIS PEG Conference 2016 (This Document is available from the Alliance for Telecommunications Industry Solutions (ATIS) <www.atis.org>).

(8) Maytum, M.J. (2014), “Magnetically induced voltages and currents in

Ethernet cables due to lightning strokes.” (9) NFPA 70®, National Electrical Code® (NEC®). (10) NFPA 780®, Standard for the Installation of Lightning Protection Systems. (11) Yseboodt, L. and Abramson, D. (2018), “Overview of 802.3bt - Power over

Ethernet standard”.

Documents

Guide for Lightning Protection of Power over Ethernet (PoE