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Industrial Ethernet Book Issue 65 / 33
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Out in a flash: networks, power surges and lightning strikes

Electronic systems can be damaged or disrupted by transient over-voltages that are much greater than the normal working voltage and which appear in a LAN for a short time. The most potent source is lightning. Chris Ground writes on lightning-induced transients, networks and their protection.

DIRECT LIGHTNING strike can cause catastrophic damage, but this is relatively rare. Far commoner is substantial damage to electronic components inflicted by a strike to ground within 1km. This can produce a surge on cables feeding vulnerable electronic equipment, resulting in damage to ICs and semiconductors. Surge protection devices (SPDs) limit surge voltages reaching protected equipment to a safe level.

SPDs are applied to network and other signal cables including telephone lines and the mains power supply. In networks, surge protection is concerned exclusively with the OSI physical layer, so SPD considerations include signal voltages, impedances, losses and bandwidth, connectors and cable types.

BS 6651: 1999, Appendix C, General advice on protection of electronic equipment within or on structures against lightning, identifies risk factors that affect equipment damage probability. These are frequency of lightning strikes to ground, size and exposure of building, soil resistivity, the number and length of ALL copper cables entering a building, and equipment vulnerability.

For small networks, it may seem that fitting SPDs is not cost effective. However, bear in mind the potential cost of damaged hardware, downtime and lost data. Many insurance companies now advise surge protection.

Surge threat

The salient issues are mains power supply transients, plus supply switching and faults. Ground potential surge is probably the greatest threat of damage to the largest number of networks. A lightning strike to ground results in current flows of typically tens of kA through surface layers, but 100kA to 200kA are worstcase direct strikes. Such currents can develop extremely large potentials, typically 1MV across some metres of ground. This is the ground potential surge - two buildings near a ground strike but separated can briefly have a large potential difference between them. If the buildings' electrical systems are entirely within their structures, this is not a problem. If, however, a cable such as a LAN cable links the buildings, equipment in one will be subjected to the relative ground potential of the other, which may cause damage (Fig.1).


Fig.1: Ground potential surge caused by a cloud-toground strike. If a LAN cable links the buildings, equipment in one will be subjected to the relative ground potential of the other, possibly causing damage.

It is, therefore, prudent to fit SPDs when cables longer than a few metres link buildings. ALL copper cables taking signals from points not tied to the building's electrical earth carry threat of damage from the ground potential surge; surge protection should be considered for each.

Direct strike

Direct strike happens relatively rarely, yet for tall buildings, the assumption must be that it will happen. With a direct lightning strike, the full strike current flows through the building's lightning conductor, resulting in possible sideflash. A voltage of 1.5MV can be developed along a 30m lightning conductor, so damage may be severe. A metal-skinned building - with all parts bonded - provides good protection; those with large glass areas are not so good. A less extreme case involves voltages induced by capacitive or inductive coupling (Fig.2, 3). Reduce the effect by running cables within earthed metallic ducting.


Fig.2: Capacitive and inductive coupling to building internal wiring. Reduce the effect by running cables within earthed metallic ducting.


Fig. 3. How a side flash can arise. Lightning strikes an air termination and travels down a conductor on the surface of the building. The voltage across a length of cable due to a rapidly changing current flowing through it depends on the rate of change of current and the cable's self inductance. Calculation indicates that a voltage up to 1.5MV may be developed along a 30m lightning conductor in some circumstances.

BS 6651, Appendix C, section C.7.2.1 advises locating electronic equipment towards the centre of the building. Avoid top floor near air terminations. Data and power lines should ideally be run in adjacent ducts to minimise loop areas.

So far, the main concentration has been on surges entering equipment via LAN signal cables. However, the mains power supply presents another route, and a direct strike to a HV power line can produce a damaging mains power supply surge.

SPD installation

SPDs are only effective if correctly installed. First, the guidelines for a network having LAN cables run between buildings that have either structural lightning protection fitted, or for which direct strike risk is negligible:

LAN cable routing - Run cables that pass between buildings close to the mains power distribution boards from which the electrical supply earth for each building is derived.

Fit a network SPD in the LAN cable close to the mains power distribution board in each building. Each building should be treated alike, since the concern is with potential differences.

Earth the SPD at the mains power distribution board with the shortest possible length of cable with a minimum CSA of 2.5mm2. Better still, use several cables electrically in parallel, spaced apart. Best of all, fit the SPD on earthed metal panels.

Apart from a direct building strike, the result will be protection for the entire LAN cable in the building, using only one LAN SPD / building. If the LAN cable cannot be routed as described, the entire network cannot be protected with such great confidence. Individual hardware items can be fully protected, however, with lesser protection offered to the rest (Fig. 4).


Fig.4: The use of SPDs to protect individual items of LAN equipment. Use short connections to the equipment earth point, and pay particular attention to the integrity of the earth bonding

When a LAN cable is subject to a surge, SPDs protect the equipment to which they are attached. However, depending on wiring layout, the current pulse generates an inductive transient voltage, possibly causing damage to other devices Even so, the network will still benefit from fitting even partial protection.

LAN SPDs

Some LANs have cable data rates ten times that of standard Ethernet, or more. Fibre is one answer, but is expensive, so modern copper cable systems have been developed. SDPs are available for use on unshielded twisted pair cables carrying data at up to and beyond 100Mbps. Protection is provided for all four pairs of standard cable, including Cat 5. Some types are single units, protecting one cable, while others are 32-way rackmountable units protecting up to 64 cables.

Selecting an SPD requires consideration of cable and connector type, signal levels, system impedance, allowable attenuation and/or series resistance, data rate / bandwidth required, earthing arrangement, size constraints and installation method. A checklist should be made using risk analysis. Consider fitting SPDs to the following cables: mains power electricity supply; telephone lines feeding modems; LAN data; telemetry/instrumentation control; antenna; security camera; and outdoor lighting.

Installation

Cables routed close to the building earth connection at the distribution board where the mains power electrical supply enters the building:

Fit SPDs close to this earth connection on earthed metalwork.

Earth cable from SPD to the supply earth to be as short as possible (CSA 2.5mm2 recommended).

Cables routed remote from the building earth connection:

Fit SPD(s) to the relevant cable(s) close to this equipment's earth.

SPD earth cables to be as short as possible (CSA 2.5mm2 recommended).

Fit a mains power SPD.

The equipment should then be fully protected, the rest partially protected. SPDs are maintenance-free for around 20 years' life, but periodically check that the earth connections are sound. Note that LAN cable testing using time domain reflectometry (TDR) should be carried out without SPDs installed in the cabling.

Chris Ground is Product Line Manager at MTL Instruments, Luton, UK


www.mtl-inst.com
Source: Industrial Ethernet Book Issue 65 / 33
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