In the first part of this series, George Georgevits explained lightning, what causes it and the results of a lightning strike. He now outlines its effects on
communications circuits – and how to mitigate them.

Fortunately, most lightning strokes occur between clouds; unfortunately, associated electromagnetically induced surges can seriously damage unprotected nearby communications circuits.

The danger is very real. Many years ago, in inner-eastern Sydney, it was common for our old rotary dial telephone bell to sound a single ‘ding’ in unison with lightning flashes. And back then all telephone circuits in the area were buried, most in lead-sheathed cables.

LIGHTNING INDUCED SURGES

The nature of lightning and its effects on communications (and power) circuits has been the subject of extensive research for many years.

Simulation models for surges on communications circuits have been developed and generally agreed by the various international Standards bodies (e.g. CCITT, IEC, ANSI).

These models define the voltage and current waveforms (shape, rise time, peak value, etc.) to be used for simulating lightning induced surges in cables. They also provide recommended designs for the various impulse generators required to generate the waveforms.

It is worth noting that the waveforms induced into communications circuits are not the same shape as those occurring in the lightning stroke. There are time constants associated with the parasitic components in the cables (e.g. pair to ground capacitances, coupling inductances, etc.) and these tend to act like a low pass filter on the induced surges.

By using waveforms defined in the Standards, protection devices and circuits may be evaluated in a controlled environment. This also provides a level of confidence that protection devices will do the job under most conditions likely to be encountered in the field.

The test waveforms are specified in terms of exponential rise and fall times (in microseconds), the rise time being specified from 10% to 90% of peak value, and the fall time from peak value to 50% of peak value.

For example, an 8/20 waveform is used for induced current, meaning 8ms rise time and 20ms fall to half the peak value. The peak amplitude is adjusted to suit the specified power-handling capability of the device under test.

With communications circuits, it is important to understand that the emphasis is on protecting against induced surges rather than against direct strikes. The energy in a typical lightning stroke is so great and the incidence of a direct hit is so infrequent (particularly for buried cables) that strike protection is generally not viable or necessary.

SURGES AS INTERFERENCE SOURCES

Surges appearing in communications circuits can be regarded as a type of interference or noise, albeit harmful and sometimes even injurious.

The mechanisms for coupling with communications circuits are much the same as in other types of interference, the main differences being that voltage levels are usually much higher and the duration is generally short.

The effect of surges on communications circuits can be to:
> cause injury (electric shock, acoustic shock, etc.), and in rare cases even death;
> damage the cable and/or terminating equipment (e.g. fire, damage to components, etc); and
> prevent the circuit from functioning correctly for the duration of the surge (interference).

INTERFERENCE MECHANISMS

Interference can affect communications circuits via electric field coupling (electrostatic induction), magnetic field coupling (electromagnetic induction) and direct contact (conduction). Different mitigation measures apply to each of these interference types.

INDUCED SURGES

Induced surges have diff erent properties to surges caused by direct contact.

In particular, surges in twisted pair communications circuits can be induced only in common or longitudinal mode.

This means that the surge voltage (and current) induced on each leg of the circuit will be roughly the same in amplitude and polarity.

As the surge travels away from the source of induction and along the circuit in both directions, physical irregularities and imperfections in the cable pair will convert some of this longitudinally induced surge to a differential voltage.

If the terminating equipment at each end of the circuit is not well balanced, it too will convert some of the longitudinal surge into a differential mode signal. In general, the longitudinal surge will cause the damage. Most communications circuits are balanced to better than 1% (40dB). Thus the differential mode surge generated due to mode conversion will always be much smaller than the longitudinal mode surge. The effect of differential mode surges will generally be limited to causing interference, possibly preventing the circuit from functioning for the duration of the surge.

CONDUCTED LIGHTNING SURGES

As mentioned, direct lightning strikes on communications cables are relatively rare.

However, conducted interference in communications cable circuits does arise more frequently as a result of nearby strikes to ground.

When lightning strikes the ground, very large currents flow away from the strike point, along the ground surface and also into the body of the earth.

By Ohm’s Law, paths of least resistance are favoured. The strike current sets up an enormous potential rise gradient area in the vicinity of the strike point (remember that typical peak currents of about 30,000A can be expected, and the ground is usually not all that conductive).

This gradient area diminishes exponentially with distance from the strike point.

The insulation on a communications cable passing through such a high-voltage gradient area may easily break down, as different parts of the cable will be at vastly different potentials.

Outdoor telecommunications cable sheaths typically have a breakdown strength of 20-30kV. Individual conductors break down at 3-5kV, depending on cable type.

The net result of such breakdown will be to conduct some of the lightning current away along affected copper communications circuits. Also, the amount of current entering each leg of each pair may well be very different, depending on which conductors break down first.

In such instances, the cable will usually suffer permanent pinhole sheath damage, as well as probable carbon tracking between conductors. Rain or flood may cause more damage in future due to water ingress, depending on cable type (filled or unfilled, pressurised, etc) and the severity of the sheath damage.

As another example, if an exchange building is struck, the building’s earth potential will rise for the duration of the stroke. This potential rise is fed out on all copper communications circuits leaving the exchange, either via the attached lightning protection devices (e.g. gas arresters that will work backwards, which is what happens when the earth potential rises) or via the centre tap on the balanced pair line feed transformers.

SURGE PROTECTION TECHNIQUES

Surge protection can be accomplished by shielding (for induced surges) and/or fi tting suitable surge protection devices at appropriate points along each circuit.

Shielding is usually cost effective only for the purpose of electrostatic shielding on cables. This is accomplished by providing a continuous metallic cover that envelops the entire cable run.

For electric field interference, the shield is not required to carry appreciable current, so shielding can be achieved simply by providing a conductive foil that envelopes all pairs in the cable. The grounding of this foil at only one end of the cable run is sufficient to provide an effective electrostatic screen, and it prevents detrimental circulating currents from fl owing.

Effective electromagnetic shielding is more difficult to achieve, particularly for low frequencies (e.g. for 50Hz power line interference, as may occur during the phase to earth when a nearby power line is struck by lightning).

To shield effectively against electromagnetic induction, the communications cable will require:

> some form of enveloping ferromagnetic material (such as steel tape armouring);
> running in a galvanised iron conduit; or
> a very low resistance conductive sheath that is well earthed at regular intervals (e.g. lead sheath).

If the cable is a plastic jacketed moisture barrier type, one or more thick copper guard wires buried directly beside the cable for the length of the run will also serve the purpose.

SURGE PROTECTION

Various surge protection techniques and devices are available for protecting communications and power circuits. The next article will examine them in detail, along with the need for earthing (most surge protection devices require a good earth to operate eff ectively). We will also briefl y outline the basic principles behind earthing system design and construction.