Application note – basic principles.
Abstract
This application note discusses the nature of the threat to electronic instrumentation and communications networks posed by voltage surges induced by lightning or other causes.
The practical application of surge protection devices (SPDs) designed to prevent damage from such sources is described. It is the intention to discuss suitable techniques to protect electronic circuits and equipment from high voltages and surge currents induced by lightning and other forms of transients.
The need for surge protection
Most process control or telemetry installations are interconnected by power and signal cables which run on trays, in ducting or via overhead poles. Lightning strikes, static discharges and induction from power cabling are typical sources of transient voltages which can be coupled into signal cables and hence transmitted to electronic equipment. Field transmitters, computer terminals, etc, containing low-power semiconductor devices can be damaged by over voltages of only tens of volts. The longer the cables, the more frequent the occurrence of high voltage transients through shifts in ground potential, so devices controlling or monitoring events in remote locations are more likely to suffer from over voltages and consequent component failures. Significant damage can also be found in equipment connected by relatively short cables if the circuits or components are particularly sensitive – as is the case for computer data communication ports.
As an illustration, consider the effects of a lightning strike to a building housing control and telemetry equipment, of which the fabric is protected from a direct strike by lightning conductors and ground rods. The conductor carries the very large strike current to ground where the charge transfer is dissipated into the mass of the earth.
The effect of this current is to elevate the reference potential at the building. For example, if the strike current is 100 kA and the conductor/ground impedance is 10 Ω, then the potential above ground is 1 million volts. Exposed metalwork within the building is bonded to the same reference potential and so only small voltage differences exist that pose little risk to personnel.
However, consider a field transmitter pole-mounted away from the control building but connected to the telemetry electronics by signal cabling. Most transmitters incorporate some level of isolation from structural earth, typically 500 V. This level of isolation now has to withstand the transient voltage between the new building reference potential and its local earth potential. Many transmitters can be destroyed in this way, even though the actual lightning strike was to a protected building.
Readers wishing to find out more about surge protection devices and strategies can view the full application note at: http://instrumentation.co.za/+C13509
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