Surge Protection for Process Systems (Telematic)
While the importance of surge protection for industrial and process plant has been appreciated for many years, the rapid development of computerized control, monitoring, and security systems has made it even more imperative for modern process systems which recognize that any externally-cabled connections are possible sources of potentially damaging surges. Complete protection can only be provided by protecting all cable routes into plant structures.

The figure illustrates a typical industrial process layout and highlights those areas which are most at risk from lightning and surge damage. In areas that are most likely to pass induced currents into sensitive or critical equipment, surge protection should be considered essential.

The threat to process systems
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 a lightning conductors and ground rods as shown in the figure above. The conductor carries the very large strike current into the earth termination and dissipates the charge transfer 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 100kA and the conductor/ground impedance, is 10 ohms, 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 exists, posing little risk to personnel.

The field transmitter is 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 500V. This level of isolation now has to withstand the transient voltage between the new building reference potential and its local earth potential. Many transmitters are destroyed in this way, even though the actual lightning strike was to a protected building.

Loop protection
For complete protection, it is usually necessary to protect both ends of a loop, as any surge protection device can only provide local protection. SPDs control voltage and divert surge currents relative to their local earth points and therefore remote devices need their own individual protection. However, in average industrial plants many individual items of remote field instrumentation are relatively inexpensive and easy to replace, the cost of loop failure is not great, and the risk of damage from local surge currents is slight. Therefore, in these circumstances, it may be thought desirable to confine protection to the control room end of the loop where much more vital control equipment affecting the whole process (or a major part of it) may be at risk.

However, there are some areas of application where field instrumentation should be protected. These include loops which are vital to the process, field devices which are inherently expensive (such as some types of transmitters), and loops in which the field instrumentation is located in very remote or inaccessible locations. Major users of SPDs for remote field instrumentation include the utility companies which maintain what are often far-flung distribution and monitoring systems. For these companies it is both inconvenient and expensive to send an engineer many miles to replace fuses or failed sensors, so it is economically good sense for them to provide remote instrumentation with modern maintenance-free and auto-resetting SPDs.

Telematic supply some unique SPDs designed specifically for use with particular items of field instrumentation . These devices incorporate proven hybrid surge protection circuits and are designed for mounting within or on the instrumentation for which they are specified. Some specific applications for advanced surge protection technology from Telematic is discussed below.

Field SPDs for process transmitters
SPDs are not necessarily needed for transmitters when the loop is within a covered plant. However, if the transmitter variable is particularly vital to the process or if it is remote and unprotected by any surrounding steelwork, then protection is advisable. Transmitters on tall structures such as distillation columns are also vulnerable to high voltages between the case and the circuitry caused by lightning currents flowing down the structure, as also are transmitters located close to structural steelwork used as lightning conductors, an increasingly common practice. The transmitter illustrated in the Figure below would be subject to a voltage stress of more than 100kV between its case and the internal electronics. Any or all these factors must be taken into account when deciding if protection is desirable. 

SPDs for comprehensive transmitter loop protection
When a transmitter or other field-mounted equipment is protected by an SPD then the parallel paths created by the installation make it necessary also to protect the control-room end of the loop. The majority of installations take the form of the circuit shown in the figure below. The end-to-end resistance of the two suppression circuits is very low (less than 10 ohms in total) and hence does not appreciably affect the circuit operation. For example, with a 24V supply, a transmitter requiring a minimum of 12V and a computer requiring 5V, the available voltage for line resistance and other accessories is 7V, which is more than adequate for most applications.



The circuit shown in the above figure illustrates the use of an SD32 SPD at the control-room end to provide fuse protection to prevent a field short-circuit fault affecting the operation of the rest of the circuit sharing the common power supply. The circuit disconnect facility of the SD32 is useful for maintenance purposes. If this latter facility is not needed, then an SD32X (which does not include the replaceable fuse/disconnect link facility) can be used instead of the SD32.

The maximum supply voltage can be allowed to exceed 32V by a small margin since leakage current from the power supply rail to the 0V of the system does not affect the measurement accuracy. However, voltages in excess of 35V could blow the fuse. In a relatively small number of applications it is necessary to increase the loop voltage for operational reasons. This can be because of: extremely long land lines, additional equipment such as indicators or trip amplifiers at the transmitter end of the loop, or control-room equipment that needs a signal voltage of more than 5V.

In this case the Telematic SD55 can be deployed, to provide a maximum working voltage of 48V. If it is not necessary to be concerned about achieving a very low circuit current for detecting an open circuit transmitter, then a voltage higher than 48V can be applied. Operationally, the transmitter always consumes more than 4mA and the voltage drop created by this current can be used to increase the supply volts. Generally however, it is usually practical (and less complicated!) to use a supply voltage of 48V or less.

SPDs for use with vibration sensors
The 3-wire transmitters used with vibration monitoring equipment are invariably supplied by a 24V dc power supply, so the recommended Telematic SPD choice to protect the control-room end of the loop is an SD32 or SD32X unit. Where the probe and its driver must also be protected, then a suitable field-mounted SPD such as the Telematic mSA30/2 should be used. Direct connection of the field wiring to ground at more than one point is not recommended since the resulting circulating current will cause measurement problems. If it is considered desirable to 'isolate' the system from earth and all three wires need protecting then this can be done by using the 4-channel mSA30/2. Each channel has a resistance of 43 ohms and hence the most effective result is achieved by paralleling two channels and using them in the 0V line which is most affected by resistance.

SPDs for temperature sensors
Sensors commonly used for temperature measurement are relatively simple devices such as thermocouples (THCs) and resistance temperature detectors (RTDs). While these are hardly immune to damage and destruction caused by high-voltage transients and surge currents, the replacement cost is generally so low that protection for them in the field is rarely provided unless they are difficult to replace or the particular temperature measured is so vital to the process that the cost or consequences of any downtime makes the installation of an SPD worthwhile. In the control-room however, the receiving and control equipment is also liable to damage from surges and the replacement and downtime cost will almost certainly be more than enough to warrant the installation of an SPD.

Signals from temperature sensors of the type described are usually of low voltage and the end-to-end resistance of SPD channels is only significant for RTDs. Temperature measurement with RTDs is resistance sensitive to the extent that 3- and 4-wire RTD connections are used to eliminate the effects of lead resistance changes on the measurement resistance change. RTDs in protected circuits must be either 2-wire types (ie, RTDs which are not particularly inherently accurate and are therefore mainly suitable for use as an over-temperature trip) or a 4-wire type in which a constant-current source is used to compensate for variations in lead and SPD resistance. The working voltage selected for an SPD to protect instrumentation connected to field temperature sensors is not critical since the leakage specification voltage is likely to be orders of magnitude greater than the system operating voltage.

SPDs for temperature monitoring of large motors
Temperature monitoring of large motors is a case where SPDs should be specified to protect panel instrumentation from power faults and transients on the motor windings. The figure below illustrates a typical installation of this type in which a thermocouple is used for temperature sensing. If the thermocouple is insulated, then the transient potential between the thermocouple and the motor structure is determined by the current flowing through the structure and other return paths. The potential is therefore the supply voltage potentially divided between the return path impedance and the source impedance plus the fault voltage. Hence the return path must be of low impedance or the voltage developed can be high. With a 440V 3-phase motor, the 250V with respect to earth is likely to have a transient voltage of 100V or so until the protective network operates. On higher voltage motors, unless the fault current is restricted, the transient voltage is correspondingly higher and further precautions such as installing an SPD as shown are necessary to protect the instrumentation/monitoring circuits.



In general, Telematic SD07/SD07X SPDs are suitable for protecting THCs and RTDs in the field and SD16/SD16X SPDs for protecting the related control-room instrumentation.

SPDs for weighing installations
Weighbridges are frequently located in exposed conditions and the load cells associated with them are therefore susceptible to lightning-induced surges and it is advisable to protect both the 'field' load-cells as well as the associated control-room equipment. Telematic provides specialist SPDs (the LC30 system) designed for mounting under weigh-bridges and between silo legs and which are suitable for virtually all strain-gauge, load-cell, weighbridge cabins, silos, and process storage facilities.

The system covers working voltages up to 30V and handles maximum surge currents of 10kA. The LC30 system components and their applications are described in detail in Telematic Application Note1006, Surge protection for weighing systems (available for free download in the Departments area of this site).

SPDs for miscellaneous low-voltage analogue circuits
Apart from temperature sensors, other low-voltage analogue loop field devices which may need control-room and field protection include ac sensors, photocells, microphones and turbine flowmeters. Suitable SPDs for these are the Telematic SD07 and SD16 units. For slidewire displacement transducers, the recommended choices are usually the Telematic SD07 and SD07X.

SPDs for protecting instrumentation systems
Incoming signal cabling to electronic systems is usually by twisted pairs or co-axial lines. Because of the wide choice of applications, a variety of data transmission speeds and system characteristics, there are a number of choices of SPDs based on fast hybrid circuitry.

Apart from the near 'universal' DIN-mounting Telematic SD Series discussed elsewhere, other suitable ranges include busbar-mounting units ( Telematic 375 range) Eurocard 19-inch rack mounting devices (Telematic DP Series) and backplane-mounting units for close integration into DCSs (Telematic SP4300 Series). Most of these cover virtually the same range of applications as the SD Series. Additionally, there is the Telematic CA range of 'in-line' co-axial SPDs which insert into co-axial lines with minimal insertion loss and VSWR with wide bandwidth. The CA range is available with a wide choice of terminations to suit virtually all applications (including panel-bulkhead fitting).

SPDs for hazardous-area applications
Intrinsic safety is the most common and generally preferred technique of explosion protection for measurement and control instrumentation in process hazardous areas, ie, areas where a potentially explosive atmosphere may occur. Measurement Technology Limited (a sister company of Telematic Limited in the MTL Instruments Group) is the world’s leading supplier of intrinsically safe instrumentation and co-operates closely with Telematic in the development of surge protection devices suitable for use in protecting intrinsically safe circuits against lightning-induced transients or other high-power fault surges.

The technique of intrinsic safety works basically by ensuring that under all circumstances the amount of electrical power that can reach hazardous-area process equipment from safe-area control equipment is limited to a maximum of approximately 1W. To make sure this occurs, intrinsically safe (IS) interfaces are generally included in the control loop at the safe-area end. IS interfaces are of two kinds, either shunt-diode (ie, Zener) safety barriers or galvanic isolators. The former shunt fault currents to earth while the latter, as the name suggest, 'isolate' fault currents. Safety barriers are less expensive but isolators have the advantage that they can incorporate additional signal processing circuits to provide a double function.

This brief introduction to surge protection in IS circuits has not touched on the various IS certification requirements of national and international standards authorities which can affect the implementation of surge protection in these circuits, nor the stringent earthing requirements (except in the USA!) imposed upon surge protection systems. Those with a particular interest in this application are advised to read Telematic Application Note (TAN) 1004, Surge protection for intrinsically safe systems and also TAN1005, Surge suppression for Zone 0 locations. All Telematic Application Notes are available for free download in the Departments area of this web site.

SPDs for telemetry systems
As the first providers of distributed cabled networks (much of it out-of-doors and covering long distances) telephone systems were also one of the first major users of surge protection devices, hence the application of surge protection to telemetry is widely accepted and, generally, well understood.

Many telemetry systems use telephone lines (either private 'dedicated' or public dial-up) for signal transmission and SPDs used for these applications must be approved by a PTT (Post Telephone and Telegraph Organization) such as BT. Bearing in mind the openly distributed nature of many telemetry installations, it is clearly advisable to protect equipment at both ends of the line and Telematic has borne this in mind with the product range available for these applications.

Telephone systems use fairly high dc voltages for line supply and bell operation. Typical system working voltages are of the order of 40 to 50V dc. In the UK, ringing voltages are 120 to 140V but some systems can impose ringing voltages of up to 270V. Electronic telecommunications equipment includes 'subscriber line interface circuits,' which have voltage withstands of the order of 60V or so. SPD’s used in public telephone systems are frequently required to be approved by a national post, telephone and telegraph (PTT) organization. In the U.K. this was the British Approvals Board for Telecommunications (BABT) until the recent liberalization of the requirements.

Standard Zener or surge diodes with breakdown voltages of the order of 180V can provide clamping of transient surges but the power dissipation in the component is high and leads either to an unacceptably high cost or to a reduced life expectancy for the network. To solve this problem, 'foldback' diodes have been designed which behave as conventional Zener diodes below a critical voltage known as the ‘voltage breakdown level’ or VBR, i.e., a small amount of reverse leakage current. Above VBR, the device begins to conduct very rapidly, the changeover taking place in picoseconds (10^-12 seconds). With a conventional Zener diode, as voltage increases across it, current increases through it with a slope resistance of typically 1 or 2 ohms. With foldback diodes however, the voltage across the unit collapses to a much lower value when the current is flowing through it, thereby significantly reducing the internal power dissipation.

Specialist SPDs for telemetry applications include the DP200/4 SPD and the PX SPDs - both of which are PTT approved. The former is designed for installation between line jacks and telephone socket outlets to protect equipment such as fax machines, modems, extension telephones, etc; while the latter units are designed to protect local PABX exchanges against surges on incoming lines. All these units handle surge currents up to 10kA.

Assessing protection requirements
In determining protection needs, it is necessary to balance the relative cost of providing protection against the probability of damage and the costs and consequences of such damage. Except in very exceptional circumstances, the possibility of a lightning strike directly hitting electronic instrumentation is usually discounted. Extreme cases such as wind gauges on the highest point of an offshore rig are an example of a conspicuous exception. In such a case, the gauge itself is destroyed but the equipment connected to it can be protected by suitable SPDs.

The principal assessment factors are:



• The risk of lightning-induced or other surges occurring on interconnecting cables.

• The cost of damage to equipment directly or indirectly connected to the cables. This should include an assessment of the availability of spares and the accessibility of the equipment should repairs be necessary.

• The consequential cost of downtime caused before damage can be rectified, such as loss of production or work in progress on a computer system.

• The safety implications of damage. This factor is frequently difficult to assess in purely financial terms if there is the possibility of human injuries or fatalities. Thus, emergency shut-down (ESD) systems and fire alarm monitors are typical of the systems which call for a high degree of protection for safety reasons.

SPDs for comprehensive transmitter loop protection
When a transmitter or other field-mounted equipment is protected by an SPD then the parallel paths created by the installation make it necessary also to protect the control-room end of the loop. The majority of installations take the form of the circuit shown in the figure below. The end-to-end resistance of the two suppression circuits is very low (less than 10 ohms in total) and hence does not appreciably affect the circuit operation. For example, with a 24V supply, a transmitter requiring a minimum of 12V and a computer requiring 5V, the available voltage for line resistance and other accessories is 7V, which is more than adequate for most applications.





The circuit shown in the above figure illustrates the use of an SD32 SPD at the control-room end to provide fuse protection to prevent a field short-circuit fault affecting the operation of the rest of the circuit sharing the common power supply. The circuit disconnect facility of the SD32 is useful for maintenance purposes. If this latter facility is not needed, then an SD32X (which does not include the replaceable fuse/disconnect link facility) can be used instead of the SD32.

The maximum supply voltage can be allowed to exceed 32V by a small margin since leakage current from the power supply rail to the 0V of the system does not affect the measurement accuracy. However, voltages in excess of 35V could blow the fuse. In a relatively small number of applications it is necessary to increase the loop voltage for operational reasons. This can be because of: extremely long land lines, additional equipment such as indicators or trip amplifiers at the transmitter end of the loop, or control-room equipment that needs a signal voltage of more than 5V.

In this case the Telematic SD55 can be deployed, to provide a maximum working voltage of 48V. If it is not necessary to be concerned about achieving a very low circuit current for detecting an open circuit transmitter, then a voltage higher than 48V can be applied. Operationally, the transmitter always consumes more than 4mA and the voltage drop created by this current can be used to increase the supply volts. Generally however, it is usually practical (and less complicated!) to use a supply voltage of 48V or less.

SPDs for use with vibration sensors
The 3-wire transmitters used with vibration monitoring equipment are invariably supplied by a 24V dc power supply, so the recommended Telematic SPD choice to protect the control-room end of the loop is an SD32 or SD32X unit. Where the probe and its driver must also be protected, then a suitable field-mounted SPD such as the Telematic mSA30/2 should be used. Direct connection of the field wiring to ground at more than one point is not recommended since the resulting circulating current will cause measurement problems. If it is considered desirable to 'isolate' the system from earth and all three wires need protecting then this can be done by using the 4-channel mSA30/2. Each channel has a resistance of 43 ohms and hence the most effective result is achieved by paralleling two channels and using them in the 0V line which is most affected by resistance.

SPDs for temperature sensors
Sensors commonly used for temperature measurement are relatively simple devices such as thermocouples (THCs) and resistance temperature detectors (RTDs). While these are hardly immune to damage and destruction caused by high-voltage transients and surge currents, the replacement cost is generally so low that protection for them in the field is rarely provided unless they are difficult to replace or the particular temperature measured is so vital to the process that the cost or consequences of any downtime makes the installation of an SPD worthwhile. In the control-room however, the receiving and control equipment is also liable to damage from surges and the replacement and downtime cost will almost certainly be more than enough to warrant the installation of an SPD.

Signals from temperature sensors of the type described are usually of low voltage and the end-to-end resistance of SPD channels is only significant for RTDs. Temperature measurement with RTDs is resistance sensitive to the extent that 3- and 4-wire RTD connections are used to eliminate the effects of lead resistance changes on the measurement resistance change. RTDs in protected circuits must be either 2-wire types (ie, RTDs which are not particularly inherently accurate and are therefore mainly suitable for use as an over-temperature trip) or a 4-wire type in which a constant-current source is used to compensate for variations in lead and SPD resistance. The working voltage selected for an SPD to protect instrumentation connected to field temperature sensors is not critical since the leakage specification voltage is likely to be orders of magnitude greater than the system operating voltage.

SPDs for temperature monitoring of large motors
Temperature monitoring of large motors is a case where SPDs should be specified to protect panel instrumentation from power faults and transients on the motor windings. The figure below illustrates a typical installation of this type in which a thermocouple is used for temperature sensing. If the thermocouple is insulated, then the transient potential between the thermocouple and the motor structure is determined by the current flowing through the structure and other return paths. The potential is therefore the supply voltage potentially divided between the return path impedance and the source impedance plus the fault voltage. Hence the return path must be of low impedance or the voltage developed can be high. With a 440V 3-phase motor, the 250V with respect to earth is likely to have a transient voltage of 100V or so until the protective network operates. On higher voltage motors, unless the fault current is restricted, the transient voltage is correspondingly higher and further precautions such as installing an SPD as shown are necessary to protect the instrumentation/monitoring circuits.



In general, Telematic SD07/SD07X SPDs are suitable for protecting THCs and RTDs in the field and SD16/SD16X SPDs for protecting the related control-room instrumentation.

SPDs for weighing installations
Weighbridges are frequently located in exposed conditions and the load cells associated with them are therefore susceptible to lightning-induced surges and it is advisable to protect both the 'field' load-cells as well as the associated control-room equipment. Telematic provides specialist SPDs (the LC30 system) designed for mounting under weigh-bridges and between silo legs and which are suitable for virtually all strain-gauge, load-cell, weighbridge cabins, silos, and process storage facilities.

The system covers working voltages up to 30V and handles maximum surge currents of 10kA. The LC30 system components and their applications are described in detail in Telematic Application Note1006, Surge protection for weighing systems (available for free download in the Departments area of this site).

SPDs for miscellaneous low-voltage analogue circuits
Apart from temperature sensors, other low-voltage analogue loop field devices which may need control-room and field protection include ac sensors, photocells, microphones and turbine flowmeters. Suitable SPDs for these are the Telematic SD07 and SD16 units. For slidewire displacement transducers, the recommended choices are usually the Telematic SD07 and SD07X.

SPDs for protecting instrumentation systems
Incoming signal cabling to electronic systems is usually by twisted pairs or co-axial lines. Because of the wide choice of applications, a variety of data transmission speeds and system characteristics, there are a number of choices of SPDs based on fast hybrid circuitry.

Apart from the near 'universal' DIN-mounting Telematic SD Series discussed elsewhere, other suitable ranges include busbar-mounting units ( Telematic 375 range) Eurocard 19-inch rack mounting devices (Telematic DP Series) and backplane-mounting units for close integration into DCSs (Telematic SP4300 Series). Most of these cover virtually the same range of applications as the SD Series. Additionally, there is the Telematic CA range of 'in-line' co-axial SPDs which insert into co-axial lines with minimal insertion loss and VSWR with wide bandwidth. The CA range is available with a wide choice of terminations to suit virtually all applications (including panel-bulkhead fitting).

SPDs for hazardous-area applications
Intrinsic safety is the most common and generally preferred technique of explosion protection for measurement and control instrumentation in process hazardous areas, ie, areas where a potentially explosive atmosphere may occur. Measurement Technology Limited (a sister company of Telematic Limited in the MTL Instruments Group) is the world’s leading supplier of intrinsically safe instrumentation and co-operates closely with Telematic in the development of surge protection devices suitable for use in protecting intrinsically safe circuits against lightning-induced transients or other high-power fault surges.

The technique of intrinsic safety works basically by ensuring that under all circumstances the amount of electrical power that can reach hazardous-area process equipment from safe-area control equipment is limited to a maximum of approximately 1W. To make sure this occurs, intrinsically safe (IS) interfaces are generally included in the control loop at the safe-area end. IS interfaces are of two kinds, either shunt-diode (ie, Zener) safety barriers or galvanic isolators. The former shunt fault currents to earth while the latter, as the name suggest, 'isolate' fault currents. Safety barriers are less expensive but isolators have the advantage that they can incorporate additional signal processing circuits to provide a double function.

This brief introduction to surge protection in IS circuits has not touched on the various IS certification requirements of national and international standards authorities which can affect the implementation of surge protection in these circuits, nor the stringent earthing requirements (except in the USA!) imposed upon surge protection systems. Those with a particular interest in this application are advised to read Telematic Application Note (TAN) 1004, Surge protection for intrinsically safe systems and also TAN1005, Surge suppression for Zone 0 locations. All Telematic Application Notes are available for free download in the Departments area of this web site.

SPDs for telemetry systems
As the first providers of distributed cabled networks (much of it out-of-doors and covering long distances) telephone systems were also one of the first major users of surge protection devices, hence the application of surge protection to telemetry is widely accepted and, generally, well understood.

Many telemetry systems use telephone lines (either private 'dedicated' or public dial-up) for signal transmission and SPDs used for these applications must be approved by a PTT (Post Telephone and Telegraph Organisation) such as BT. Bearing in mind the openly distributed nature of many telemetry installations, it is clearly advisable to protect equipment at both ends of the line and Telematic has borne this in mind with the product range available for these applications.

Telephone systems use fairly high dc voltages for line supply and bell operation. Typical system working voltages are of the order of 40 to 50V dc. In the UK, ringing voltages are 120 to 140V but some systems can impose ringing voltages of up to 270V. Electronic telecommunications equipment includes 'subscriber line interface circuits,' which have voltage withstands of the order of 60V or so. SPD’s used in public telephone systems are frequently required to be approved by a national post, telephone and telegraph (PTT) organisation. In the U.K. this was the British Approvals Board for Telecommunications (BABT) until the recent liberalisation of the requirements.

Standard Zener or surge diodes with breakdown voltages of the order of 180V can provide clamping of transient surges but the power dissipation in the component is high and leads either to an unacceptably high cost or to a reduced life expectancy for the network. To solve this problem, 'foldback' diodes have been designed which behave as conventional Zener diodes below a critical voltage known as the ‘voltage breakdown level’ or VBR, i.e., a small amount of reverse leakage current. Above VBR, the device begins to conduct very rapidly, the changeover taking place in picoseconds (10^-12 seconds). With a conventional Zener diode, as voltage increases across it, current increases through it with a slope resistance of typically 1 or 2 ohms. With foldback diodes however, the voltage across the unit collapses to a much lower value when the current is flowing through it, thereby significantly reducing the internal power dissipation.

Specialist SPDs for telemetry applications include the DP200/4 SPD and the PX SPDs - both of which are PTT approved. The former is designed for installation between line jacks and telephone socket outlets to protect equipment such as fax machines, modems, extension telephones, etc; while the latter units are designed to protect local PABX exchanges against surges on incoming lines. All these units handle surge currents up to 10kA.

Assessing protection requirements
In determining protection needs, it is necessary to balance the relative cost of providing protection against the probability of damage and the costs and consequences of such damage. Except in very exceptional circumstances, the possibility of a lightning strike directly hitting electronic instrumentation is usually discounted. Extreme cases such as wind gauges on the highest point of an offshore rig are an example of a conspicuous exception. In such a case, the gauge itself is destroyed but the equipment connected to it can be protected by suitable SPDs.  The principal assessment factors are:

• The risk of lightning-induced or other surges occurring on interconnecting cables.

• The cost of damage to equipment directly or indirectly connected to the cables. This should include an assessment of the availability of spares and the accessibility of the equipment should repairs be necessary.

• The consequential cost of downtime caused before damage can be rectified, such as loss of production or work in progress on a computer system.

• The safety implications of damage. This factor is frequently difficult to assess in purely financial terms if there is the possibility of human injuries or fatalities. Thus, emergency shut-down (ESD) systems and fire alarm monitors are typical of the systems which call for a high degree of protection for safety reasons.