Electrical Airbag
Protects Field Devices and Control Systems
The control and instrumentation system cable is usually distributed over wide areas of buildings and open spaces. The result is that the advantages of the increasing networking of systems and the unlimited flow of information are also subject to risks. Just as the usable data is propogated via data networks, undesirable occurrences, such as overvoltages, follow the same track, without regard to zone divisions and can spread throughout the control and instrumentation installations.
When damage occurs, an entire process plant can be rendered non-operational in fractions of a second. Far-sighted plant operators do not trust to luck and protect their systems by taking suitable precautions.
Overvoltage Suppressors are Electrical Airbags
Limiting the Energy of Sparks and Surface Temperature Effects
In all areas of process engineering, control and instrumentation signals are protected through “Intrinsic safety” in accordance with IEC 60079-11 and EN 50020. Intrinsic safety prevents the occurrence of sparks in a potentially explosive atmosphere by limiting the amount of energy generated in the circuit. Thus, electrical sparks are prevented from occurring and also the surface temperatures of connected components are kept below those that are likely to cause danger and well below the respective “Minimum ignition energy”.
This technology of energy limitation is supported by the decreasing signal levels used in communicating with modern sensors and actuators. Progressive miniaturisation means that in future the signal levels will be decreased still further and the proportion of intrinsically safe signal circuits will increase.
Overvoltage is Often "Home-made"
Overvoltages are frequently held responsible for system failures and damage to hardware on sensitive electronic assemblies. The reasons and sources of the overvoltages are found from time to time in the operating sequence of the system itself. For example, electromagnetic fields arise due to charge transfers in the processing of foils and during the conveying of granulates and solvents. The high field intensities discharge on exposed objects and then reach the electronic assemblies via signal cables. Likewise, transients occur during electro-welding, switching of large consumer items or inductive loads, which propagate along the cables in the system.
Since the user can prevent the occurrence of the interference signals named above through suitable physical precautions applied directly at their source, the real risk and potential for danger lies in the direct or indirect effects of a lightning strike.
Lightning Strikes are Unavoidable
The consequences of a lightning strike may assume frightening proportions for the business involved in terms of the cost of plant downtime. Fault elimination and re-commissioning can quickly lead to enormous expense.
Due to the current lightning protection standard with lightning conductors, building precautions such as Faraday cages and quality equipotential bonding, the direct lightning strike has long since lost its primeval terror, but for unprotected electronic components it remains a rapid and frequently silent cause of their demise.
Around 700,000 Lightning Strikes Recorded in Germany Every Year
Exposed locations, widely spread plant and signal transfer networks outside and between buildings are particularly prone to their effects. Counted among these threatened installations are oil and gas pipelines, refineries, tank storage facilities, clarification plant and tanker vehicle loading/unloading terminals. During a lightning strike, in the ideal case, the lightning conductor conducts the entire energy of the discharge to earth. The complete sequence lasts around 0.3 ms at an average current intensity of 45 kA and with voltages of up to 400 MV.
Since the earth potential is incapable of accepting the total discharge within this short period of time, an electrical saturation effect takes place in the case of approx. 50 % of the discharge, in the immediate vicinity of the primary strike. This saturation then hinders further equipotential bonding via the direct path. The remaining 50 % of the energy now has to seek a path to other and more distant earth potential that remains able to accept it, via cables and conducting parts of the system.
Danger from Induced Secondary Currents
All conducting constructional parts, the process pipe work and, naturally, the electrical cabling, function as an inductive or galvanic coupled network for the transportation of the remaining energy. Despite poor coupling factors and large distances, relatively high induction currents occur due to the enormous amount of energy involved in the lightning strike.
The energy transfer intensifies due to the extremely short rise times and resulting physical coupling effects. The voltage in the lightning channel increases by up to 12 kV in 1 µs, whilst the current increases by up to 200 kA.
Such a rapid “signal change” from 0 to 1, associated with the high current intensity, induces secondary currents up to 5 kA and voltages of 10 kV in the electrical cables in the vicinity. Energy of this order of magnitude is now present on cables, which were not primarily affected by the initial lightning strike. This even includes cables within the Faraday cage-protected area and, not least, those that terminate in expensive electronic assemblies. Since this is now a matter of signals on cables, the distances involved no longer have much part to play and as a consequence, the whole plant system must be classed as a hazardous area.
Both Lead Ends Transmit Overvoltage into the Connected Devices
If a transient pulse is induced on one sensor lead, both leads will be affected. Simultaneously, it reaches the sensor and the process control system signal input. The possible consequences range from the destruction of the sensor to the destruction of the input assembly and even to the destruction of the entire control system. An effective protection against this eventuality can be achieved by means of overvoltage conductors. They conduct the interference signal to earth and limit the voltage for the duration of the interference.
Overvoltage Conductors:
the "Airbag" for Control and Instrumentation Circuits
A comparison with the well known airbag is close to reality: Like the airbag, the overvoltage conductor “springs into action” as soon as it is needed. On the other hand, both have to be invisible in “standby” mode, i.e. they have quasi no part to play. And in both cases these devices are required to trigger in fractions of a second when a critical threshold is reached.
However, the overvoltage conductors from Pepperl+Fuchs have a clear advantage compared with the airbag: after having triggered, the overvoltage conductor self-reverts to ‘Standby’ mode, is again ‘invisible’ to instrumentation signals and waits to be called into action again. The overvoltage conductors are optimised for the special requirements of intrinsically safe C&I signal circuits. A distinction is made between two groups of conductors:
- Application on, or in close proximity to the field device—i.e., directly in the Ex area if necessary.
- Installation in the control cabinet, ahead of the Ex-i circuit control components.
Overvoltage Suppressor, Type F-LB
To protect the field devices, the F-LB type is provided with a high-grade steel sleeve and internal thread. The principle is as easy as it is effective: On installation, the item is simply screwed into a free cable gland on the field device and three cables are attached in the terminal compartment. In intrinsically safe circuits the overvoltage barriers can be installed directly in the Ex area for protection of the field devices.
In the event that the terminal compartment of the field device is unsuitable for this type of mounting, the F-LB screw-in types or, alternatively, the extra slim K-LB types, are mounted on a terminal box and series-connected to the field devices.
Overvoltage Suppressor, Type K-LB
The K-LB family of devices in the 12.5 mm wide housings are likewise used for the protection of control cabinet equipment. The housing is available both in 1 and 2-channel versions and facilitates free wiring to the protected components.
Overvoltage Protection of a Piece
The K-Series device and the overvoltage protection form a closed unit within the control cabinet. This is a very special benefit, because this principle is simple and reliable. The user avoids field wiring in these areas and saves planning and wiring time. But the decisive benefit of the snap-on solution is the fixed wiring. The connection between the overvoltage conductor and the functional module is short, durable and suited to industrial applications.
Overvoltage Suppressor, Type P-LB
The danger of a subsequent coupling in of transients due to unsatisfactory cabling between the protected and unprotected wiring is much reduced in comparison to the free wiring arrangement. In the case of an Ex-i control cabinet, the spacing between the wiring within the cabinet is already prescribed and ideally satisfies the requirements of overvoltage protection.
Characteristics of the overvoltage conductor:
- Response time from 1 ns
- Discharge capacity 10 kA
- Automatic resumption of standby after responding
- Insulation voltage > 500 VDC (important for intrinsically safe, galvanically isolated circuits)
Conclusion
A functional overvoltage protection system requires a consistent separation of the wiring in addition to fast and reliable performance from the conducting elements. If overvoltage conductors are too densely installed or, for reasons of space, are positioned between the devices that are to be protected, then there will be the danger of coupling because of the cramped installation and as a consequence of coupling caused by the positioning of the wiring. The installation regulations for the execution of intrinsically safe circuits fundamentally demand separated installation of the wiring. Thus, the functional capability and the correct execution of overvoltage protection within a control cabinet—for the purpose of Ex-i isolation with appropriate interface modules—is sometimes simpler to achieve than when free wiring is adopted.
