Earthing –bonding and all that
The perennial problem of earthing and bonding is raised from time to time. The basic principles that are applicable to all bonding problems is that it is potential difference that causes problems and explosions and that all currents should be provided with a well defined low impedance return path. Within Europe there is a strong movement to train cows to stand on one leg during thunderstorms so that they avoid the potential difference which could ignite the methane in their stomachs. These learned ‘certified ‘ beasts will be branded with the appropriate ATEX marking.
A recent situation where these principles were applied was to consider the bonding of an IS installation on an off-shore rig which was subject to lightning induced surges, the primary concern being the bonding of the screen. The installation, which emerged, is illustrated in this diagram
NOTE. The numbers shown on the diagram and used throughout the text are indicative of the order of values commonly encountered. They are intended to assist in understanding the subject and are not intended to be precise or applicable in all circumstances.
Almost all offshore installations are electrically well-bonded structures, which normally carry the relatively small [500 mA] leakage currents associated with heavy current equipment. There are tales of significant voltages appearing between deck plates but these are outside the author’s experience. Significant circulating currents have been experienced in the immediate vicinity of the electrical generators on some less well constructed installations but these are very localised and do not affect the major part of the installation.
In normal operation the circuit when connected as illustrated follows the usual rules for intrinsically safe circuits with the screen being bonded at one point only. In this case, multiple bonding of the screen would not cause an incendive current to flow in the screen because the voltage between the bonding points A and B would not be high [50 mV]. It remains good practice to bond the screen at one point in order to avoid low frequency interference being induced in the measuring circuit. In this type of circuit the measuring circuit, the screen and the structure are maintained at or about the same potential by SPD 2 and its bond to point B. The measuring circuit within the field device is usually isolated from the enclosure, [except for RF interference capacitors which are normally negligible from an IS viewpoint], however the SPD is usually bonded to the enclosure and its bond is common with that of the enclosure.
When a lightning strike occurs then a significant current flows in the rig structure, and this current has a fast rise time. [100 kA/ 10ms]. The return path for this current is either the discharge of a locally accumulated induced charge between cloud and sea or part of a circulating current through the earth and troposphere or a combination of both. The usual simplification is that it disappears into the sea or seabed and the rig makes a good connection for it to disappear through. The current sub-divides in the structure and the lower current [10kA] generates a voltage difference in the structure, which is almost entirely due to the inductance of the structure [0,1mH/m] and the rise time of the current [10ms]. Using the suggested figures and assuming a distance of a 100m between A and B gives a voltage difference of 10kV between the two points. The SPDs prevent this voltage difference damaging the measuring equipment but allow a further subdivided current to flow through the interconnecting cable [100A]. It follows that during the short period of this transient current [100ms] the cable is not intrinsically safe, but this is considered an acceptable risk in Zones 1 and 2. An advantage of a well-bonded structure is that the SPD bonds can be kept short [1m] and straight thus minimising their inductance [1mH] which results in a small transient voltage drop [10v] which can largely be ignored. The bonds need to be mechanically robust and secure. The forces generated by these transient currents are not negligible. In practice any transient voltage difference between the field cable and the computer is absorbed by the isolation in the interface.
In normal operation the screen is bonded to the structure and provides an effective electrostatic screen.When the screen is connected to the cable via SPD2 as illustrated, it tracks the potential of that end of the cable during the transient. Consequently there is a potential difference between the screen and the cable at the field end. This is not quite the full voltage difference between points A and B [10 Kv] because the transient voltage is attenuated by the distributed capacitive and inductive of the screen and cable but it is still significant.[5kV] The cable insulation will usually easily withstand this transient but it is important that flashover does not occur at the field termination. It is desirable therefore that the screen should be carefully cut back and insulated and preferably secured in an unused terminal so as to minimise this probability.
The system illustrated describes an effective bonding technique, which is adequately safe in normal and abnormal conditions. A practical installation will almost certainly differ in detail because of the difficulty of ensuring adequate bonding connections. However providing that the basic principle of minimising potential difference is considered then an adequately safe installation should be created.
