supply will contain a significant amount of surges but if for any reason the supply is particularly exposed to invasion from lightning induced surges then some consideration to suppressing the main supply should be given.
A practical economic solution is to protect the supply input to the com- puter system as indicated in figure 5.
A similar argument can be made if a data link is made to any remote location. This is less likely to directly affect the intrinsically safe circuit but can be very damaging to the computer.
7 CONCLUSION
The solution shown in figure 5 is therefore the best practical solution to achieve safety for circuits entering Zone 0 where there is a significant probability of the circuit being influenced by adjacent lightning strikes.
It is probable that this solution is not directly applicable to all installa- tions but a solution based on a similar analysis is usually achievable. MTL and Telematic are in an almost unique position to give advice on this problem and consider that they have the competence to assist in preparing the relevant documentation.
APPENDIX A
This appendix is comprised of extracts from the draft IEC
6.3Potential equalisation
Potential equalisation is required for installations in hazardous areas. For TN, TT and IT systems all exposed and extraneous conductive parts shall be connected to the equipotential bonding system. The bonding system may include protective conductors, metal conduits, metal cable sheaths, steel wire armouring and metallic parts of structures, but shall not include neutral conductors. Connections shall be secure against
Exposed conductive parts need not be separately connected to the equipotential bonding system if they are firmly secured to and are in metallic contact with structural parts or piping which are connected to the equipotential bonding system. Extraneous conductive parts, which are not part of the structure or of the electrical installation, need not be connected to the equipotential bonding system, if there is no danger of voltage displacement, for example frames of doors or windows.
For additional information see clause 413 of IEC
Metallic enclosures of
NOTE - Potential equalisation between vehicles and fixed installations may require special arrangements, for example, where insulated flanges are used to connect pipelines.
6.5Lightning protection
In the design of electrical installations, steps shall be taken to reduce the effects of lightning.
NOTE - In the absence of IEC standards on protection against light- ning, national or other standards should be followed.
Subclause 12.3 gives details of lightning protection requirements for Ex ‘ia’ apparatus installed in Zone 0.
12.3Installations for Zone 0
In installations with
12.2.4and any mains powered apparatus connected to the safe area terminals are isolated from the mains by a double wound transformer, the primary winding of which is protected by an appropriately rated fuse of adequate breaking capacity. The circuit (including all simple components, simple electrical apparatus,
Simple apparatus installed outside the Zone 0 shall be referred to in the system documentation and shall comply with the requirements on IEC
If earthing of the circuit is required for functional reasons the earth connection shall be made outside the Zone 0 but as close as is reason- ably practicable to the Zone 0 apparatus.
If part of an
The surge protection device shall be capable of diverting a minimum peak discharge current of 10 kA (8/20 μs impulse to IEC
The
NOTE - The use of a surge protection device with a
The cable between the
APPENDIX B
Requirements of simple apparatus extracted from EN50020:1994.
5.4Simple apparatus
The following apparatus shall be considered to be simple apparatus:
a)passive components, e.g. switches, junction boxes, potentiometer and simple semiconductor devices.
b)source of stored energy with well defined parameters, e.g. capacitors or inductors, whose values shall be considered when determining the overall safety of the system.
c)sources of generated energy, e.g. thermocouples and photo- cells, which do not generate more than 1,5 V, 100 mA and 25 mW. Any inductance or capacitance present in these sources of energy shall be considered as in b).
Simple apparatus shall conform to all relevant requirements of this stand- ard but need not be certified and need not comply with clause 12. In particular the following aspects shall always be considered.
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