Western Telematic TAN-1005 manual Conclusion, Appendix a, Appendix B

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 79-14 code of practice of electrical installations in hazardous areas dated October 1994. It may still be modified in detail but it is not probable that the principles will change.

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 self-loosening.

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 364-4-41.

Metallic enclosures of intrinsically-safe apparatus need not be connected to the equipotential bonding system, unless required by the apparatus documentation. Installations with cathodic protection shall not be con- nected to the equipotential bonding system unless the system is specifi- cally designed for this purpose.

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

Intrinsically-safe circuits shall be installed in accordance with 12.2 except where modified by the following special requirements.

In installations with intrinsically-safe circuits for Zone 0 the intrinsically- safe apparatus and the associated apparatus shall comply with IEC 79-11 category ‘ia’. Associated apparatus with galvanic isolation between the intrinsically-safe and non-intrinsically-safe circuits is pre- ferred. Associated apparatus without galvanic isolation may be used provided the earthing arrangements are in accordance with item 2) of

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, intrinsically-safe apparatus, associated apparatus and the maximum allowable electrical param- eters of interconnecting cables) shall be of category ‘ia’.

Simple apparatus installed outside the Zone 0 shall be referred to in the system documentation and shall comply with the requirements on IEC 79-11, category ‘ia’.

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 intrinsically-safe circuit is installed in Zone 0 such that apparatus and the associated equipment are at risk of developing hazardous potential differences within the Zone 0, for example through the presence of atmospheric electricity, a surge protection device shall be installed between each non-earth bonded core of the cable and the local structure as near as is reasonably practicable, preferably within 1 m, to the entrance to the Zone 0. Examples of such locations are flammable liquid storage tanks, effluent treatment plant and distillation columns in petrochemical works. A high risk of potential difference generation is generally associated with a distributed plant and/or ex- posed apparatus location, and the risk is not alleviated simply by us- ing underground cables or tank installation.

The surge protection device shall be capable of diverting a minimum peak discharge current of 10 kA (8/20 μs impulse to IEC 60-1, 10 operations). The connection between the protection device and the local structure shall have a minimum cross-sectional area equivalent to 4 mm2 copper.

The spark-over voltage of the surge protection device shall be deter- mined by the user and an expert for the specific installation.

NOTE - The use of a surge protection device with a spark-over voltage below 500 V a.c. 50 Hz may require the intrinsically-safe circuit to be regarded as being earthed.

The cable between the intrinsically-safe apparatus in Zone 0 and the surge protection device shall be installed such that it is protected from lightning.

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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