Three conditions, flammable media, oxygen and temperature need to exist for fire or explosion to take place. In a Hydrocarbon handling installation or mines having flammable/explosive materials, the presence of explosive mixtures are a cause of major fire /explosion hazard. Various methodologies and techniques as stipulated in IS/IEC 60079 are being adopted by the installation owner to handle this hazards. Depending on the type of system and severity of the hazard, owners use different type of techniques such as Explosion proof (Exd), Increased safety (Exe), intrinsically safe (Exi) pressurized enclosure (Exp), Special protection (Exs) etc. are selected prevalently. Each of these systems have their own advantages ,disadvantages and limitations, however Intrinsic Safety (Exi) is the most preferred choice for the low voltage handling systems such as instrumentation ,SCADA and control systems. IS/ IEC 60079-11; IS/IEC 60079-25 lays down guidelines of Intrinsic Safety (Exi) to limit energy in the hazardous area below the ignition energy of the explosive hazard present thereby preventing explosions.
Guidelines as laid out by the standards must be followed while implementing safety systems used to safe-guard against explosion hazard. We can imagine what can happen if there is a casual approach while implementing or maintaining such systems. Out of all the protective systems specified by IEC 60079, Intrinsic Safety is the only protection that considers faults of the field wiring and offers live working without the need for a safety clearance permit to perform maintenance. It does require some design and planning to ensure that the system loop analysis is acceptable.
To ensure that an intrinsically safe system performs with safety against explosion hazard, user has to ensure following:
Ensure intrinsically safe certification of the equipment installed in hazardous area and the associated equipment installed in safe area with the area classification of area of equipment installation.
Ensure parameter compatibility between field devices, associated equipment in safe area and the interconnecting cable i.e.
V (t)max ≥ V(a), I(t)max ≥ I(a), P(t)max ≥ P(a)
C(a)max ≥ C(t) +C(c) , L(a)max ≥L(t) + L(c)
Where V stands for Voltage, I Stands for current, P stands for Power, C stands for capacitance and L stands for inductance. While (t) stands for transmitter parameter, (a) stands for associated equipment parameter and (c) stands for cable parameter.
There must be a minimum separation between Intrinsically safe and non-intrinsically safe terminals/wires/equipment.
In order to ensure Life cycle safety of the intrinsically safe loop, it is necessary that user must maintain loop-wise record of intrinsically loop design , certified GA drawing, Field cable routing drawing, Junction box drawing and loop wiring drawing with component model number etc. These drawings must be controlled for future use i.e. in case there is a change or, replacement of any component or wiring, this activity must be carried out after proper approvals by the user. There is a need to adopt a procedure similar to that described under IS/ IEC 61508 or IS/IEC 61511. Users must have proper records and life cycle safety procedure for intrinsically safe system , which must be meticulously followed.
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