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Intrinsic Safety

Sécurité Intrinsèque les principes de base

    In many industrials processes, the presence of flammable materials (gases, vapours, liquids, dusts, fibres and flyings) requires the adoption of safety practices to protect both, plant and personnel, from the risk of fires and explosions. An explosion or fire can occur when, in certain areas at certain times, an explosive or flammable mixture and a means of ignition, thermal or electrical, are present.
    Flammable materials are grouped according to the ignition energy (Gas Groups) and classified for their minimum ignition temperature (Temperature Class), while Area classification ("Zone" in Europe, "Division" in the USA) takes into account the probability of the presence of an explosive mixture. Electrical equipment, in Hazardous Areas ("Locations" in the USA) , constitute potential sources of danger because they may generate arcs or sparks or hot surfaces which could ignite the explosive atmosphere.

Ignition Triangle

    From a chemical point of view, oxidation, combustion and explosion are all exothermic reactions with different reaction speeds. For such reactions to take place, it is essential that the following three components be present simultaneously in due proportions:

    • Fuel: flammable vapors, liquids or gases, or combustible dusts or fibers;
    • Oxidizer: generally, air or oxygen;
    • Ignition Energy: electrical or thermal.

Protection Method

    Basic safety concept is to avoid the simultaneous existence of a dangerous atmosphere and a source of ignition by:

    • Containing the explosion within a well-defined space where it will not cause any harm.
    • Physically segregating the sources of energy from the explosive mixtures.
    • Preventing the release of sufficient energy to ignite any explosive mixture.

    According to the safety concept and the way to apply it, there are different explosion protection methods to enable electrical equipment to be used in hazardous Area. All these techniques are ruled by national and international standards, as well as codes of practice, that define how to design and install the equipment, while recognized authorities issue the conformity certificates of the apparatus or systems. Among the protection methods, the simplest and most effective, applied to the electrical and electronic instrumentation, is Intrinsic Safety.

    The basic principle, on which intrinsic safety works, is to limit, under normal conditions, the amount of electrical energy in Hazardous Area circuits such that any sparks or arcs or high surface temperatures cannot ignite the explosive atmosphere.
    Electrical equipment, in Hazardous Area, as well as the interconnected instrumentation in Safe Area, must be designed to reduce the open circuit voltage (Voc) and short circuit current (Isc) to values that cannot cause ignition by opening, closing or earthing the circuit or by heating of any parts belonging to the circuit.

Resistive Circuits

    A circuit is considered as resistive when the reactive part, inductance and capacitance, is zero or negligible (figure A). The energy released by this type of circuit depends essentially on the power supply source V and the current limitation due to the presence of resistor R. In this case, it is difficult to correlate the minimum ignition energy (MIE) with a circuital situation that generates the spark. The experimental tests on this type of circuit have demonstrated that the capacity for igniting a dangerous mixture depends on the open-circuit voltage (Voc=V) and the short-circuit current (Isc=V/R). The ignition curve for resistive circuits is shown in Figure B. The above graph shows the ignition curve relative to the group of gases that are considered by the standards. By the trend of the curve, note that the lower the open-circuit voltage, the greater the amount of power that can be used safely. This characteristic allows process instrumentation that works with voltages on the order of 20-30 V to be used efficiently in intrinsic safety applications.

 Figure A

 Figure B

    The inherent lo power involved, even in unfavourable circumstances, gives some advantages that can not be obtained with other techniques:
    • Intrinsic Safety is the only method accepted for the most Hazardous Areas (Zone 0).
    • Maintenance and calibration of field equipment can be carried out while the plant is in operation and the circuit "live". Low voltages are also safe for personnel.
    • No special mechanical protection of field wiring is required but ordinary instrument cabling is acceptable.

    In Intrinsically Safe applications three basic parts have to be considered:

    • Hazardous Area devices (Simple Apparatus), or equipment (Intrinsically Safe Apparatus)
    • Safety interfaces (Associated Apparatus)
    • Interconnecting cables.

Simple Apparatus

    Passive components (switches, resistive sensors, potentiometers), simple semiconductor (LEDs, phototransistors) and simple generating devices (thermocouples, photocells) are regarded as Simple Apparatus if they do not generate or store more than: 1,5 V, 100 mA, 25 mW.
    Simple Apparatus can be used in Hazardous Area without certification; they have to be assessed for the temperature classification on the basis of the matched output power of the interface device.

Intrinsically Safe Apparatus

    Transmitters, I/P converters, solenoid valves and any other "energy-storing" devices must be certified as Intrinsically Safe Apparatus suitable for use in Hazardous Area, according to the zone classification and gas characteristics (group and temperature class).

Associated Apparatus

    Interfaces between field and control room equipment, usually called "Barriers", protect the Hazardous Area circuits by limiting the voltage and current in normal and in fault conditions.
    Two types of intrinsically safe interface exist: "Zener Barriers" and "Galvanic Isolator Barriers"; they basically differ on the way the potentially dangerous energy, from control room equipment, is diverted to prevent it from passing through to the Hazardous Area circuits.
    Barriers must be designed and certified as Associated Apparatus suitable for connection to intrinsically safe or simple apparatus in hazardous Area. Associated apparatus are the key to any intrinsically safe system because they define maximum allowable safety parameters of the circuits connected the Hazardous Area terminals of the barriers.

Interconnecting Cables

    Low voltage and current, in intrinsically safe circuits, allow the use of ordinary instrumentation cables provided that capacitance and inductance are taken into account in assessing the safety of the system; cable parameters seldom are a problem and long distances can be easily achieved.

Explosive Mixture Characteristics

    The risk of an ignition of an air/gas mixture depends on the probability of the simultaneous presence of the following two conditions:

    • Formation of flammable or explosive vapors, liquids or gases, or combustible dusts or fibers with atmosphere or accumulation of explosive or flammable material;
    • Presence of an energy source - electrical spark, arc or surface temperature - that is capable of igniting the dangerous mixture present.

    It is possible to draw an ignition characteristics for each type of fuel. The characteristic curves of hydrogen and propane are illustrated in the previous page. A minimum ignition energy (MIE) exists for every fuel that represents the ideal ratio of fuel to air. At this ratio, the mixture is most easily ignited. Below the MIE, ignition is impossible for any concentration. For a concentration lower than the one corresponding to the MIE, the quantity of energy required to ignite the mixture increases until a concentration value is reached below which the mixture cannot be ignited due to the low quantity of fuel. This value is called the lower explosive limit (LEL). In the same way, when increasing the concentration the energy requirement increases, and a concentration value is identified above which ignition cannot occur due to the low quantity of an oxidizer. This value is called the upper explosive limit.(UEL)

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