The basics of explosion protection - STAHL - #25

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Minimum ignition energy Associated apparatus The minimum ignition energy of a gas/air and vapour/air mixture is the smallest level of electri- cal energy which occurs while a capacitor is discharging, and which may still be sufficient to ignite the most ignitable mixture of a gas or vapour and air at atmospheric pressure and 20°C.An essential aspect of the type of protection“Intrinsic Safety” is reliability with regard to the observance of voltage and current limit values, even if determined faults may occur. Intrinsically safe apparatus and intrinsically safe components from related equipment are classified in different levels of protection “ia”, “ib” or “ic” with regard to infallibility.The level of protection “ia” is a prerequisite for category 1 equipment and suitable for use in Zone 0, the level of protection “ib” for category 2 equipment and suitable for use in Zone1. The new level of protection “ic” for cate- gory 3 is suitable for use in Zone 2. Electrical apparatus which contains circuits, some of which are intrinsically safe and some are not, and which is constructed such that the non-intrin- sically safe circuits cannot negatively adversely affect the intrinsically safe circuits (table 14). 25 4.4.2 Applications of Type of Protection“Intrinsic Safety” An electric circuit in which neither a spark nor the effect of heat can cause a defined explosive atmosphere to ignite. Intrinsically safe apparatus Terms and Definitions Intrinsically safe electrical circuit Electrical apparatus in which all circuits are intrinsically safe. The type of protection “Intrinsic Safety” is basedon the principle of energy limitation within an electric circuit.The energy from a power circuit capable of causing an explosive atmosphere to ignite is thus limited to such an extent that the surrounding explosive atmosphere cannot ignite as a result of sparks or inadmissible surface heat- ing of the electrical components. The type of protection “Intrinsic Safety” is partic-ularly used in measurement and control tech- nology, as no high currents, voltage and power are required here.
Table 14: Difference between Intrinsically Safe and Associated Apparatus Intrinsically safe apparatusAssociated apparatusThese contain intrinsically safe circuits onlyThese contain both intrinsically safe and non-intrinsically safe electric circuitsEEx ib IIC T6[EEx ib] IIC T6EEx de [ib] IIC T6 All necessary information such as category, ex-The square brackets indicate that the associated electrical apparatus contains an intrinsicallyplosion group and temperature class is provided.safe electric circuit that may be introduced into Zone 1, gas groups IIA, IIB and IIC.The apparatus may be used in Zone 1.The apparatus has to be installed outsideThanks to being integrated in a flameproof of the potentially explosive area.enclosure (“d”), the apparatus may be usedin Zone 1. R.STAHL explosion protection