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explosion protection

The basics of explosion protection - 9224 4.technical principles

Normal safety Safety is required under normal operation. The failure of the zener diode is not taken into account. (level of protection “ic”: one single zener diode). 26 Isolation of Intrinsically Safe Circuits fromNon-intrinsically Safe Circuits An important measure for intrinsically safe circuits is the safe isolation of all intrinsically safe circuits from non intrinsically safe circuits (fig. 6). Safe electrical isolation is always required, with the exception of safety barriers.Electric isolation is generally recommanded forZone 0. Zener diodes, used for limiting voltage, as well as other semiconductor components are considered to be fallible and must therefore be safeguarded by means of redundant components. Wire wound or sheet resistors for current limi- tation are considered to be infallible components (they have high resistivity in the event of a fault). Therefore one single component is sufficient.

Fig.6: Electric Isolators IS pac Table 15: Levels of protection of intrinsically safe electrical circuits Level of protection “ia”Electrical apparatus of level of protection ”ia”shall not be capable of causing ignition in normal operation and when one fault occurs or when a combination of any two faults occurs.Safety factor1.5:During normal operation and in case of one faultSafety factor 1.0:Two independent faults Level of protection “ib”Electrical apparatus of level of protection ”ib”shall not be capable of causing ignition in normal operation or when one fault occurs.Safety factor1.5:During normal operation and in case of one faultSafety factor 1.0:In case of one fault, if the electrical apparatus does not have unprotected switching contacts in those components, which may be exposed to an explosive atmosphere, and when the fault is monitored. Level of protection “ic”Electrical apparatus of level of protection “ic”shall not be capable of causing ignition in normal operation.Safety factor1.0:During normal operation R.STAHL explosion protection