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Safety Standard IEC 60079
intrinsic safety “i”
"type of protection based on the restriction of electrical energy within equipment and of interconnecting wiring exposed to the explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects"
8.5 Current-limiting resistors
Current-limiting resistors shall be one of the following types:
a) film type;
b) wire wound type with protection to prevent unwinding of the wire in the event of breakage;
c) printed resistors as used in hybrid and similar circuits covered by a coating conforming to 6.3.9 or encapsulated in accordance with 6.6.
An infallible current-limiting resistor shall be considered as failing only to an open-circuit condition which shall be considered as one countable fault.
A current-limiting resistor shall be rated in accordance with the requirements of 7.1, to withstand at least 1,5 times the maximum voltage and to dissipate at least 1,5 times the maximum power that can arise in normal operation and under the fault conditions defined in Clause 5. Faults between turns of correctly rated wire wound resistors with coated windings shall not be taken into account. The coating of the winding shall be assumed to comply with the required CTI value in accordance with Table 5 at its manufacturer’s voltage rating."