In order to specify SIL, the IEC61508 standard classifies the operation modes of SRSs into two categories, low demand mode of operation and high demand/continuous mode of operation. However, it is not clear which modes of operation should be applied to the protection devices of the heaters. Then, the present paper investigates the relationships among failures of the protection devices, demand to operate the heater and the hazardous event, i.e., breaking out of fire by use of fault tree analyses involving sequential failure logic AND-gates (priority AND-gates). Finally SILs of the protection devices are specified using the ratios of hazardous event to demand frequencies. Thus, the present paper demonstrates the procedure how to combine SILs of the protection devices with risk analyses of the overall electric heater-systems.
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